Chapter 11 Anatomy and Physiology

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FIGURE 11.21 Postsynaptic Potentials

(a) An excitatory postsynaptic potential (EPSP) is closer to threshold.

FIGURE 11.10 Graded Potentials

(a) Graded potentials are proportional to the stimulus strength. A weak excitatory stimulus applied briefly causes a small depolarization, which quickly returns to the resting membrane potential. (1). Progressively stronger stimuli result in larger depolarizations (2-4).

What is anterograde?

Axon transport mechanisms move cytoskeletal proteins, mitochondria and synaptic vesicles down the axon toward the presynaptic terminals. Movement away from the cell body is called anterograde.

Temporal summation.

Two action potentials arrive in close succession at the postsynaptic cell from he presynaptic terminal. The first action potential causes the production of a graded potential in the postsynaptic cell that does not reach threshold at the trigger zone. The second action potential results in the production of a second graded potential that summates with the first to reach threshold, resulting in the production of an action potential.

How many types of summation are possible?

Two types of summation are possible: spatial summation and temporal summation.

What are the 5 functions that neurons and glial cells allow for the nervous system?

1. Maintaining homeostasis 2. Receiving sensory input 3. Integrating information. 4. Controlling muscles and glands 5. Establishing and maintaining mental activity

What is a nerve?

A nerve is a collection of many axons bundled together outside the brain and the spinal cord.

What is a plexus?

A plexus is a bundle of nerves outside the brain and the spinal cord.

What is the third nervous system of our body?

A third division of the nervous system is the enteric nervous system, which consists of neuronal networks within the wall of the digestive tract.

What are Pseudo-unipolar neurons?

Pseudo-unipolar neurons, also known as pseudo-unipolar, start out as bipolar neurons during development, but the two processes fuse into a single process. These neurons have a single process extending form the cell body, which divides into two branches a short distance form the cell body. The two branches function as a single axon. One branch, the peripheral process, extends to the periphery and has dendrites. These dendrites either act as a sensory receptor or communicate with a sensory receptor. The other branch, called the central process, extends to the CNS. In this way, stimuli that occur at a sensory receptor generate action potentials that are conducted along the peripheral process to the central process and ultimately to the CNS. Most sensory neurons are unipolar.

When astrocytes help form endothelial cells what do the endothelial cells help form?

Recall from chapter 4 that tight junctions create an impermeable barrier between adjacent cells. The endothelial cells with their tight junctions form the blood-brain barrier.

FIGURE 11.5 Structural Classes of Neurons

(a) A multipolar neuron has many dendrites and an axon.

Ependymal cells

(a) Ciliated ependymal cells lining the ventricles of the brain and the central canal of the spinal cord help move cerebrospinal fluid. (b) Ependymal cells on the surface of the choroid plexuses secrete cerebrospinal fluid.

FIGURE 11.3 Divisions of the Peripheral Nervous System

(a) Sensory division. A neuron with its cell body (green dot) in a dorsal root ganglion.

FIGURE 11.4 Neuron

(a) The structural features of a neuron are a cell body and two types of cell projections: dendrites and an axon. The neuron depicted here is a multipolar neuron. (b) Photomicrograph of a multipolar neuron.

FIGURE 11.6 Responses to Injury in an Axon

(a) When a nerve is injured, there are two possible outcomes. Regardless, the muscle will initially atrophy (shrink in size). (b) Without stimulation from the nerve, the muscle is paralyzed and atrophies. (c) When the two ends of an injured axon are aligned in close proximity, healing and regeneration of the axon are likely to occur. After rein nervation, the muscle can become functional and hypertrophy (increase in size). (d) When the two ends of an injured axon are not aligned in close proximity, regeneration is unlikely to occur. Without innervation from the nerve, muscle function is completely lost, and the muscle remains atrophied.

FIGURE 11.5 Structural Classes of Neurons

(b) A bipolar neuron has a dendrite and an axon.

FIGURE 11.3 Divisions of the Peripheral Nervous System

(b) Somatic nervous system. The neuron (purple) extends from the spinal cord (CNS) to skeletal muscle.

FIGURE 11.5 Structural Classes of Neurons

FIGURE 11.3 Divisions of the Peripheral Nervous System

(c) Autonomic nervous system. Two neurons are in series between the CNS and the effector organ or cells (smooth muscle or glands). The first neuron has its cell body (red dot) in the CNS, and the second neuron has its cell body in an autonomic ganglion.

FIGURE 11.5 Structural Classes of Neurons

(d) An anaxonic neuron has multiple branches but no axons.

Branch of Nervous System: CNS

Components: Brain and spinal cord Division: Direction of Signal: Branch of PNS: Type of Control: Subdivisions: Effectors: Respons at Effector:

Branch of Nervous System: PNS

Components: Receptors, nerves, ganglia, plexuses Division: Sensory Motor Direction of Signal: Afferent Efferent Branch of PNS: Somatic Autonomic Type of Control: Voluntary Involuntary Subdivisions: Sympathetic Parasympathetic Effectors: Skeletal muscle Cardiac and smooth muscle; glands Cardiac and smooth muscle; glands Responses at Effector: Stimulates contraction Readies body for physical activity Regulates resting functions.

3. Integrating information.

The brain and spinal cord are the major organs of processing sensory input and initiating responses. The input may produce an immediate response, be stored as memory, or be ignored.

PROCESS FIGURE 11.8 Resting Membrane Potential.

(a) In a resting cell, there is a higher concentration of K+ inside the cell and a higher concentration of Na+ outside the cell. In a resting cell, only the leak ion channels are open; the gated ion channels are closed. There are many more K+ leak ion channels than Na+ leak ion channels. As a result, K+ diffuses out of the cell down its concentration gradient. The membrane is not permeable to the negatively charged proteins inside the cell. The tendency for the K+ to diffuse to the outside of the cell down its concentration gradient is opposed by the tendency for the positively charged K+ to be attracted back into the cell by the negatively charged proteins. A small amount of Na+ diffuses into the cell (not shown). The sodium-potassium pump helps maintain the differential levels of Na+ and K+ by pumping three Na+ out of the cell in exchange for two K+ into the cell. The pump is drive by ATP hydrolysis.

PROCESS FIGURE 11.20 Removal of Neurotransmitters from the Synaptic Cleft

(a) In some synapses, neurotransmitters are broken down by enzymes and recycled into the presynaptic terminal. (b) In other synapses, neurotransmitters are taken up whole into the presynaptic terminal.

FIGURE 11.7 Comparison of Myelinated and Unmyelinated Axons

(a) Myelinated axon with two Schwann cells forming part of the myelin sheath around a single axon. Each Schwann cell surrounds part of one axon.

FIGURE 11.22 An Axoaxonic Synapse

(a) The inhibitory neuron of the axoaxonic synapse is inactive and has no effect of the release of neurotransmitter from the presynaptic terminal.

FIGURE 11.10 Graded Potentials

(b) A stimulus applied to a cell causes a small depolarization. When a second depolarizing stimulus is applied before the first disappears, the second stimulus is added to the first to result in an even larger depolarization.

FIGURE 11.21 Postsynaptic Potentials

(b) An inhibitory postsynaptic potential (IPSP) is farther from threshold.

FIGURE 11.22 An Axoaxonic Synpase

(b) The inhibitory neuron of the axoaxonic synapse releases a neuromodulator, which reduces the amount of neurotransmitter released from the presynaptic terminal.

PROCESS FIGURE 11.8 Resting Membrane Potential

(b) The recording electron is inside the cell; the reference electron is outside. A potential difference of about -70 mV is recorded, with the inside of the plasma membrane negative with respect to the outside of the membrane.

FIGURE 11.7 Comparison of Myelinated and Unmyelinated Axons

(b) Unmyelinated axons with two Schwann cells surround several axons in parallel formation. Each Schwann cell surrounds part of several axons.

What are the four phases of action potentials?

Action potentials have four phases: (1) a depolarization phase, (2) a repolarization phase, (3) an after potential, and (4) the return to rising membrane potential.

What are Characteristics of Graded Potentials

1. A stimulus causes ion channels to open, increasing the permeability of the membrane to Na+, K+, or Cl- 2. Increased permeability of the membrane to Na+ results in depolarization. Increased permeability of the membrane to K+ or Cl- results in hyperpolarization. 3. The size of the graded is proportional to the strength of the stimulus. Graded potentials can also summate. Thus, a graded potential produced in response to several stimuli is larger than one produced in response to a single stimulus. 4. Graded potentials are conducted in a decremental fashion, meaning that their magnitude decreases as they spread over the plasma membrane. Graded potentials cannot be measure a few millimeters from the point of stimulation. 5. Depolarizing graded potentials can combine (summate) to cause an action potential.

What are the six chemical classes of neurotransmitters?

1. Acetylcholine 2. Biogenic Amines 3. Amino Acids 4. Purines 5. Neuropeptides 6. Gases and Lipids

List the steps of Acetylcholine removal from the Synaptic Cleft.

1. Acetylcholine molecules bind to their receptors. 2. Acetylcholine molecules unbind from their receptors. 3. Acetylcholinesterase splits acetylcholine into choline and acetic acid, which prevents acetylcholine form again binding to its receptors. Choline is taken up by the presynaptic terminal 4. Choline is used to make new acetylcholine molecules that are packaged into synaptic vesicles. 5. Other acetylcholine molecules simply diffuse into the extracellular fluid away from the synaptic cleft.

What are Characteristics of Action Potentials?

1. Action potentials are produced when a graded potential reaches threshold. 2. Action potentials are all-or-none. 3. Depolarization is a result of increased membrane permeability to Na+ and movement of Na+ into the cell. Activation gates of the voltage-gated Na+ channels open. 4. Repolarization is a result of decreased membrane permeability to Na+ and increased membrane permeability to K+, which stops Na+ movement into the cel land increases K+ movement out of the cell. The inactivation gates of the voltage-gated Na+ channels close, and the voltage-gated K+ channels open. 5. During the absolute refractory period, no action potential is produced by a stimulus, no matter how strong. During the relative refractory period, a stronger-than-threshold stimulus can produce an action potential. 6. Action potentials are propagated and, for a given axon or muscle fiber, the magnitude of the action potential is constant. 7. Stimulus strength determines the frequency of action potentials.

List the steps of A Chemical Synapse.

1. Action potentials arriving at the presynaptic terminal cause voltage-gated Ca2+ channels to open. 2. Ca2+ diffuses into the cell and stimulates exocytosis of the synaptic vesicles, which releases neurotransmitter molecules. 3. Neurotransmitter molecules diffuse form the presynaptic terminal across the synaptic cleft. 4. Neurotransmitter molecules combine with their receptor sites and cause ligand-gated Na+ channels to open. Na+ diffuses into the cell and causes depolarization.

List the steps of the Production of Action Potential.

1. Action potentials in the communicating neuron stimulate graded potentials in a receiving neuron that can summate at the trigger zone. 2. Action potentials are propagated down the axon to the axon terminal 3. Action potentials result in communication of the neuron with its target.

List the steps of Action Potential Propagation in an Unmyelinated Axon.

1. Action potentials propagate in one direction along the axon. 2. An action potential (orange part of the membrane ) generates local currents of Na+ (black arrows0 that tend to depolarize the membrane immediately adjacent to the action potential. 3. When depolarization caused by the Na+ entry reaches threshold, a new action potential is produced adjacent to where the original action potential occurred. 4. Action potential propagation occurs in one direction because the absolute refractory period of the previous action potential prevents generation of an action potential in the reverse direction.

Give the steps of Saltatory Conduction.

1. An action potential (orange) at a node of Ranvier generates local currents of Na+ (black arrow). The Na+ flows to the next node of Ranvier because the myelin sheath of the Schwann cell insulates the axon of the internode. 2. When the depolarization caused by the Na+ entry reaches threshold at the next node of Ranvier, a new action potential is produced (orange). 3. Action potential propagation is rapid in myelinated axons because the action potentials are produced at successive nodes of Ranvier (1-5) instead of at every part o the membrane along the axon.

List the steps of Electrical Synapse

1. Electrical synapses connect cardiac muscle cells. 2. An electrical synapse is a gap junction where the membranes of two cells are separated by a gap but connected by proteins called connexons. 3. An action potential (orange arrow) in the plasma membrane generates local currents (black arrows). Na+ ions from one cell flow to adjacent parts of the plasma membrane and through the gap junction. 4. The Na+ movement stimulates the production of another action potential. Thus, the action potential propagates along the plasma membrane. 5. As Na+ flows through a gap junction, it stimulates the production of an action potential in the adjacent cardiacs muscle cell. Thus, the action potential propagates to the adjacent cell.

List the steps of the resting membrane potential.

1. In a resting cell, there is a higher concentration of K+ (purple circles) inside the cell membrane and a higher concentration of Na+ (pink circles) outside the cell membrane. Because the membrane is not permeable to negatively charged proteins (green) they are isolated to inside of the cell membrane. 2. There are more K+ leak channels than Na+ leak channels. In the resting cell, only the leak channels are opened; the gated channels (not shown) are closed. Because of the ion concentration differences across the membrane, K+ diffuses out of the cell down its concentration gradient. The tendency for K+ to diffuse out of the cell is opposed by the tendency of the positively charged K+ to be attracted back into the cell by the negatively charged proteins. 3. The sodium-potassium pump helps maintain the differential levels of Na+ and K+ by pumping three Na+ out of the cell in exchange for two K+ into the cell. The pump is drive by ATP hydrolysis. The resting membrane potential is established when the movement of K+ out of the cell is equal to the movement of K+ into the cell.

Define ligand, receptor, and receptor site.

1. Ligand-gated ion channels. Ligand-gated ion channels are stimulated to open by the binding of a specific molecule to the receptor site of the ion channel. The receptor site of the ion channel is located on its extracellular side, which allows it to receive signals from the environment. The membrane-spanning part forms a channel through the phospholipid bilayer. The specific molecule that binds to the receptor site can be referred to as a ligand. Ligands could be neurotransmitters or hormones, but there is one particular ligand for each ligand-gated ion channel.

List and give examples of the general functions of the nervous system.

1. Maintaining homeostasis. The trillions of cells int he human body do not function independently of each other but must work together to maintain homeostasis. 2. Receiving sensory input. Sensory receptors monitor numerous external and internal stimuli. 3. Integrating information. The brain and spinal cord are the major organs for processing sensory input and initiating responses. 4. Controlling muscles and glands. Skeletal muscles normally contract only when stimulated by the nervous system; thus, the nervous system controls the major movements of the body by controlling skeletal muscle. 5. Establishing and maintaining mental activity. The brain is the center of mental activities, including consciousness, thinking, memory, and emotions.

Describe and give the function of a neuron cell body, a dendrite, and an axon.

1. Neuron Cell Body The neuron cell body, or soma, performs the typical functions of any cell, such as protein synthesis and packaging of proteins into vesicles. Each neuron cell body contains a single, relatively large, and centrally located nucleus with a prominent nucleolus. Neurons have extensive rough endoplasmic reticulum (ER), called Nissl bodies. The abundance of Nissle bodies reflects the significant amount of protein synthesis neurons perform. The Golgi apparatuses are located near the nucleus, and mitochondria and other organelles are present. Large numbers of intermediate filaments (neurofilaments) and microtubules form bundles that organize the cytoplasm into different regions. 2. Dendrites Dendrites are extensions of the cell body and receive information from other neurons or the environment portion of the neuron. Dendrites are short, often highly branched cytoplasmic extensions that are tapered from their bases at the neuron cell body to their tips. Many dendrite surfaces have small extensions, called dendritic spines, where axons of other neurons form connections with the dendrites. When stimulated, dendrites generate small electric currents, which are conducted toward the neuron cell body. 3. Axons In most neurons, a single axon arises from a cone-shaped area of the neuron cell body called the axon hillock. As the axon hillock narrows, it transitions into the initial segment. The initial segment is the actual beginning of the axon. The combination of the axon hillock and the initial segment is called the trigger zone. The trigger zone is where action potentials are generated. Many axons remain as a single structure, but others branch to form collateral axons, or side branches. Each axon has constant diameter, but axons can vary in length from a few millimeters to more than 1 meter. The cytoplasm of an axon is sometimes called the axoplasm, and its plasma membrane can also be called the axolemma (lemma, husk).

List the steps of Norepinephrine removal from the Synaptic Cleft.

1. Norepinephrine binds to its receptor. 2. Norepinephrine unbinds from its receptor. 3. Norepinephrine is taken up by the presynaptic terminal, which prevents norepinephrine from again binding to its receptor. 4. Norepinephrine is repackaged into synaptic vesicles or broken down by monamine oxidase (MAO). 5. Other norepinephrine molecules simply diffuse into the extracellular fluid away from the synaptic cleft.

List the steps of Voltage-Gated Ion Channels and the Action Potential.

1. Resting membrane potential. Voltage-gated Na+ channels (pink) and most, but not all, voltage-gated K+ channels (purple) are closed. The outside of the plasma membrane is positively charged compared to the inside. 2. Depolarization Voltage-gated Na+ channels open. Voltage-gated K+ channels begin to open. Depolarization results because the inward movement of Na+ makes the inside of the membrane more positive. 3. Repolarization. The inactivation gates of voltage-gated Na+ channels close and additional voltage-gated K+ channels open. Na+ movement into the cell stops, and K+ movement out of the cell increases, causing repolarization. 4. End of depolarization and after potential. Voltage-gated Na+ channels are closed. Closure of the activation gates and opening of the inactivation gates reestablish the resting condition for Na+ channels (see step 1). Diffusion of K+ through open voltage-gated channels produces the after potential. 5. Resting membrane potential. The resting membrane potential is reestablished by the Na+/K+ pump after the voltage-gated K+ channels close. Na+ is pumped out fo the cell and K+ is pumped into the cell.

What are the Characteristics Responsible for the Resting Membrane Potential?

1. The concentration of K+ is higher inside the cell than outside, and the concentration of Na+ is higher outside the cell than inside. 2. Due to the K+ leak channels, the plasma membrane is 50-100 times more permeable to K+ than to other positively charged ions, such as Na+ 3. The plasma membrane is impermeable to large, intracellular, negatively charged molecules, such as proteins. In other words, these anions are "trapped" inside the cell. 4. Potassium ions tend to diffuse across the plasma membrane from the inside to the outside of the cell. 5. Because negatively charged molecules cannot follow the positively charged K+, a small negative charged develops inside the plasma membrane. 6. The negative charge inside the cell attracts positively charged K+. When the negative charge inside the cell is great enough to prevent additional K+ from diffusing out of the cell through the plasma membrane, an equilibrium is established. 7. The charge difference across the plasma membrane at equilibrium is reflected as a difference in potential, which is measured in millivolts (mV). 8. The resting membrane potential is proportional to the potential for K+ to diffuse out of the cell but not to the actual rate of flow for K+. 9. At equilibrium, very little movement of K+ or other ions takes place across the plasma membrane.

What does a neurotransmitter do?

A neurotransmitter can stimulate some cells but inhibit others. More than one type of receptor molecule exists for some neurotransmitters. Different cells respond differently to a neurotransmitter when these cells have different receptors.

If the duration of the absolute refractory period of a neuron is 1 millisecond (ms), how many action potentials are generated by a maximal stimulus in 1 second?

1000

What is the function of the trigger zone?

3. Axons In most neurons, a single axon arises from a cone-shaped area of the neuron cell body called the axon hillock. As the axon hillock narrows, it transitions into the initial segment. The initial segment is the actual begging of the axon. The combination of the axon hillock and the initial segment is called the trigger zone. The trigger zone is where action potentials are generated. Many axons remain as a single structure, but others branch to form collateral axons, or side branches. Each axon has a constant diameter, but axons can vary in length from a few millimeters to more than 1 meter. The cytoplasm of an axon sometimes called the axoplasm, and its plasma membrane can also be called the axolemma (lemma, husk).

What is necessary for understanding many of the normal functions and pathologies of the body?

A basic knowledge of the electrical properties of cells is necessary for understanding many of the normal functions and pathologies of the body.

When does the chemical synapse occur?

A chemical synapse occurs where a chemical messenger, called a neurotransmitter, is used to communicate a message to an efector.

Describe the release of neurotransmitter in a chemical synapse.

A chemical synapse occurs where a chemical messenger, called a neurotransmitter, is used to communicate a message to an effector. The essential components of a chemical synapse are the presynaptic terminal, the synaptic cleft, and the postsynaptic membrane. The presynaptic terminal consist of the end of an axon of the presynaptic cell. The space separating the axon ending and the cell with which it synapses is the synaptic cleft. The membrane of the postsynaptic cell associated with the presynaptic terminal is the postsynaptic membrane. Postsynaptic cells are typically other neurons, muscle cells, or gland cells.

What is a graded potential?

A graded potential is a relatively small change in the membrane potential localized to one are of the plasma membrane. They are called graded potentials because they vary in size depending on the strength of the stimulus.

What is graded potential, and what four events can cause it? Define decremental conduction of graded potentials.

How does an action potential differ from a local potential? How do depolarizing and hyperpolarizing graded potentials affect the likelihood of generating an action potential?

A graded potential is a relatively small change in the membrane potential localized to one area of the plasma membrane. They are called graded potentials because they vary in size depending on the strength of the stimulus. A larger stimulus causes a larger graded potential, while a smaller stimulus causes a smaller graded potential. Graded potentials can be due to several types of stimuli: (1) chemicals binding to ligand-gated ion channels, (2) changes in voltage triggering opening or closing of voltage-gated ion channels, (3) mechanical stimuli opening mechanically gated ion channels, (4) temperature changes affecting specific temperature receptors (thermorecptors), or (5) spontaneous opening of ion channels. Graded potentials can be either (1) hyperpolarizing or (2) depolarizing. Hyperpolarizing graded potentials are due to either K+ exit form the cell or Cl- entry into the cell (see figure 11.9). Hyperpolarizing graded potentials are always inhibitory to the cell, meaning the cell is less likely to generate an action potential. Depolarizing graded potentials are always excitatory to the cell, which means the neuron is more likely to generate an action potential. The depolarizing graded potentials are added together in a process called summation. Summation is the combination of graded potenitials, which, if sufficiently large, will result in an action potential. Summation of depolarizing membrane graded potentials causes the neuron's membrane potential to become closer to a specific membrane potential called threshold. Threshold is the membrane potential at which an action potential is generated. An action potential is generated when voltage gated Na+ channels open. Action potentials are the means by which neurons communicate with their effectors. Without an action potential, the effector will not respond. Action potentials result from summation of graded potentials. Recall that graded potentials result from the detection of stimulus input to the neuron.

How does the stimulus affect a graded potential?

A larger stimulus causes a larger graded potential, while a smaller stimulus causes a smaller graded potential.

What is a maximal stimulus?

A maximal stimulus is just strong enough to produce a maximum frequency of action potentials.

Where does a single action potential occur?

A single action potential occurs in one very small area of the plasma membrane and does not occur over the entire membrane at one time. Additionally, the same action potential does not travel down the entire length of an axon.

What does the stimulus strength affect?

A small stimulus result in a small graded potential and, as stimulus strength increases, the size of the grade potentials increases. Thus, action potential frequency is directly proportional to stimulus strength and to the size of the graded potential.

What does a stronger stimulus result in?

A stronger stimulus will result in a greater frequency of action potentials, rather than larger magnitudes of each action potential.

Which produces the most action potentials: a prolonged threshold stimulus or a prolonged, stronger-than-threshold stimulus fo the same duration? Explain.

A stronger-than-threshold stimulus for the same duration would produce more action potentials because the stronger the stimulus the more action potentials that are produced.

What is a submaximal stimulus?

A submaximal stimulus includes all stimuli between threshold and the maximal stimulus strength. For submaximal stimuli, the action potential frequency increases in proportion to the strength of the stimulus because the size of the graded potential increases with stimulus strength.

What is a supramaximal stimulus?

A supramaximal stimulus is any stimulus stronger than a maximal stimulus. Because an axon's ability to produce action potentials is limited, these stimuli cannot produce a greater frequency of action potentials than a maximal stimulus.

PROCESS FIGURE 11.19 A Chemical Synapse

A synapse consists of the end of a neuron (presynaptic terminal), a small space (synaptic cleft), and the postsynaptic membrane of another neuron or an effector cell, such as a muscle or gland cell.

What is a threshold stimulus?

A threshold stimulus produces a graded potential that is just strong enough to reach threshold and cause the production of a single action potential.

What can initiate another action potential during the relative refractory period?

A very strong stimulus, or a stronger-than-threshold stimulus, can initiate another action potential during the relative refractory period. Thus, after the absolute refractory period, but before the relative refractory period is completed, a sufficiently strong stimulus can produce an action potential.

Acetylcholine

Acetylcholine is the most well understood of the neurotransmitters. ACh is synthesized from the precursors acetic acid and choline.

Give an analogy for unmyelinated and myelinated axons for action potential conduction.

Action potential conduction in a myelinated axon is like a child skipping across the floor; in an unmyelinated axon, it is like a child walking heal to toe across the floor. The child (action potential) moves more rapidly by skipping. The generation of action potentials at nodes of Ranvier occurs so rapidly that as many as 30 successive nodes of Ranvier are simultaneously in some phase of an action potential.

Spatial summation.

Action potentials 1 and 2 cause the production of graded potentials at two different dendrites. These graded potentials summate at the trigger zone to produce a graded potential that exceeds threshold, resulting in action potential.

Where are action potentials readily produced?

Action potentials are readily produced at the trigger zone because the concentration of voltage-gated Na+ channels is approximately seven times greater there than at the rest of the neuron cell body.

What are action potentials?

Action potentials are the means by which neurons communicate with their effectors. Without an action potential, the effector will not respond. Action potentials result from summation of graded potentials. Recall that graded potentials result from the detection of stimulus input to the neuron.

PROCESS FIGURE 11.17 Saltatory Conduction: Action Potential Propagation in a Myelinated Axon

Action potentials effectively "jump" from node to node. The gaps between the Schwann cells are exaggerate for clarity.

In step 5, why does the action potential proceed in both directions away from the point of local current entry into the adjacent cell?

Action potentials in one cell create a current where they wake neighboring cells. When the action potential initiated it wakes all the surrounding cells.

Explain the "all" and the "none" parts of the all-or-none principle of action potentials.

Action potentials occur according to the all-or-none principle. If a stimulus produces a depolarizing graded potential that is large enough to reach threshold, all the permeability changes responsible for an action potential proceed without stopping and are constant in magnitude (the "all" part). If a stimulus is so weak that the depolarizing graded potential does not reach threshold, few of the permeability changes occur. The membrane potential returns to its resting level after a brief period without producing an action potential (the "none" part). An action potential can be compared to the started in a car. Once the ignition switch is pressed (reaches threshold), the car starts (an action potential is produced) and, each time, the engine runs the same as the previous time the car starter (the "all" part). If the ignition switch is pressed, but not fully (does not reach threshold), the car does not start (the "none" part).

What is the all-or-none principle?

What are some examples of receptors existing on presynaptic membranes.

Although neurotransmitter receptors are in greater concentrations on postsynaptic membranes, some receptors exist on presynaptic membranes. For example, norepinephrine released from the presynaptic membrane binds to receptors on both presynaptic and postsynaptic decreases the release of additional synaptic vesicles. Norepinephrine can therefore modify its own release by binding to presynaptic receptors. A high frequency of presynaptic action potentials results in the release of fewer synaptic vesicles in response to later action potentials.

How do Schwann cells or oligodendrocytes connect to each other?

Although the axon at a node of Ranvier is not wrapped in myelin, Schwann cells or oligodendrocytes extend across the node and connect to each other.

What are the four basic patterns of parallel pathways?

Although their complexity varies, four basic patterns of parallel pathways can be recognized: (1) convergent pathways, (2) divergent pathways, (3) reverberating circuits, and (4) parallel after-discharging circuits.

Describe saltatory conduction.

An action potential at one node of Ranvier generates local currents that flow rapidly toward the next node of Ranvier. The lipids within the membranes of the myelin sheath act as a layer of insulation, forcing the local currents to flow from one node of Ranvier to the next. The action potentials seem to jump from node to node. In addition, voltage-gated Na+ channels are highly concentrated at the nodes of Ranvier. Therefore, the local current quickly flows to a node and stimulates the voltage-gated Na+ channels to open, resulting in the production of an action potential.

What can an action potential be compared to?

An action potential can be compared to the starter in a car. Once the ignition switch is pressed (reaches threshold), the car starts (an action potential is produced) and, each time, the engine runs the same as the previous time the car started (the "all" part). If the ignition switch is pressed, but not fully (does not reach threshold), the car does not start (the "none" part).

What are action potentials?

An action potential is a large change in the membrane potential that propagates, without changing its magnitude, over long distances along the plasma membrane. Thus, action potentials can transfer information from one part of the body to another. It generally takes 1-2 milliseconds (ms; 1 ms = 0.001 s) for an action potential to occur.

Combine spatial and temporal summation with both excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs).

An action potential is produced at the trigger zone when the graded potentials produced as a result of the EPSPs and IPSPs summate to reach threshold.

What is an excitatory effect?

An excitatory effect is one where the neurotransmitter induces a depolarization making the cell more likely to generate an action potential.

What is an inhibitory effect?

An inhibitory effect is where the neurotransmitter induces a hyperpolarizaiton making the cell less likely to generate an action potential.

What are anaxonic neurons?

Anaxonic neurons do not have axons and only have dendrites projecting from their cell body. Found within the brain and retina, these neurons communicate using only graded potentials and not action potentials.

What parts of the brain have more microglia cells?

Areas of the brain or spinal cord that have been damaged by infection, trauma, or stroke have more microglia than healthy areas. There the microglia perform phagocytosis of dead cells and pathogens. A pathologists can identify these damaged areas in the CNS during an autopsy because large numbers of microglia are found there.

What happens to the plasma membrane as K+ diffuses out of the cell?

As K+ diffuses out of the cell, the loss of positive charges makes the inside of the plasma membrane more negative. Because opposite charges attract, K+ is attracted back toward the cell.

What does the sodium-potassium pump maintain?

As already noted, the sodium-potassium pump maintains the uneven distribution of Na+ and K+ across the plasma membrane. The pump is also responsible for a small portion of the resting membrane potential, usually less than 15 mV, because it transports approximately three Na+ out of the cell and two K+ into the cell for each ATP molecule used. The outside of the plasma membrane becomes more positively charged than the inside, because more positively charged ions are pumped out of the cell than are pumped into it.

What are the components of a synapse? What is the purpose of a synapse?

As stated in section 11.2, the synapse is the junction between two cells where they communicate with each other. The cell that transmits a signal toward the synapse is called the presynaptic cell (before the synapse), and the target cell receiving the signal is called the post synaptic cell (after the synapse). The average presynaptic neuron synapses with about 1000 other neurons, but the average postsynaptic neuron has up to 10,000 synapses. Some postsynaptic neurons in the part of the brain called the cerebellum have up to 100,000 synapse. There are two types of synapses: electrical and chemical.

What is the presynaptic cell?

CNS

Astrocyte foot processes cover the surfaces of neurons, blood vessels, and the pia mater membrane of the brain and spinal cord. The astrocytes provide structural support and play a role in regulating what substances from the blood reach the neurons.

What does astrocytes aid in?

Astrocytes aid both beneficial and detrimental responses to tissue damage in the CNS. Almost all injuries to CNS tissue induce reactive astrocytosis, in which astrocytes wall of the injury site and help limit the spread of inflammation to the surround healthy tissue. Reactive scar-forming astrocytes also limit the regeneration of the axons of injured neurons.

What do astrocytes release?

Astrocytes also release chemicals that promote the development of synapses and help regulate synaptic activity by synthesizing, absorbing, and recycling neurotransmitters.

What are astrocytes?

Astrocytes are star-shaped glial cells with cytoplasmic processes extending from their cell bodies. These extensions widen and spread out to form foot processes, which cover the surfaces of blood vessels, neurons, and the Pia mater. (The pia mater is a membrane covering the outside of the brain and spinal cord.) Astrocytes have an extensive cytoskeleton of microfilaments (see chapter 3), which enables them to form a supporting framework for blood vessels and neurons.

What do astrocytes help regulate?

Astrocytes help regulate the composition of extracellular brain fluid. They do this by releasing chemicals that promote the formation of tight junctions between the endothelial cells of capillaries.

Which type of glial cell supports neurons and blood vessels and promotes formation of the blood-brain barrier? What is the blood-brain barrier, and what is it's function?

Astrocytes help regulate the composition of extracellular brain fluid. They do this by releasing chemicals that promote the formation of tight junctions between the endothelial cells of capillaries. Recall from chapter 4 that tight junctions create an impermeable barrier between adjacent cells. The endothelial cells with their tight junctions form the blood-brain barrier. The blood-brain barrier determines what substances can pass form the blood into the nervous tissue of the brain and spinal cord. The blood-brain barrier protects neurons from toxic substances in the blood, allows the exchange of nutrients and waste products between neurons and the blood, and prevents fluctuations in blood composition from affecting brain functions.

Name the different kinds of glial cells that are responsible for the following functions: production of cerebrospinal fluid, phagocytosis, production of myelin sheaths in the CNS, production of myelin sheaths in the PNS, support of neuron cell bodies in the PNS.

Astrocytes- Astrocyte foot processes cover the surfaces of neurons, blood vessels, and the pia mater membrane of the brain and spinal cord. The astrocytes provide structural support and play a role in regulating what substances from the blood reach the neurons. Ependymal cells- (a) Ciliated ependymal cells lining the ventricles of the brain and the central canal of the spinal cord help move cerebrospinal fluid. (b) Ependymal cells on the surface of the choroid plexus secrete cerebrospinal fluid. Microglia- Microglia are phagocytic cells within the CNS. Oligodendrocytes- Extensions from oligodendrocytes form part of the myelin sheaths of several axons within the CNS. Schwann cells and satellite cells- Neuron cell bodies within ganglia are surrounded by satellite cells. Schwann cells form the myelin sheath of an axon within the PNS.

Do ions pass through the membrane at equilibrium?

At equilibrium, very few of these ions pass through the plasma membrane because their movement out of the cell is opposed by the negative charge inside the cell. Still, some Na+ and K+ diffuse continuously across the plasma membrane, although at a low rate. The large concentration gradients for Na+ and K+ would eventually disappear without the continuous activity of the sodium-potassium pump.

Describe the steps of an action potential

At the beginning of the action potential, depolarization occurs when the activation gates in the voltage-gated Na+ channel open. At this time, the inactivation gates in the voltage-gated Na+ channels are already open. Depolarization ends as the inactivation gates close. As long as the inactivation gates are closed, further depolarization cannot occur. Near the end of repolarization when the inactivation gates open and the activation gates close, it is possible once again, to stimulate another action potential if the activation gates re-open.

What happens at the end of repolarization?

At the end of repolarization, the return toward resting membrane potential causes the activation gates in the voltage-gated Na+ channels to close and the inactivation gates to open. Although this change does not affect the diffusion of Na+, it does return the voltage-gated Na+ channels to their resting state.

What happens at the presynaptic terminal when an action potential arrives?

At the presynaptic terminal, voltage-gated Ca2+ channels are opened why an action potential, which allows Ca2+ to diffuse into the axon terminal. There, Ca2+ stimulates exocytosis of synaptic vesicles, which contain neurotransmitters to communicate with the neuron's target. The chemical synapse is discussed fully in section 11.6.

Medical Conditions: ADHD Characterized by an inability to focus.

Attention-Deficit Hyperactivity Disorder (ADHD) ADHD is often treated with drugs that increase the level of excitatory neurotransmitters, such as norepinephrine, in the synaptic clefts. This is achieved by using selective norepinephrine reuptake inhibitors (SNRIs) that block norepinephrine transporters (symporters) and increase in the levels of norepinephrine in the synaptic clefts. Additionally, simultaneously increasing the levels of dopamine in the synaptic cleft is a common treatment. The most familiar of the ADHD medications, Ritalin, prevents reuptake of both norepinephrine and dopamine. The most recent information suggests that enhancing the nicotinic cholinergic pathways is also an effective treatment for ADHD.

What are the four major sub categories of dendrites?

Based on the number of dendrites, there are four major structural categories. They are (1) multipolar, (2) bipolar, (3) pseudo-unipolar, and (4) anaxonic.

Explain the three types of neurons based on structure, and give an example of where each type is found.

Based on the number of dendrites, there are four major structural categories. They are (1) multipolar, (2) bipolar, (3) pseudo-unipolar, and (4) anaxonic. Most of the neurons within the CNS and motor neurons of the PNS are multipolar. Bipolar neurons are located in some sensory organs, such as in the retina of the eye and in the nasal cavity. Pseudo-unipolar neurons, also known as pseudo-unipolar, start out as bipolar neurons during development, but the two processes fuse into a single process. These neurons have a single process extending from the cell body, which divides into two branches a short distance from the cell body. The two branches function as a single axon. one branch, the peripheral process, extends to the periphery and has dendrites. These dendrites either act as a sensory receptor or communicate with a sensory receptor. The other branch, called the central process, extends to the CNS Anaxonic neurons do not have axons and only have dendrites projecting from their cell body. Found within the brain and retina, these neurons communicate using only graded potentials and not action potentials.

How does Cl- affect the opening of ligand-gated Cl- channels?

Because Cl- is in higher concentration outside the cell, opening of ligand-gated Cl- channels causes Cl- to diffuse into the cell. The introduction of the negatively charged Cl- into the cell hyper polarizes it. This is a mechanism by which some inhibitory neurotransmitters work.

Why are areas of the brain that contain gray matter darker in area?

Because gray matter consists of groups of neuron cell bodies and their dendrites, where there is very little myelin, these areas are darker in appearance.

Why is the plasma membrane referred to as being polarized?

Because there are opposite charges, or poles, across the membrane, the plasma membrane is referred to as being polarized.

Why is white matter whitish in color?

Because white matter consists of bundles of parallel myelinated axons, they are whitish in color.

What do bipolar neurons have?

Bipolar neurons have two processes: one dendrite and one axon. The dendrite is often specialized to receive the stimulus, and the axon conducts action potentials to the CNS. Bipolar neurons are located in some sensory organs, such as in the retina of the eye and in the nasal cavity.

Explain how electrical synapses function.

By allowing cytoplasm to freely flow from one cell to the next, the adjacent cells function like one cell. The gap junctions allow Na+ to flow directly from one cell to a neighboring cell. Thus, an action potential in one cell produces a local current that generates an action potential int eh adjacent cell almost as if the two cells had the same membrane. As a result, action potentials are conducted rapidly between cells, allowing the cells' activity to be synchronized.

Why is the potential difference reported as a negative number?

By convention, the potential difference is reported as a negative number because the inside of the plasma membrane is negative compared with the outside. The greater the charge difference across the plasma membrane, the greater the potential difference. A cell with a resting membrane potential of -90 mV has a greater charge difference between the inside of the cell membrane and the outside of the cell membrane than a cell with a resting membrane potential of -70 mV.

Contrast the general functions of the CNS and the PNS.

CNS receives information and transmits signals to the PNS. The PNS receives sensory input from external and internal stimuli in order to send it to the CNS. The CNS can be thought of as the key decision maker, while the PNS is the messenger that provides input about the body to the CNS and then delivers the CNS decision on how the body is to respond to a particular set of stimuli.

What other two roles do Ca2+ play in action potentials?

Ca2+ also plays two other significant roles in action potentials: (1) regulation of voltage-gated Na+ channels and (2) regulation of neurotransmitter secretion at the presynaptic terminal.

What does calcium entry into an electrically excitable cell cause?

Calcium entry into a n electrically excitable cell also causes depolarization. Calcium is in higher concentration in the extracellular fluid and when voltage-gated Ca2+ channels open, it diffuses into the cell depolarizing it. This is how some cardiac muscle cells generate action potentials.

Medical Conditions: Depression and Anxiety Serotonin is important for emotional states such as depression and anxiety.

Clinical Modulation of Neurotransmitter Metabolism: Selective serotonin uptake inhibitors (SSRIs) temporarily block serotonin transporters, which decreases serotonin transport back into presynaptic terminals. This block increases serotonin levels in the synaptic cleft. Certain illegal drugs such as LSD and Ecstasy, or its relative Molly, target serontonin receptors.

Why can't communication regarding the strength of stimuli depend on the magnitudes of action potentials?

Communication regarding the strength of stimuli cannot depend on the magnitudes of action potentials because, according to the all-or-none principle, the magnitudes of action potentials produced by weak and strong stimuli are always the same.

What do cranial nerves do?

Cranial nerves, of which there are 12 pairs, originate from the brain, while spinal nerves, of which there are 31 pairs, originate from the spinal cord.

2. Dendrites

Dendrites are extensions of the cell body and receive information from other neurons or the environment portion of the neuron. Dendrites are short, often highly branched cytoplasmic extensions that are tapered from their bases at the neuron cell body to their tips. Many dendrite surfaces have small extensions, called dendritic spines, where axons of other neurons form connections with the dendrites. When stimulated, dendrites generate small electric currents, which are conducted toward the neuron cell body.

When does depolarization occur?

What happens to cause depolarization and hyperpolarization? How do alterations in the K+ concentration gradient; changes in membrane permeability to K+, Na+, or Cl-; and changes in extracellular Ca2+ concentration affect depolarization and hyperpolarization?

Depolarization occurs when the inside of the cell becomes more positive. Recall that membrane potential is measured by comparing the charge inside the cell to the charge outside the cell. As a result, the membrane potential becomes more positive. For example, if the membrane potential increases from -70 mV to -55 mV, then an action potential is generated. We also say that depolarization is movement of the membrane potential closer to zero. Because depolarization moves the membrane potential closer to the point of action potential generation, it is always excitatory to the cell. In other words, depolarization makes a neuron more likely to generate an action potential. Hyperpolarization occurs when the inside of the cell becomes even more negative compared to the outside. As a result, the membrane potential becomes more negative. For example, if the membrane potential decreases from -70 mV to -90 mV, then the cell is less likely to generate an action potential. hyperpolarization is movement of the membrane potential further from zero. Because the cell is less likely to generate an action potential, hyperpolarization is always inhibitory to the cell. There are two major ways to hyperpolarize neurons: (1) K+ exit and (2) Cl- entry. With the transferring of ions in and out of the membrane potential, the charge changes based on the concentration.

What are depolarizations produced in postsynaptic membranes?

Depolarizations produced in postsynaptic membranes are graded potentials. Within the CNS and in many PNS synapses, a single presynaptic action potential does not cause a graded potential in the postsynaptic membrane sufficient to reach threshold and produce an action potential. Instead, many presynaptic action potentials cause many graded potentials in the postsynaptic neuron. The graded potentials combine in summation at the trigger zone of the postsynaptic neuron, which is the normal site of action potential generation for most neurons. If summation results in a graded potential that exceeds threshold at the trigger zone, an action potential is produced.

What are depolarizing graded potentials?

Depolarizing graded potentials are always excitatory to the cell, which means the neuron is more likely to generate an action potential. The depolarizing graded potentials are added together in a process called summation.

Give an example of cells that respond differently neurotransmitters.

Different cells respond differently to a neurotransmitter when these cells have different receptors. For example, norepinephrine can bind to one type fo norepinephrine receptor to cause depolarization in one synapse and to another type of norepinephrine receptor to cause hyperpolarizaiton in another synapse. Thus, norepinephrine is either stimulatory or inhibitory, depending on the type of norepinephrine receptor tow which it binds and on the effect that receptor has on the permeability of the postsynaptic membrane.

What limits the length of time the neurotransmitter molecules remain bound to their receptors?

Diffusion of neurotransmitter molecules away from the synapse and into the extracellular fluid also limits the length of time the neurotransmitter molecules remain bound to their receptors.

How can drugs modulate the action of neurotransmitters at the synapse?

Drugs can modulate the action of neurotransmitters at the synapse in ways that can be either beneficial or harmful. For example, cocaine and amphetamines increase the releasee and block the reuptake of norepinephrine, which increases norepinephrine levels in a synapse. This results in overstimulation of postsynaptic neurons and deleterious effects on the body. On the other hand, drugs that block serotonin reuptake are particularly effective at treating depression and behavioral disorders. A list of neurotransmitter and neuromodulators is presenting table 11.7.

What happens during an action potential?

During an action potential, a small amount of Na+ diffuses into the cell and a small amount of K+ diffuses out fo the cell.

What happens during the absolute refractory period?

During the absolute refractory period, a second stimulus, no matter how strong, cannot stimulate an additional action potential. However, as soon as the absolute refractory period ends, it is possible for a second stimulus to cause the production of an action potential.

What happens during the relative refractory period?

During the relative refractory period, the membrane is more permeable to K+ because many voltage-gated K+ channels are open. The relative refractory period ends when the voltage-gated K+ channels close and the membrane potential has returned to the resting level.

Why are EPSPs important?

EPSPs are important because the depolarization might reach threshold, thereby producing an action potential and a response from the cell.

What does each action potential arriving at the presynaptic terminal initiate?

Each action potential arriving at the presynaptic terminal initiates a series of specific events, which result in the release of neurotransmitters. In response to an action potential, voltage-gated Ca2+ channels open in the presynaptic cell's axon terminal. Next, Ca2+ diffuses into the presynaptic terminal. There, Ca2+ serves as an intracellular single to induce exocytosis of the synaptic vesicle. Once Ca2+ binds to the vesicle the presynaptic terminal. The vesicle fuse with the membrane of the presynaptic terminal and releases its contents into the synaptic cleft.

What do voltage-gated Na+ channels have?

Each voltage-gated Na+ channel has two voltage-sensitive gates, called activation gates and inactivation gates.

Is an action potential transmitted faster between cells connected by an electrical synapse or by a chemical synapse? Explain

Electrical because the action potential in one cell awakes a neighboring cell while in chemical synapse the process of neurotransmitter release takes more time.

Where are electrical synapses important?

Electrical synapses are also important in many types of smooth muscle. Coordinated contraction of these muscle cells occur when action potentials in one cell propagate to adjacent cells because of electrical synapses.

Are electrical synapses common?

Electrical synapses are not common in the nervous system of vertebrates, but some do exist in humans, such as between adjacent cardiac muscle cells.

How do electrical synapses occur?

What is an electrical synapse? Describe its operation.

Electrical synapses occur between cells connected by gap junctions. Recall from chapter 4 that a gap junction is a 2 nm gap between adjacent cell membranes where cytoplasm is shared through tunnel-like protein structures called connexons. The connexons are groups of six tubular proteins, each called a connexin. By allowing cytoplasm to freely flow from one cell to the next, the adjacent cells function like on cell. The gap junctions allow Na+ to flow directly form one cell to a neighboring cell. Thus, an action potential in one cell produces a local current that generates an action potential in the adjacent cell almost as if the two cells had the same membrane. As a result, action potentials are conducted rapidly between cells, allowing the cells' activity to be synchronized. Electrical synapses are not common in the nervous system of vertebrates, but some do exist in humans, such as between adjacent cardiac muscle cells. Electrical synapses are also important in many types of smooth muscle. Cordinated contractions of these muscle cells occur when action potentials in one cell propagate to adjacent cells because of electrical synapses.

PROCESS FIGURE 11.16 Action Potential Propagation in an Unmyelinated Axon

Entry of Na+ depolarizes the adjacent section of membrane to threshold, which stimulates more voltage-gated Na+ to open and more Na+ diffuses into the cell.

Where are ependymal cells found?

Ependymal cells line the ventricles (cavities) of the brain and the central canal of the spinal cord.

What can excitatory and inhibitory neurons do?

Excitatory and inhibitory neurons can synapse with the same postsynaptic neuron.

How do EPSPs and IPSPs affect the likelihood that summation will result in an action potential?

Excitatory and inhibitory neurons can synapse with the same postsynaptic neuron. Spatial summation of EPSPs and IPSPs occurs in the postsynaptic neuron, and whether a postsynaptic action potential is initiated or not depend on which type of graded potential has the greater influence on the postsynaptic membrane potential. If the EPSPs (local depolarizations) cancel the IPSPs (local hyperpolarizatons) and summate to threshold, an action potential is produced. If the IPSPs prevent the EPSPs from summating to threshold, no action potential is produced.

Oligodendrocytes

Extensions from oligodendrocytes form part of the myelin sheaths of several axons within the CNS

Diagram a convergent pathways a divergent pathway, a reverberating circuit and a parallel after-discharge circuit, and describe what is accomplished in each.

FIGURE 11.24 Neuronal Pathways and Circuits In convergent pathways, multiple neurons converge upon and synapse with a smaller number of neurons. Convergence allows different parts of the nervous system to activate or inhibit the activity of neurons. In divergent pathways, a smaller number of presynaptic neurons synapse with a larger number of postsynaptic neurons to allow information transmitted in one neuronal pathway to diverge into two or more pathways. Reverberating circuits have a chain of neurons with synapses with previous neurons in the chain, making a positive-feedback loop. This allows action potentials entering the circuit to cause a neuron farther along in the circuit to produce an action potential more than once. Parallel after-discharge circuits have neurons that stimulate several neurons in parallel organization, which all converge upon a common output cell. These circuits are involved in complex neuronal processes, including mathematics and chemical conversions.

Give an example of neuromodulators.

For example, a neuromodulator that inhibits the release of an excitatory neurotransmitter from a presynaptic terminal reduces the likelihood of the postsynaptic cell producing an action potential.

Give an example of how the action potential frequency is directly proportional to stimulus strength.

For example, a sub threshold stimulus is any stimulus not strong enough to produce a graded potential that reaches threshold. Therefore, no action potential is produced.

Give an example of how communcation regarding strength of stimuli cannot depend on the magnitudes of action potentials.

For example, a weak pain stimulus generates a low frequency of action potentials, whereas a stronger pain stimulus generates a higher frequency of action potentials. The ability to interpret a stimulus as mildly painful versus very painful depends, in part, on the frequency of action potentials generated by individual pain receptors.

Give an example of hyperpolarization.

For example, if the membrane potential decreases from -70 mV to -90 mV, then the cell is less likely to generate an action potential.

Give an example of depolarization.

For example, if the membrane potential increases from -70 mV to -55 mV, then an action potential is generated.

Give an example of Neurotransmitters having short-term effects on postsynaptic membranes.

For example, in the neuromuscular junction, the neurotransmitter acetylcholine is broken down by the enzyme acetylcholinesterase to acetic acid and choline. Choline is then transported back into the presynaptic terminal and combines with acetyl-CoA to re-form acetylcholine. Acetyl-CoA is synthesized by mitochondria as foods are metabolized to produce ATP. Acetic acid can be absorbed from the synaptic cleft into the presynaptic terminal, or it can diffuse out of the synaptic cleft and be taken up by a variety of cells. Acetic acid can be used to synthesize acetyl-CoA.

Give an example of convergent pathways.

For example, one part of the nervous system can stimulate the neurons responsible for making a muscle contract, whereas another part can inhibit those neurons. Through summation, muscle contraction can be activated if more converging neurons stimulate the production of EPSPs than converging neurons stimulate the production of IPSPs. Conversely, muscle contraction is inhibited if the converging neurons stimulate the production of more IPSPs than EPSPs.

Give an example of divergent pathways.

For example, sensory input to the central nervous system can go to both the spinal cord and the brain.

Give an example of presynaptic inhibition.

For example, sensory neurons for pain can release neurotransmitters from their presynaptic terminals and stimulate the postsynaptic membranes of neurons in the brain or spinal cord. Awareness of pain occurs only if action potentials are produced in the postsynaptic inhibitory neurons of axoaxonic synapses can reduce or eliminate pain sensations by inhibiting the release of neurotransmitter from the presynaptic terminals of sensory neurons. Enkephalins and endorphins can block voltage-gated Ca2+ channels. Consequently, when action potentials reach the presynaptic terminal, the influx of Ca2+ that normally stimulates neurotransmitter release is blocked.

Give an example of how neurotransmitters are released from the presynaptic terminal and bind to ligand-gated ion channels.

For example, the binding of acetylcholine to ligand-gated Na+ channels causes them to open , allowing Na+ to diffuse into the postsynaptic cell. If the resulting depolarizing graded potential reaches threshold, an action potential is produced. On the other hand, the opening of K+ or Cl- channels results in a hyperpolarizing graded potential .

Lets follow a chemical stimulus.

For simplicity, lets follow a chemical stimulus to a neuron as it arrives at the dendrite. There, binding of a chemical stimulus quite often results in the opening of ligand-gated ion channels. If these channels are Na+ channels, the receiving cell experiences a depolarizing graded potential. If enough Na+ enters, the graded potentials summate at the trigger zone in the axon hillock. When the graded potentials summate to threshold, an action potential results. Because the trigger zone contains a much higher proportion of voltage-gated channels than other parts of the cell body, action potentials are initiated there.

FIGURE 11.15 Stimulus Strength and Action Potential Frequency

From left to right, each stimulus in the figure is stronger than the previous one. As stimulus strength increases, the frequency of action potentials increases until a maximal rate is produced. Thereafter, increasing stimulus strength does not increase action potential frequency due to the refractory period.

Medical Conditions: Epilepsy Characterized by excessive neuron function. Lack of GABA induces convulsions.

GABA targets ligand-gated Cl- channels and inhibits its target. Barbiturates enhance the binding of GABA to its receptor thereby prolonging the inhibition. GABA helps minimize epileptic seizures and is also sedative and anesthetic.

PROCESS FIGURE 11.8 Electrical Synapse

Gap junctions allow ions to flow directly from and into the adjacent cell.

Explain gated ion channels.

Gated ion channels are closed until opened by specific signals. By opening and closing, these channels can change the permeability of the plasma membrane. There are three major types of gated ion channels: 1. Ligand-gated ion channels. 2. Voltage-gated ion channels 3. Other gated ion channels

3. Other gated ion channels.

Gated ion channels that respond to stimuli other than ligands or voltage changes are present in specialized electrically excitable tissues. For example, touch receptors of the skin respond to mechanical stimulation using mechanically gated ion channels. Temperature receptors respond to temperature changes in the skin.

What kinds of stimuli cause gated ion channels to open or close?

What are glial cells?

Glial cells are supportive cells that serve many functions for the neurons. Glial cells are fully discussed in section 11.3. Together, neurons and glial cells allow the nervous system to serve a multitude of functions for the body.

What are glial cells?

Glial cells are the major support cells for neurons in the CNS. There are four types of CNS glial cells: (1) astrocytes, (2) ependymal cells, (3) microglia, and (4) oligodendrocytes.

Which glial cells are found in the CNS? In the PNS?

Glial cells are the major support cells for neurons in the CNS. There are four types of CNS glial cells: (1) astrocytes, (2) ependymal cells, (3) microglia, and (4) oligodendrocytes. There are two types of glial cells in the PNS: (1) Schwann cells and (2) satellite cells.

Medical Conditions: Stroke Damage in the brain that results in a disruption in or lack of blood flow to the brain.

Glutatmate is the major excitatory neurotransmitter of the CNS. Some glutamate receptors are ligand-gated Ca2+ channels. When stimulated, Ca2+ channels open, causing depolarization of postsynaptic membranes. Some glutamate is removed from the synapse by transporters in presynaptic terminals, whereas the bulk of it is removed by transporters (symporters) in neighboring astrocytes. When a person suffers a stroke, brain tissue is deprived of oxygen, and ATP levels decrease. This causes the secondary active transport of glutamate by the glutamate transporters to fail temporarily. As a result, glutamate accumulates in the synaptic clefts and causes excessive stimulation of postynaptic neurons. Excessive movement of Ca2+ into neurons activates a variety of destructive processes, which can cause cell death.

Why are graded potentials important?

Graded potentials are important because they can summate to generate action potentials, or in anaxonic neurons, can be primary means of communication. The characteristics of graded potentials are summarized in table 11.5.

What are graded potentials due to?

Graded potentials can be due to several types of stimuli: (1) chemicals binding to ligand-gated ion channels (2) changes in voltage triggering opening or closing of voltage-gated ion channels, (3) mechanical stimuli opening mechanically gated ion channels, (4) temperature changes affecting specific temperature receptors (thermoreceptors), or (5) spontaneous opening of ion channels.

What can graded potentials be?

Graded potentials can be either (1) hyperpolarizing or (2) depolarizing.

What does it mean to say a graded potential can summate and then spread in a decremental fashion?

Graded potentials spread, or are conducted, over the plasma membrane in a decremental fashion. That is, they rapidly decrease in magnitude as they spread over the surface of the plasma membrane, much as a teacher's voice spreads through a large lecture hall. At the front of the class, the teacher's voice can be heard easily but, the farther away a student sits, the harder it is to hear the teacher. Normally, a graded potential cannot be detected more than a few millimeters from the site of stimulation.

How do graded potentials spread?

Graded potentials spread, or are conducted, over the plasma membrane in a decremental fashion. That is, they rapidly decrease in magnitude as they spread over the surface of the plasma membrane, such as a teacher's voice spreads through a large lecture hall. At the front of the class, the teacher's voice can be heard easily but, the father away a student sits, the harder it is to hear the teacher. Normally, a graded potential cannot be detected more than a few millimeters from the site of stimulation. As a consequence, a graded potential cannot transfer information over long distances from one part of the body to another.

What happens if K+ concentration increases outside the neuron?

If K+ concentration increases outside the neuron, intracellular K+ stays inside the cell because the concentration gradient is now less. When K+ stays inside the cell, rather than diffusing out through K+ leak channels as normal, the cell becomes depolarized.

What are the two types of glial cells in the PNS?

There are two types of glial cells in the PNS: (1) Schwann cells and (2) satellite cells.

What is hyperpolarization?

Hyperpolarization is movement of the membrane potential further from zero. Because the cell is less likely to generate an action potential, hyperpolarization is always inhibitory to the cell.

When does hyperpolarization occur?

Hyperpolarization occurs when the inside of the cell becomes even more negative compared to the outside. As a result, the membrane potential becomes more negative.

What are hyper polarizing graded potentials due to?

Hyperpolarizing graded potentials are due to either K+ exit from the cell or Cl- entry into the cell. Hyperpolarizing graded potentials are always inhibitory to the cell, meaning the cell is less likely to generate an action potential.

How is hypokalemia caused?

Hypokalemia can be caused by K+ depletion during starvation, alkalosis, and certain kidney diseases.

At the end of the action potential, the cell will regain resting conditions of ions inside the cell. Will this process be active or passive? Explain your answer.

I think it will be passive because the cell has ions leave and enter to regulate when changes occur to the membrane potential for an example K+ leaves and enters to maintain an equilibrium.

Why are IPSPs important?

IPSPs are important because they move the membrane potential farther from threshold, which decreases the likelihood of an action potential being generated.

Give an example of EPSPs causing depolarizing graded potentials to reach threshold so an action potential is produced.

If EPSPs cause a depolarizing graded potential that reaches threshold, an action potential is produced. For example, awareness of pain can occur only if action potentials generated by sensory neurons stimulate the production of action potentials generated by sensory neurons stimulate the production of acton potentials in CNS neurons. Local anesthetics, such as procaine (novocaine), act at their site of application to prevent pain sensations. They do so by blocking voltage-gated Na+ channels, which prevent pain sensations. They do so by blocking voltage-gated Na+ channels, which prevents action potentials from propagating along sensory neurons. Consequently, neurotransmitters are not released from the presynaptic terminals of the sensory neurons, and EPSPs are not produce in CNS neurons.

What happens if EPSPs cause a depolarizing graded potential that reaches threshold?

If EPSPs causes. depolarizing graded potential that reaches threshold, an action potential is produced.

What happens if a neuron produces more than one neurotransmitter?

If a neuron does produce more than one neurotransmitter, it secretes all of them from each of its presynaptic terminals. The physiological significance of presynaptic terminals that secrete more than one type of neurotransmitter has not been clearly established.

PROCESS FIGURE 11.14 Production of an Action Potential

If a neuron reaches threshold, an action potential is generated and is propagated down the axon to the axon terminals. At the axon terminals, the action potential allows the neuron to communicate with its target.

What happens if an action potential is initiated at one end fo an axon?

If an action potential is initiated at one end of an axon, it is propagated in one direction down the axon. The absolute refractory period ensures one-way propagation of an action potential because it prevents the local current from stimulating the production of an action potential in the reverse direction.

What prevents an action potential from reversing its direction of propagation?

What happens when the EPSPs cancel the IPSPS and summate to threshold?

If the EPSPs (local depolarizations) cancel the IPSPs (local hyperpolarizaitons) and summate to threshold, an action potential is produced. If the IPSPs prevent the EPSPs from summation to threshold, no action potential is produced.

Predict the effect of a reduced extracellular concentration of Na+ on the magnitude of the action potential in an electrically excitable cell.

If there is less Na+ coming into a cell trying to reach threshold, the amount may be insufficient to provide a strong enough stimulus. Specifically, less Na+ means weaker stimulus which causes a weak action potential or not even an action potential to occur.

Some forms of anxiety are thought to be associated with insufficient levels of norepinephrine. Explain how an MAO inhibitor could help with treatment of anxiety.

If we inhibit MAO then the synaptic cleft will have more norepinephrine which will help treat anxiety.

What are the two different types of action potential propagation?

In a neuron, action potentials are normally produced at the trigger zone and propagate in one direction along the axon. The location at which the next action potential is generated is different for unmyelinated and myelinated axons. There are two types of action potential propagation: (1) continuous conduction and (2) saltatory conduction.

What else affects the speed of action potential conduction?

In addition to myelination, the diameter of an axon affects the speed of action potential conduction. Large-diameter axons conduct action potentials more rapidly than small-diameter axons because large-diameter axons hav a greater surface area. Consequently, at a give site on an axon, more voltage-gated Na+ channels open during depolarization, resulting in a greater local current flow, which more rapidly stimulates adjacent membrane areas.

What does the frequency of action pontential provide information about?

In addition to the frequency of action potentials, how long the action potentials are produced provides important information. For example, a pain stimulus of 1 second is interpreted differently than a pain stimulus applied for 30 seconds.

What is retrograde?

In addition, damaged, organelles, recycled plasma membrane, and substances taken in by endocytosis can be transported up the axon toward the neuron cell body. Movement toward the cell body is called retrograde.

What is the resting membrane potential?

In an unstimulated, or resting, cell, the potential difference is called the resting membrane potential.

If the middle of an axon were depolarized to threshold, in which direction could the action potential propagate?

In both directions.

What makes up gray matter and white matter?

In both the CNS and the PNS, nervous tissue is organized such that axons are group together, forming bundles, while neuron cell bodies and dendrites are grouped together. These groupings give nervous tissue distinctive areas, called gray matte and white matter. Because gray matter consists of groups of neuron cell bodies and their dendrites, where there is very little myelin, these areas are darker in appearance. In the CNS, the cortex consists of gray matter on the surface of the brain. Nuclei are clusters of gray matter located deeper within the brain. In the PNS, gray matter consists of clusters of neuron cell bodies, or ganglia. Conversely, because white matter consists of bundles of parallel myelinated axons, they are whitish in color.

How is nervous tissue organize?

In both the CNS and the PNS, nervous tissue is organized such that axons are grouped together, forming bundles, while neuron cell bodies and dendrites are grouped together.

Cations (Positive) Potassium (K+) Sodium (Na+) Calcium (Ca2+) Others

Intracellular Fluid (mEq/L) 148 10 <1 41 Total = 200 Extracellular Fluid (mEq/L) 5 142 5 3 Total = 155

Anions (Negative) Proteins Chloride (Cl-) Others

Intracellular Fluid (mEq/L) 56 4 140 Total= 200 Extracellular Fluid (mEq/L) 16 103 36 Total= 155

What characteristic makes glial cells different from neurons?

Neurons are the electrically excitable cells of the nervous system. Glial cells are the major support cells for neurons in the CNS.

Describe and state the location of the following: nerve tracts, nerves, the brain cortex, nuclei, ganglia.

In both the CNS and the PNS, nervous tissue is organized such that axons are grouped together, forming bundles, while neuron cell bodies and dendrites are grouped together. These groupings give nervous tissue distinctive areas, called gray matter and white matter. Because gray matter consists of groups of neuron cell bodies and their dendrites, where there is very little myelin, these areas are darker in appearance. In the CNS, the cortex consists of gray matter on the surface of the brain. Nuclei are clusters of gray matter located deeper within the brain. In the PNS, gray matter consists of clusters of neuron cell bodies, or ganglia. Conversely, because white matter consists of bundles of parallel myelinated axons, they are whitish in color. White matter of the CNS forms nerve tracts, which propagate action potentials form one area of the CNS to another. In contrast, in the PNS, bundles of axons and their connective tissue sheaths are simply called nerves.

In chemical synapses, do action potentials pass directly from the presynaptic terminal to the postsynaptic membrane?

In chemical synapses, action potentials do not pass directly from the presynaptic terminal to the postsynaptic membrane. Instead, the action potentials in the presynaptic terminal cause the release of neurotransmitters from its terminal.

Explain convergent pathways.

In convergent pathways, multiple neurons converge upon and synapse with a smaller number of neurons. Convergence allows different parts of the nervous system to activate or inhibit the activity of neurons.

FIGURE 11.9 Depolarization and Hyperpolarization of the Resting Membrane Potential.

In depolarization, the charge inside the plasma membrane becomes more positive. In hyperpolarization, the charge inside the plasma membrane becomes more negative.

Explain divergent pathways.

In divergent pathways, a smaller number of presynaptic neurons synapse with a larger number of postsynaptic neurons to allow information transmitted in one neuronal pathway to diverge into two or more pathways. Diverging pathways allow one part of the nervous system to affect more than one part of the nervous system.

Give an example of how EPSP occur because the membrane has become more permeable to Na+

In general, an EPSP occurs because the membrane has become more permeable to Na+. For example, when glutamate in the brain and acetylcholine in skeletal muscle bind to their receptors, they cause Na+ channels to open and Na+ diffuses into the cell causing depolarization

Describe the after potential and its cause?

In many cells, a period of hyperpolarization, called after potential, follows each action potential. The after potential occurs because the voltage-gated K+ channels remain open for a slightly longer time than it takes to bring the membrane potential back to its original resting level. This allows extra K+ to leave the cell, hyperpolarizing it. As the voltage-gated K+ channels close, the original resting membrane potential is reestablished by the sodium-potassium pump.

What is aftepotential?

3. Axons

In most neurons, a single axon rises from a cone-shaped area of the neuron cell body called the axon hillock. As the axon hillock narrows, it transitions into the initial segment. The initial segment is the actual beginning of the axon. The combination of the axon hillock and the initial segment is called the trigger zone. The trigger zone is where action potentials are generated. Many axons remain as a single structure, but others branch to form collateral axons, or side branches. Each axon has a constant diameter, but axons can vary in length from a few millimeters to more than 1 meter. The cytoplasm of an axon is sometimes called the axoplasm, and its plasma membrane can also be called the axolemma (lemma, husk).

What is a myelin sheath? How is it formed in the CNS? In the PNS?

In myelinated axons, Schwann cells in the PNS or oligodendrocyte extension in the CNS repeatedly wrap around a segment of an axon to form a series of tightly wrapped membranes rich in phospholipids, with little cytoplasm sandwiched between the membrane layers. One way to picture the overlapping wrappings, especially for Schwann cells, is to imagine rolling up a hot dog (axon) inside a tortilla (Schwann cell). The tightly wrapped membranes of the Schwann cells constitute the myelin sheath and give myelinated axons a white appearance because of the high lipid concentration. The myelin sheath is not continuous but contains gaps every 0.3-1.5 mm. At these locations are slight constrictions where the myelin sheaths of adjacent cells dip toward the axon but do not recover it, leaving an area where the myelin sheath is much thinner and about 2-3 micrometers in length. These gaps in the myelin sheath are the nodes of Ranvier. Although the axon at a node of Ranvier is not wrapped in myelin. Schwann cells or oligodendrocytes extend across the node and connect to each other. The myelin sheath protects and electrically insulates each axon. These characteristics help myelinated axons conduct electrical signals more rapidly than unmyelinated axons.

Describe myelinated axons.

In myelinated axons, Schwann cells in the PNS or oligodendrocyte extensions in the CNS repeatedly wrap around a segment of an axon to form a series of tightly wrapped membranes in rich in phospholipids, with little cytoplasms sandwiched between the membrane layers. One way to picture the overlapping wrappings, especially for Schwan cells, is to imagine rolling up a hot dog (axon) inside a tortilla (Schwann cell).

Why is the intracellular fluid electrical neutral?

FIGURE 11.5 Structural Classes of Neurons

Neurons are classified structurally by the number of cellular projections extending from their cell bodies. Dendrites and sensory receptors are specialized to receive stimuli, and axons are specialized to conduct action potentials.

How do myelinated axons differ from unmyelinated axons?

In myelinated axons, Schwann cells in the PNS or oligodendrocyte extensions in the CNS repeatedly wrap around a segment of an axon to form a series of tightly wrapped membranes rich in phospholipids, with little cytoplasm sandwiched between the membrane layers. One way to picture the overlapping wrappings, especially for Schwann cells, is to imagine rolling up a hot dog (axon) inside a tortilla (Schwann cell). The tightly wrapped membranes of the Schwann cells constitute the myelin sheath and give myelinated axons a white appearance because of the high lipid concentration. The myelin sheath is not continuous but contains gaps every 0.3 - 1.5 mm. At these locations are slight constrictions where the myelin sheaths of adjacent cells dip toward the axon but do not cover it, leaving an area where the myelin sheath is much thinner and about 2-3 micrometers in length. These gaps in the myelin heath are the nodes of Ranvier. Although the axon at a node of Ranvier is not wrapped in myelin, Schwann cells or oligodendrocytes extend across the node and connect to each other. The myelin sheath protects and electrically insulates each axon. These characteristics help myelinated axons conduct electrical signals more rapidly than unmyelinated axons. Unmyelinated axons are not devoid of myelin, as their name suggests. Instead, the axons rest in invaginations of the Schwann cells or oligodendrocytes. The glial cell's plasma membrane surrounds each axon but does not wrap around it many times. Thus, each axon is surrounded by a series of Schwann cells, and each Schwann cell can simultaneously surround more than one unmyelinated axon.

What criteria must a molecule meet to be considered a neurotransmitter?

In order to be considered a neurotransmitter, a molecule must meet very specific criteria: (1) they must be synthesized by a neuron and stored with in synaptic vesicles in presynaptic terminals (2) an action potential must stimulate their exocytosis into the synaptic cleft, (3) they must bind to a specific receptor on the postsynaptic membrane, and (4) they must evoke a response in the postsynaptic cell.

What happens in presynaptic facilitation?

In presynaptic facilitation, the amount of neurotransmitter released from he presynaptic terminal is elevated. For example, serotonin, released in certain axoaxonic synapses, functions as a neuromodulator that increases the release of neurotransmitter from the presynaptic terminal by causing voltage-gated Ca2+ channels to open.

Give an example of presynaptic inhibition. Describe presynaptic facilitation.

In presynaptic inhibition, the amount of neurotransmitter released form the presynaptic terminal is reduced. For example, sensory neurons for pain can release neurotransmitters from their presynaptic terminals and stimulate the postsynaptic membranes of neurons in the brain or spinal cord. Awareness of pain occurs only if action potentials are produced in the postsynaptic membrane of the CNS neurons. In presynaptic facilitation, the amount of neurotransmitter released from the presynaptic terminal is elevated. For example, serotonin, released in certain axoaxonic synapses, functions as a neuromodulator that increases the release of neurotransmitter from the presynaptic terminal by causing voltage-gated Ca2+ channels to open.

What happens in presynaptic inhibition?

In presynaptic inhibition, the amount of neurotransmitter released from the presynaptic terminal is reduced.

What does the cortex of the CNS consist of?

In the CNS, the cortex consists of gray matter on the surface of the brain.

What does gray matter consist of in the PNS?

In the PNS, gray matter consists of clusters of neuron cell bodies, or ganglia.

What is the resting membrane potential? What does it result from? Is the outside of the plasma membrane positively or negatively charged relative to the inside?

Intracellular fluid is electrically neutral because the number of positively charged cations is equal to the number of negatively charged anions. Similarly, extracellular fluid is electrically neutral. However, there is a difference in charge across the plasma membrane because of the uneven distribution of positive and negative ions across it. For simplicity, we can say that the inside of the cell is negative compared with the outside of the cell. because there are opposite charges, or poles, across the membrane, the plasma membrane is referred to as being polarized. This electrical charge difference across the plasma membrane is called a potential difference. In an unstimulated, or resting, cell, the potential difference is called the resting membrane potential. It can be measured using an oscilloscope or a voltmeter connected to microelectrodes positioned inside and outside the plasma membrane.

How is the resting membrane potential measured?

It can be measured using an oscilloscope or a voltmeter connected to micro electrodes positioned inside and outside the plasma membrane.

What are the characteristics of electrically excitable cells and the generation of action potentials due to?

It cannot be overemphasized that the characteristics of electrically excitable cells and the generation of action potentials are each due to the fundamental principle of diffusion. Intracellular and extracellular ions diffuse down their concentration gradients into or out of the cell. Because of the charge these ions posses, their movement results in an electrical current and the resting membrane potential is altered.

What are the three nerve fiber types?

It is not surprising that the structure of nerve fibers reflects their functions. There are three never fiber types: (1) type A, (2) type B, and (3) type C.

Gases and Lipids

It is now well known that gases nitric oxide (NO) and carbon monoxide (CO) serve as neurotransmitters and are sometimes referred to as gasotransmitters. The lipid-derived neurotransmitters include endocannabanoids, chemicals that bind to the same receptors as the active indgredietn in marijauna.

Give an example of action potentials in one cell stimulating action potentials in another cell.

Just as the fire from one lit torch can light another torch, action potentials in one cell can stimulate action potentials in another cell, thereby allowing communication that produces action potentials in sensory nerve fibers. The action potentials are propagated along the sensory fibers form the finger toward the CNS. For the CNS to get this information, the action potentials of the sensory neurons must produce action potential in CNS neurons. After the CNS has received the information, it produces a response. One response is the contraction of the appropriate skeletal muscles that causes the finer to move away from the hot pan. CNS action potentials cause motor neurons to produce action potentials that are then transmitted by the motor neurons toward skeletal muscles. The action potentials of the motor neuron produce skeletal muscle action potentials, which are the stimuli that cause muscle fibers to contract.

What are leak ion channels?

Describe leak ion channels and gated ion channels. How are they responsible for the permeability of a resting versus a stimulated plasma membrane?

Leak ion channels, or non gated ion channels, are always open and are responsible for the permeability of the plasma membrane to ions when the plasma membrane is unstimulated, or at rest. Each ion channel is specific for one type of ion, although the specificity is not absolute. The number of each type of leak ion channel in the plasma membrane determines the permeability characteristics of the resting plasma membrane to different types of ions. The plasma membrane is more permeable to K+ and Cl- and much less permeable to Na+ because the membrane has many more K+ and Cl- leak ion channels than Na+ leak ion channels. Gated ion channels are closed until opened by specific signals. By opening and closing, these channels can change the permeability of the plasma membrane. There are three major types of gated ion channels: 1. Ligand-gated ion channels 2. Voltage-gated ion channels 3. Other gated ion channels.

1. Ligand-gated ion channels.

Ligand-gated ion channels are stimulated to open by the binding of a specific molecule to the receptor site of the ion channel. The receptor site of the ion channel is located on its extracellular side, which allows it to receive signals from the environment. The membrane-spanning part forms a channel through the phospholipid bilayer. The specific molecule that binds to the receptor site can be referred to as a ligand. Ligands could be neurotransmitters or hormones, but there is one particular ligand for each ligand-gated ion channel. When the ligand binds to the receptor site, the ion channel opens or closes. For example, the neurotransmitter acetylcholine released from the presynaptic terminal of a neuron is the ligand that binds to a ligand-gated Na+ channel in the membrane of a muscle fiber. As a result, the Na+ channel opens, allowing Na+ to enter the fiber. Ligand-gated ion channels exist for Na+, K+, Ca2+, and Cl-, and these channels are common in nervous and muscle tissues, as well as in glands.

Chemical Class: Gases and Lipids Endocannabinoids

Location Secreted: Widespread in CNS Effects on Target: Inhibitory Mechanism of Action: Metabotropic Clinical Connections: Important in learning and memory as well as appetite and nausea control; receptors targeted by the active indgredient in marijuana.

Clinical Class: Amino Acids GABA

Locations Secreted: CNS synapses Effect on Target: Mostly inhibitory Mechanism of Action: Ionotropic and metabotropic Clinical Connections: Alcoholism renders GABA receptors less sensitive and exacerbates alcohol withdrawal symptoms such as anxiety and tremors; see table 11. 8

Chemical Class: Biogenic Amines Serotonin

Locations Secreted: CNS synapses Effects on Target: Mostly inhibitory Mechanism of Action: Metabotropic and ionotropic Clinical Connections: Mood regulator (see table 11.8)

Chemical Class: Amino Acids Glycine

Locations Secreted: CNS synapses Effects on target: Inhibitory Mechanism of Action: Ionotropic Clinical Connections: The poison strychnine, such as in rat poison, increases the excitability of certain neurons and prevents them from being inhibited. Strychnine poisoning results in muscle convulsions and tetanus. Tetanus of respiratory muscles leads to death.

Chemical Class: Acetylcholine

Locations Secreted: CNS synapses, ANS synapses, neuromuscular junction. Effects on Target: CNS: Excitatory ANS: Excitatory or Inhibitory Neuromuscular Junction: Excitatory Mechanism of Action: Ionotropic Clinical Connections: Myasthenia graves results from destruction of ACh receptors; botulism is due to inhibition of ACh secretion; tetanus is due to inhibition of ACh breakdown.

Chemical Class: Purines Adenosine

Locations Secreted: CNS synapses; important in learning and memory Effects on Target: Inhibitory Mechanism of Action: Metabotropic Clinical Connections: Prevents glutamate release during a stroke providing a neuroprotective effect; caffeine blocks adenosine's drowsiness effect

Chemical Class: Amino Acids Glutamate

Locations Secreted: CNS synapses; major excitatory neurotransmitter in the brain. Effects on Target: Excitatory Mechanism of Action: Ionotropic Clinical Connections: Important in learning and memory; used to treat Alzheimer disease; linked to enhanced taste of umami by taste buds

Chemical Class: Neuropeptides Substance P

Locations Secreted: CNS: descending pain pathways Effects on Target: Excitatory Mechanism of Action: Metabotropic Clinical Connections: Morphine reduces pain by blocking release of substance P.

Chemical Class: Biogenic Amines Dopamine

Locations Secreted: Some CNS synapses; some ANS synapses Effects on Target: Excitatory or inhibitory Mechanism of Action: Metabotropic Clinical Connections: Cocaine increases extracellular dopamine levels by blocking reuptake, which leads to euphoria

Chemical Class: Biogenic Amines Norepinephrine

Locations Secreted: Some CNS synapses; some ANS synapses; most sympathetic targets Effects on Target: Excitatory Mechanism of Action: Metabotropic Clinical Connections: Amphetamines increase extracellular norepinephrine and dopamine levels by blocking reuptake, which leads to euphoria.

Chemical Class: Neuropeptides Endorphins

Locations Secreted: Widespread in CNS Effects on Target: Inhibitory Mechanism of Action: Metabotropic Clinical Connections: Endorphins reduce pain by blocking release of substance P; produces feelings of euphoria.

What is a characteristic of multiple sclerosis?

Loss of the myelin sheath surrounding neurons in the CNS is characteristic of multiple sclerosis.

For action potential what is the analogy for the movement of action potentials?

Movement of action potentials down an axon is like a row of toppling dominoes. A single domino does not travel along the entire row. Rather, each domino must trigger the next domino to fall over, and so on. The row of dominoes represents the axon, and each domino represents an individual action potential.

Explain the functions of the nervous system.

Maintaining homeostasis. The nervous system acts to keep a constant state in order for us to survive. Integrating information. The body is constantly receiving information from internal and external stimuli. The brain and spinal cord along with nerves help integrate this information to carry out processes. Controlling muscles and glands. When you move your limbs, the nervous system happens. Smooth muscle can occur more rapidly or slowly due to the nervous system. Nervous system does not initiate contraction in smooth muscle. Receiving sensory input- the body can detect stimuli internally and externally. Establishing and maintaining mental activity. The body is able to keep a constant state of mental activity or establish one.

What are axoaxonic synapses?

Many of the synapses of the CNS are axoaxonic synapses, meaning that the axon of one neuron synapses with the presynaptic terminal (axon) of another.

What are axoaxonic synapses?

Many of the synapses of the CNS are axoaxonic synapses, meaning that the axon of one neuron synapses with the presynaptic terminal (axon) of another. Through axoaxonic synapses, one neuron can release a neuromodulator that influences the release of a neurotransmitter form the presynaptic terminal of another neuron.

A 75-year-old man was found unconscious in his bathroom after falling and hitting his head. He survived for several hours but died later in the hospital. An autopsy was performed to determine the exact cause of death. Evidence indicated that the man had suffered two strokes, both due to blocked blood vessels. One had occurred a few weeks earlier; the other had occurred very recently and may have led to the fall. Autopsy findings also indicated that, when the man hit his head, some damaged to his brain occurred as well. Based on what you know about inflammation and the cellular structure of the brain, describe what the pathologist found in each of the damaged areas of the brain.

Microglia appears in parts of the brain that are damaged from trauma, strokes or other injuries. Microglia are in the CNS, CNS has gray matter that forms on the brain in areas. The pathologist found gray matter and microglia in the damaged parts of the brain.

What are microglia?

Microglia are CNS-specific immune cells derived from he same embryonic tissue as other immune cells within the blood. However, once these cells are established within the CNS, they are the sole source of new microglia.

Microglia

Microglia are phagocytic cells within the CNS

When do microglia become mobile and phagocytic?

Microglia become mobile and phagocytic in response to inflammation. They phagocytize necrotic tissue, microorganisms, and other foreign substances that invade the CNS (table 11.2).

What are most pathways?

Most pathways are more complex and are called parallel pathways, where the input travels along several pathways.

Once she turned 21, Amanda expected good times ahead. So why could she barely manage to climb the two flights of steps to her chemistry class? When she started experiencing weakness in her left hand, Amanda consulted a physician. After conducting numerous tests, Amanda's physician told her she had multiple sclerosis (MS), a condition in which the myelin sheaths of motor and sensory neurons in the brain and spinal cord are gradually destroyed. By combining what you learned about the histology, physiology, and gross anatomy of the muscular system in chapters 9 and 10 with new information about nervous tissue organization in this chapter, explain why MS made it difficult for Amanda to walk up stairs and led to her hand weakness. Also, predict how Amanda's condition is likely to change over the next several years.

Multiple sclerosis (MS) results from the destruction of myelin sheaths around axons of CNS neurons. Although the exact mechanisms causing MS are still under scrutiny, MS is characterized as an immune-mediated disease, with the majority of MS specialist considering it to be an auto-immune disorder. Specific immune cells have been demonstrated to attack and destroy oligodendrocytes in the CNS. This is associated with a response by astrocytes called astrogliosis in which the blood-brain barrier is weakened, allowing further entry of the immune cells into the CNS. As a result of MS, Amanda is experiencing muscle weakness in her legs and left hand. In chapter 9 we learned that skeletal muscle contractions are stimulated by the nervous system and that the amount of muscle tension produced is determined by the frequency of the stimulation. This chapter explained that myelin sheaths increase the frequency of muscle stimulation. We can then conclude that the destruction of the myelin sheaths reduced the ability of Amanda's brain and spinal cord to communicate with the motor neurons supplying her leg and hand muscles. Because the brain and spinal cord neurons were conduction action potentials more slowly due to a reduction in saltatory conduction, the motor neurons were not being stimualted as quickly as normal. Therefore, Amanda's muscles could not produce as much tension as they could before the damage to the brain and spinal cord neurons occurred. Amanda's difficult with climbing the steps suggests that the damaged spinal cord neurons include those that stimulate neurons that innervate muscles of her hip, thigh, and leg. As the degeneration of myelin sheaths continues, Amanda is likely to experience increasing muscle weakness. Some muscle groups may become so weak that they cannot support Amanda's weight. She may have to use crutches, a cane, or eventually a wheelchair. Other muscle groups, including those involved in swallowing and breathing, will probably be compromised. In addition to motor functions, if Amanada's sensory neurons in her brain and spinal cord are affected, she may experience numbness, pain, and vision problems.

Describe multipolar neurons.

Multipolar neurons have many dendrites and a single axon. The dendrites vary in number and in their degree of branching. Most of the neurons within the CNS and motor neurons of the PNS are multipolar.

When do Myelin sheaths begin to form?

Myelin sheaths begin to form late in fetal development. The process continues rapidly until the end of the first year after birth and continues more slowly thereafter.

What is saltatory conduction?

Myelinated axons employ saltatory conduction (saltare, to leap) in which, an action potential is conducted from one node of Ranvier to another.

Describe saltatory conduction of an action potential.

Myelinated axons employ saltatory conduction (saltare, to leap) in which, an action potential is conducted from one node of Ranvier to another. An action potential at one node of Ranvier generates local currents that flow rapidly toward the next node of Ranvier

How are nerve fibers classified?

Nerve fibers (axons) are classified according to their size and degree of myelination.

What is nervous tissued composed of?

Nervous tissue is composed of two main cell types: (1) neurons and (2) glial cells.

What are neuromodulators?

Neuromodulators are substances released from neurons that influence the likelihood of an action potential being produced in the postsynaptic cell.

Schwann cells and satellite cells

Neuron cell bodies within ganglia are surrounded by satellite cells. Schawann cells form the myelin sheath of an axon within the PNS.

How many types of synapses are there?

There are two types of synapses: electrical and chemical.

What are neurons?

Neurons are the electrically excitable cells of the nervous system. A neuron consist of a cell body with several processes. Neurons send electrical signals to other cells using long extensions called axons.

What are neurons and what are their three parts?

Neurons are the electrically excitable cells of the nervous system. There are three parts to most types of neuron: (1) a neuron cell body, (2) dendrites, and (3) a single axon.

Describe the three types of neurons based on function.

Neurons can be classified based on either their function or their structure. There are three functional classifications based on the direction of action potentials conduction: (1) sensory neurons (afferent neurons) conduct action potentials toward the CNS; (2) motor neurons (efferent neurons) conduct action potentials away from the CNS toward muscles or glands. (3) Interneurons conduct action potentials within the CNS from one neuron to another.

What are the three functional classifications of neurons?

Neurons can be classified on either their function or their structure. There are three functional classifications based on the direction of action potentials conduction: (1) sensory neurons (afferent neurons) conduct action potentials toward the CNS; (2) motor neurons (efferent neurons) conduct action potentials away from the CNS toward muscles or glands. (3) interneurons conduct action potentials within the CNS from one neuron to another.

What are excitatory neurons?

Neurons releasing neurotransmitter substances that cause EPSPs are excitatory neurons. In general, an EPSP occurs because the membrane has become more permeable to Na+.

What are inhibitory neurons?

Neurons releasing neurotransmitter substances that cause IPSPs are called inhibitory neurons.

Neuropeptides

Neuropeptides are short chains of amino acids, ranging form 10 amino acids to 40 amino acids. These molecules include substance P and endorphins.

what are two ways neurotransmitters mediate their excitatory or inhibitory effects?

Neurotransmitter mediate their excitatory or inhibitory effects in a variety of ways. However, these mechanisms can be placed into two broad categories: (1) binding to ion channels, having an inotropic effect, or (2) binding to G-protein linked receptors, having a metabotropic effect.

What are neurotransmitters?

Neurotransmitters are the chemical messengers secreted from neurons. However, neurons can secrete more than one type of neurotransmitter.

Why does a given type of neurotransmitter affect only certain types of cells? How can a neurotransmitter stimulate one type of cell but inhibit another type?

Neurotransmitters are the chemical messengers secreted from neurons. However, neurons can secrete more than one type of neurotransmitter. Neurotransmitter can be classified on the basis of (1) their chemical structure, (2) their effect on the postsynaptic membrane, and (3) their mechanism of action at their target. In order to be considered a neurotransmitter, a molecule must meet very specific criteria: (1) they must be synthesized by a neuron and stored with in synaptic vesicles in presynaptic terminals, (2) an action potential must stimulate their exocytosis into the synaptic cleft, (3) they must bind to a specific receptor on the postsynaptic membrane, and (4) they must evoke a response in the postsynaptic cell. There are at least 100 different identified neurotransmitters. Some neurotransmitter molecules can be both excitatory at one target cell and inhibitory at a different target cell. In addition, some neurotransmitters can be excitatory at one target by binding to an int channel and excitatory at a different target by binding to a G-protein-linked receptor. Thus, a critical concept to remember is that the function of a neurotransmitter is determined by its receptor in the target cell.

How are neurotransmitters classified?

Neurotransmitters can be classified on the basis of (1) their chemical structure, (2) their effect on the postsynaptic membrane, and (3) their mechanism of action at their target.

What are the two effects neurotransmitters can have on the postsynaptic cell?

Neurotransmitters can have one of two effects on the postsynaptic cell: (1) excitatory or (2) inhibitory.

Why do neurotransmitters have short-term effects on postsynaptic membranes?

Neurotransmitters have short-term effects on postsynaptic membranes because the neurotransmitter is rapidly destroyed or removed form the synaptic cleft.

Name three ways to stop the effect of a neurotransmitter on the postsynaptic membrane. Give an example of each.

Neurotransmitters have short-term effects on postsynaptic membranes because the neurotransmitters is rapidly destroyed or removed from the synaptic cleft. For example, in the neuromuscular junction, the neurotransmitter acetylcholine is broken down by the enzyme acetylcholinesterase to acetic acid and choline. Choline is then transported back into the presynaptic terminal and combines with acetyl-CoA to re-form acetylcholine. Acetyl-CoA is synthesize by mitochondria as foods are metabolized to produce ATP (see chapter 25). Acetic acid can be absorbed from the synaptic cleft into the presynaptic terminal, or it can diffuse out of the synaptic cleft and be taken up by a variety of cells. acetic acid can be used to synthesize acetyl-CoA. When the neurotransmitter norepinephrine is released into the synaptic cleft, most of it is transported back into the presynaptic terminal, where it is packaged into synaptic vesicles for later use. The enzyme monamine oxidase breaks down some fo the norepinephrine. Diffusion of neurotransmitter molecules away from the synapse and into the extracellular fluid also limits the length of time the neurotransmitter molecules remain bound to their receptors. When norepinephrine is secreted into the blood from the adrenal medulla, it functions as a hormone. In the circulation norepinephrine is taken up primarily by liver and kidney cells, where the enzymes monoamine oxidase and catechol-O-methyltransferase convert it into inactive metabolites.

What other ions have a influence on the resting membrane potential?

Other ions, such as Na+, Cl-, and Ca2+, have a minor influence on the resting membrane potential, but the major influence is from K+.

What do action potentials do for us?

Our ability to perceive our environment, perform complex mental activities, and respond to stimuli depends on action potentials. For example, the brain interprets action potentials received from sensory cells as vision, hearing, or touch. Complex mental activities, such as conscious thought, memory, and emotions, result from action potentials. The contraction of muscles and the secretion of certain glands occur in response to action potentials generated within them.

Does the movement of Na+ into the cell shown in step 4 immediately result in an action potential? Why or why not?

No because cells require a certain level of positivity in order to reach threshold to produce an action potential.

What does K+ normally due to its concentration gradient?

Normally, due to its concentration gradient, K+ diffuses out of the cell. However, changes in the extracellular concentration of K+ can affect the resting membrane potential.

What are nuclei?

Nuclei are clusters of gray matter located deeper within the brain.

What do Oligodendrocytes form?

Oligodendrocytes form an insulating layer around axons. Oligodendrocytes have cytoplasmic extensions that wrap many times around axons forming the myelin sheath. One oligodendrocyte forms myelin sheaths for axons of multiple neurons.

What is local current?

On the outside of the membrane, Na+ from the adjacent area is attracted to the negative charges at the site of the action potential. Once inside the cell, Na+ diffuses away from the entry point. This diffusion of Na+ is called local current. As a result of the local current, the part of the membrane immediately adjacent to the action potential depolarizes.

What is the refractory period?

Once an action potential is produced at a given point on the plasma membrane, that area becomes less sensitive to further stimulation. This time period is called the refractory period.

Describe the absolute and relative refractory periods. Relate them to the depolarization and repolarization phases of the action potential.

Once an action potential is produced at a given point on the plasma membrane, that area becomes less sensitive to further stimulation. This time period is called the refractory period. The first part of the refractory period, during which complete insensitivity exists to another stimulus, is called the absolute refractory period. In many cells, it occurs from the beginning of the action potential until the end of repolarization. At the beginning of the action potential, depolarization occurs when the activation gates int eh voltage-gated Na+ channel open. At this time, the inactivate gates in the voltage-gated Na+ channels are already open. Depolarization ends as the inactivation gates close depolarization cannot occur. Near the end of repolarization when he inactivate gates open and the activation gates close , it is possible, once again, to stimulate another action potential if the activation gates re-open. The existence of the absolute refractory period guarantees that, once an action potential is begun, both the depolarization and the repolarizaiton phases will be completed, or nearly completed, before another action potential can begin and that a strong stimulus cannot lead to prolonged depolarization of the plasma membrane. The absolute refractory period has important consequences for the rate at which action potentials can be generated and for the propagation of action potentials. The second part of the refractory period, called the relative refractory period, follows the absolute refractory period. A very strong stimulus, or a stronger-than-threshold stimulus, can initiate another action potential during the relative refractory period. Thus, after the absolute refractory period, but before the relative refractory period is completed, a sufficiently strong stimulus can produce another action potential. During the relative refractory period, the membrane is more permeable to K+ because of many voltage-gated K+ channels are open. The relative refractory period ends when the voltage-gated K+ channels close and the membrane potential has returned to the resting level.

What happens once neurotransmitters are released from the presynaptic terminal?

Once neurotransmitters are released from the presynaptic terminal, they diffuse rapidly across the synaptic cleft, which is about 20 nm wide, and bind reversibly to specific receptors, such as ligand-gated ion channels, in the postsynaptic membrane. Depending on the ion channel type, this binding produces a depolarizing or hyperpolarizign graded potential in the postsynaptic membrane.

Amino Acids

One role for certain amino acids is to serve as neurotransmitters rather than just biosynthetic precursors for proteins synthesis. The amino acids that function as neurotransmitters include gamma (y)-aminobutyric acid (GABA), glycine, and glutamate.

What do Parallel after-discharge circuits have?

Parallel after-discharge circuits have neurons that stimulate several neurons in parallel organization, which all converge upon a common output cell. These circuits are involved in complex neuronal processes, including mathematics and chemical conversion. The intricate functions carried out by the CNS are affected by the numerous circuits operating together and influencing the activity of one another.

Medical Conditions: Parkinson Disease Loss of muscular contraction control.

Parkison Disease Parkinson disease results from the destruction of dopamine-producing neurons, and it is characterized by tremors and decreased voluntary motor control. Parkinson disease is treated with the drug L-Dopa, which increases the production of dopamine in the presynaptic terminals of remaining neurons. Another treatment option involves drugs that mimic the action of dopamine.

What are postsynaptic cells?

Postsynaptic cells are typically other neurons, muscle cells, or gland cells.

How are presynaptic terminal specialized?

Presynaptic terminals are specialized to produce and release neurotransmitters.

When does repolarization occur?

Repolarization occurs as the membrane potential approaches its maximum depolarization. There, the inactivation gates of the voltage-gated Na+ channels are triggered to close by the specific membrane potential and Na+ entry stops. The voltage-gate K+ channels are now fully open and K+ exits the cell. The increased diffusion of K+ out of the cell causes repolarization.

What is controlled by reverberating circuits?

Respiration appears to be controlled by a reverberating circuit. In addition, neurons that spontaneously produce action potentials are common int eh CNS and may activate reverberating circuits, which remain active awhile and include the sleep-wake cycle.

Explain reverberating circuits.

What is after-discharge?

Reverberating circuits have a chain of neurons with synapses with previous neurons in the chain, making a positive-feedback loop. This allows action potentials entering the circuit to cause a neuron farther along in the circuit to produce an action potential more than once. This response, called after-discharge, prolongs the response to a stimulus. Once a reverberating circuit is stimulated, it continues to discharge until the synapses involved become fatigued or are inhibited by other neurons.

What role do reverberating circuits play?

Reverberating circuits play a role in neuronal circuits that control rhythmic activities.

What are satellite cells?

Satellite cells surround neuron cell bodies in sensory and autonomic ganglia. Besides providing support and nutrition to the neuron cell bodies, satellite cells protect neurons from heavy-metal poisons, such as lead and mercury, by absorbing them and reducing their access to the neuron cell bodies.

What are Schwann cells?

Schwann cells form myelin sheaths. However, unlike oligodendrocytes, each Schwann cell forms a portion of the myelin sheath around only one axon. The outermost layer of each Schwann cell is called the neurilemma. It contains the majority of the Schwann cell cytoplasm, nucleus, and organelles.

What are the following: sensory receptor, nerve, ganglion, plexus?

Sensory receptors detect stimuli and then send input along nerves, which extend from the receptor to the brain or spinal cord. Sensory receptors can be neuron endings or specialized cells that detect external and internal environmental stimuli, such as temperature, pain, touch, pressure, and light, among others. A nerve is a collection of many axons bundled together outside the brain and the spinal cord. Some nerves carry electrical signals from the body to the brain and spinal cord. Other nerves carry electrical signals away from the brain and spinal cord out to body organ, such as the heart or the skeletal muscles. Some neurons form clusters of cell bodies outside the brain and spinal cord called ganglia. A plexus is a bundle of nerves outside the brain and the spinal cord.

What are sensory receptors?

Sensory receptors detect stimuli and then send input along nerves, which extend from the receptor to the brain or spinal cord. Sensory receptors can be neuron endings or specialized cells that detect external and internal environmental stimuli, such as temperature, pain, touch, pressure, and light, among others. Sensory receptors are distributed around the body within muscles, skin, joints, eyes, ears, and many other locations. These receptors constantly monitor body conditions and communicate that information to the brain and the spinal cord. These specialized receptors are located throughout the body, such as in the skin or specializing organs like the eyes, and detect temperature, pain, touch, pressure, light, odors, and many other stimuli. These stimuli are communicated to the CNS, which will process the information and initiate a response by the body. For example, when turning on a bright light in a dark room, the CNS generates a message to constrict the pupil of the eye. It is the motor division of the PNS that will actually communicate with the eye to cause constriction of the pupil.

2. Receiving sensory input.

Sensory receptors monitor numerous external and internal stimuli. We are aware of sensations from some stimuli, such as sight, hearing, taste, smell, touch, pain, body position, and temperature. Other stimuli, such as blood pH, blood gases, and blood pressure, are processed at an unconscious level.

What is a neuromodulator? Give some examples of how drugs can modulate the action of neurotransmitters.

Several substance have been identified as neurotransmitters, and other are suspected neurotransmitters. Scientists once thought that each neuron contained only one type of neurotransmitter; however, they now know that some neurons can secrete more than one type. If a neuron does produce more than on neurotransmitter, it secretes all of them from each of its presynaptic terminals. The physiological significance of presynaptic terminals that secrete more than one type of neurotransmitter has not been clearly established. a summary of several representative neurotransmitter is provided in table 11.6 Table 11.7 provides information on clinical uses of selected neurotransmitters. Serotonin is important for emotional states such as depression and anxiety. Selective serotonin uptake inhibitors (SSRIs) temporarily block serotonin transporters, which decreases serotonin transport back into presynaptic terminals. This block increases serotonin levels in the synaptic cleft. Certain illegal drugs such as LSD and Ecstasy, or its relative Molly, target serotonin receptors.

What did scientists once believe about each neuron?

Several substances have been identified as neurotransmitters, and others are suspected neurotransmitters. Scientists once thought that each neuron contained only one type of neurotransmitter; however, they now know that some neurons can secrete more than one type.

4. Controlling muscles and glands.

Skeletal muscles normally contract only when stimulated by the nervous system; thus, the nervous system controls the major movements of the body by controlling skeletal muscle. Some smooth muscle, such as that in the walls of blood vessels, contracts only when stimulated by the nervous system or by hormones. Cardiac muscle and some smooth muscle, such as that in the wall of the stomach, contract autorhythmically-that is, no external stimulation is necessary for each contraction event. Although the nervous system does not initiate contraction in these muscles, it can cause the contractions to occur more rapidly or more slowly. Finally, the nervous system controls the secretions from many glands, including sweat glands, salivary glands, and glands of the digestive system.

Differntiate between the somatic and the autonomic nervous systems.

Somatic is voluntary while the autonomic is involuntary. Somatic controls our muscles that we move with conscious thought while the autonomic system controls our organs, glands and other involuntary processes that act unconsciously.

What do nerves do?

Some nerves carry electrical signals from the body to the brain and spinal cord. Other nerves carry electrical signals away from the brain and spinal cord out to body organs, such as the heart or the skeletal muscles.

What do some neurons form?

Some neurons form clusters of cell bodies outside the brain and spinal cord called ganglia.

Can neurotransmitter molecules be both excitatory and inhibitory?

Some neurotransmitters molecules can be both excitatory at one target cell and inhibitory at different target cell. In addition, some neurotransmitters can be excitatory at on target by binding to an ion channels and excitatory at a different target by binding to a G-protein-linked receptor. Thus, a critical concept to remember is that the function of a neurotransmitter is determined by its receptor in the target cell.

What is hypocalcemia?

Sometimes, an individual may experience hypocalcemia, a lower-than-normal level of Ca2+ in the blood. Because normal levels of Ca2+ are required to keep voltage-gated Na+ channels closed, hypocalcemia allows for their spontaneous opening. Thus, hypocalcemia symptoms include nervousness and uncontrolled skeletal muscle contraction. Hypocalcemia can result due to a lack of dietary Ca2+ or vitamin D or insufficient PTH levels.

What is spatial summation?

Spatial summation occurs when multiple action potentials from separate neurons arrive simultaneously at the same postsynaptic neuron. In the postsynaptic neuron, each action potential causes a depolarizing graded potential that undergoes summation at the trigger zone. If the summated depolarization reaches threshold, an action potential is produced.

When does spatial summation of EPSPs and IPSPs occur?

Spatial summation of EPSPs and IPSPs occurs in the postsynaptic neuron, and whether a postsynaptic action potential is initiated or not depends on which type of graded potential has the greater influence on the postsynaptic membrane potential.

What are choroid plexuses?

Specialized ependymal cells and blood vessels form structures called choroid plexuses, which are located within certain regions of the ventricles. The choroid plexuses secrete cerebrospinal fluid, which flows through the ventricles of the brain.

PROCESS FIGURE 11.12 Voltage-Gated Ion Channels and the Action Potential

Step 1 illustrates the status of voltage-gated Na+ and K+ channels in a resting cell. Steps 2-5 show how the channels open and close to produce an action potential. Next to each step (far right), a graph shows in red the membrane potential resulting form the condition fo the ion channels.

Excitatory neurons A and B both synapse with neuron C. Neuron A releases a neurotransmitter, and neuron B releases the same type and amount of neurotransmitter plus a neuromodulator that produces EPSPs in neuron C. Action potentials produced in neuron A axon can result in action potential production in neuron C. Action potentials produced in neuron B alone also can cause action potential production in neuron C. Which results in more action potentials in neuron C, stimulation by only neuron A or stimulation by only neuron B? Explain.

Stimulation of neuron B because this neuron releases neurotransmitters and neuromodulaters which will increase the amount of action potential being produced.

Why are stimuli that do not result in action potential transmission across synapses not perceived?

Stimuli that do not result in action potential transmission across synapses are not perceived because the information never reaches the cerebral cortex. The brain doesn't perceive a large amount of sensory information as a result of complex integration.

Chemical Class: Gases and Lipids Nitric Oxide (NO)

Structure: :N=O Locations Secreted: CNS; PNS; adrenal glands; penis Effects on Target: Excitatory Mechanism of Action: Metabotropic Clinical Connections: NO release causes vasodilation of blood vessels supplying the penis. Viagra, an erectile dysfunction treatment, prolongs the effects of NO

What do subsequent action potentials cause?

Subsequent action potentials cause depolarizations that summate with previous depolarizations. If the summated depolarizing graded potentials reach threshold at the trigger zone, an action potential is produced in the postsynaptic neuron.

Describe subtrheshold, threshold, maximal, sub maximal, and supranmaximal stimuli. What determines the maximum frequency of action potential generation?

Subthreshold stimulus is any stimulus not strong enough to produce a graded potential that reaches threshold. Therefore, no action potential is produced. A threshold stimulus produces a graded potential that is just strong enough to reach threshold and cause the production of a single action potential. A maximal stimulus is just strong enough to produce a maximum frequency of action potentials. A submaximal stimulus includes all stimuli between threshold and the maximal stimulus strength. For submaximal stimuli, the action potential frequency increases in proportion to the strength of the stimulus because the size of the graded potential increases with stimulus strength. A supramaximal stimulus is any stimulus stronger than a maximal stimulus. Because an axon's ability to produce action potentials is limited, these stimuli cannot produce a greater frequency of action potentials than a maximal stimulus. The duration of the absolute refractory period determines the maximum frequency of action potentials generated in an excitable cell. During the absolute refractory period, a second stimulus, no matter how strong, cannot stimulate an additional action potential. However, as soon as the absolute refractory period ends, it is possible for a second stimulus to cause the production of an action potential.

What is summation?

Summation is the combination of graded potentials, which, if sufficiently large, will result in an action potential.

What does summation of depolarizing membrane graded potentials cause?

Summation of depolarizing membrane graded potentials causes the neuron's membrane potential to become closer to a specific membrane potential called threshold.

What are symptoms of hypokalemia?

Symptoms of hypokalemia include muscular weakness, abnormal heart function, and sluggish reflexes.

When does temporal summation in the postsynaptic cell result?

Temporal summation in the postsynaptic cell results when the second action potential from the presynaptic neuron initiates a second grade depolarization before the postsynaptic cell's membrane potential returns to tis resting value.

When does temporal summation result?

Temporal summation results when two or more action potentials arrive very close together at the postsynaptic cell from eh presynaptic terminal of a particular neuron. The first action potential causes a depolarizing graded potential in the postsynaptic membrane that remains for a few milliseconds before it disappears, although its magnitude decreases through time.

What are the two subdivisions of the ANS?

The ANS has two subdivisions: (1) the sympathetic division and (2) the parasympathetic division.

What are the subcategories of the ANS?

The ANS has two subdivisions: (1) the sympathetic division and (2) the parasympathetic division. The sympathetic division readies the body for physical activity, and is called the fight-or-flight division. The parasympathetic division regulates resting functions, such as digesting food or slowing the heart rate, and is called the rest-and-digest division.

Give a analogy for the two systems communicating with each other?

The CNS can be thought of as the key decision maker, while the PNS is the messenger that provides input about the body to the CNS and then delivers the CNS decision on how the body is to respond to a particular set of stimuli.

What does the CNS consist of?

Name the components of the CNS and the PNS.

The CNS consists of the brain and the spinal cord. The brain is housed within the skull and the spinal cord is housed within the vertebral canal of the vertebral column. The brain and spinal cord are continuous with each other, transitioning from brain to spinal cord at the foramen magnum of the skull. The PNS consists of all the nervous tissue outside the CNS, which includes nerves, ganglia, and sensory receptors. The PNS has two primary divisions: (1) the sensory division and (2) the motor division.

What does the IPSP result from?

The IPSP results from an increase in the permeability of the plasma membrane to Cl- or K+, resulting in hyperpolarization of the postsynaptic cell.

Give an example of IPSP resulting form an increase in the permeability of the plasma membrane to Cl- or K+

The IPSP results from an increase in the permeability of the plasma membrane to Cl- or K+, resulting in hyperpolarization of the postsynaptic cell. For example, in the spinal cord, glycine binds to its receptors, directly causing Cl- channels to open and Cl- diffuses into the cell, causing the inside of the cell to become more negative and resulting in hyperpolarization. Acetylcholine can bind to its receptors in the heart, causing G-protein-mediate opening of K+ channels. The concentration of K+ is greater inside the cell than outside, and increased permeability of the membrane to K+ allows K+ to diffuse out of the cell. Consequently, the outside of the cell becomes more positive than the inside, resulting in hyperpolarization.

What does the PNS consist of?

The PNS consist of all the nervous tissue outside the CNS, which includes nerves, ganglia, and sensory receptors. The PNS has two primary divisions: (1) the sensory division and (2) the motor division.

Based on the direction they transmit action potentials, what are the two subcategories of the PNS?

The PNS consists of all the nervous tissue outside the CNS, which includes nerves, ganglia, and sensory receptors. The PNS has two primary divisions: (1) the sensory division and (2) the motor division. The sensory division transmits electrical signals from specialized receptors in the body toward the CNS. For this reason, the sensory division is also called the afferent division. The motor division, as described in the bright light example, transmits electrical signals from the CNS to effector organs, such as muscles in the eyes and glands. Thus, the motor division is also called the efferent division (efferent = away). The motor division consists of two branches: (1) the somatic nervous system and (2) the autonomic nervous system.

What else is the action potential frequency proportional to?

The ability to stimulate muscle or gland cells also depends on action potential frequency. A low frequency of action potentials produces a weaker muscle contraction or less secretion than does a higher frequency. For example, a low frequency of action potentials in a muscle results in incomplete tetanus, and a high frequency results in complete tetanus.

FIGURE 11.3 Refractory Period

The absolute and relative refractory periods of an action potential. In some cells, the absolute refractory period ends during the repolarization phase of the action potential

What does the absolute refractory period have?

The absolute refractory period has important consequences for the rate at which action potentials can be generated and for the propagation of action potentials.

What does the accompanying reduction in action potential speed account for?

The accompanying reduction in action potential speed accounts for a reduced ability to regulate skeletal muscle movements.

What does the accumulation of K+ outside the plasma membrane do?

The accumulation of K+ outside the plasma membrane makes the outside of the plasma membrane positive relative to the inside.

FIGURE 11.11 Action Potential

The action potential consists of a depolarization phase and a repolarization phase, often followed by a short period of hyperpolarization, called the after potential.

What is the action potential frequency?

What is action potential frequency? What two factors determines action potential frequency?

The action potential frequency is the number of action potentials produced per unit of time in response to a stimulus. Recall that the size of the graded potential is dependent on the strength of a stimulus. A small stimulus result in a small graded pontential and, as stimulus strength increases, the size of the graded potential increases. Thus, action potential frequency is directly proportional to stimulus strength and to the size of the graded potential.

Though not shown in this figure, Cl- concentrations are higher outside the cell. However, Cl- movement across the membrane is limited. Given that the membrane is relatively permeable to Cl-, explain what factor limits the movement of Cl- across the membrane.

The amount of negativity within a cell is at a constant level, this would deter the amount that should be inside. Cl- being a negative ion would make the membrane too negative.

What is the autonomic nervous system?

The autonomic nervous system is the involuntary division of the motor division. It regulates activities without our conscious control such as contractions of smooth muscle, cardiac muscle, and secretions by certain glands. For example, your heart rate increases when you hear an unexpected, loud noise, which startles you. The ANS has two sets of neurons between the CNS and the effector. Cell bodies of the first neuron are within the CNS. Their axons connect with the cell bodies of the second neuron in an autonomic ganglion. Axons of the second neuron extend to the effector.

How many synapses does an average presynaptic neuron have and postsynaptic neuron have?

The average presynaptic neuron synapses with about 1000 other neurons, but the average postsynaptic neuron has up to 10,000 synapses. Some postsynaptic neurons in the part of the brain called the cerebellum have up to 100,000 synapses.

What can an axon of a neuron do?

The axon of a neuron can branch repeatedly to form synapses with many other neurons, and hundreds or even thousands of axons can synapse with the cell body and dendrites of a single neuron.

Where does the axon project?

The axon projects away from the cell body until it reaches the cell membrane of the effector. The point of contact between the axon ending and its effector is called a synapse.

What is the role of a neurotransmitter? Where is it stored?

The axon projects away from the cell body until it reaches the cell membrane of the effector. The point of contact between the axon ending and its effector is called a synapse. Usually, axons branch many times and contact multiple effector cells. The axon ending at the synapse is called the presynaptic terminal. There, the axon endings have many synaptic vesicles, which store the signal molecules produced by the neuron. These signal molecules control the effectors and area called neurotransmitters.

What is the blood-brain barrier?

The blood-brain barrier determines what substances can pass from the blood into the nervous tissue of the brain and spinal cord. The blood-brain barrier protects neurons from toxic substances in the blood, allows the exchange of nutrients and waste products between neurons and the blood and prevents fluctuations in blood composition from affecting brain functions.

5. Establishing and maintaining mental activity.

The brain is the center of mental activities, including consciousness, thinking, memory, and emotions.

FIGURE 11.1 Nervous System

The central nervous system (CNS) consists of the brain and spinal cord. The peripheral nervous system (PNS) consists of cranial nerves, which arise form the brain, and spinal nerves, which arise from the spinal cord. The nerves, which are shown cut in the illustration, actually extend throughout the body.

What is the CNS?

The central nervous system (CNS) receives information from and sends information to the body.

Compare the general functions of the CNS and the PNS.

The central nervous system (CNS) receives information from and sends information to the body. The peripheral nervous system (PNS) is responsible for detecting stimuli in and around the body and sending that information to the CNS and then communicating messages from the CNS to the body. The CNS an be thought of as the key decision maker, while the PNS is the messenger that provides input about the body to the CNS and then delivers in CNS decisions on how the body is to respond a particular set of stimuli.

What does the combination of neurotransmitters with their specific receptors cause?

The combination of neurotransmitters with their specific receptors causes either depolarization or hyperpolarization of the postsynaptic membrane.

Explain the production of EPSPs and IPSPs. Why are they important?

The combination of neurotransmitters with their specific receptors causes either depolarization or hyperpolarization of the postsynaptic membrane. When depolarization of the postsynaptic cell occurs, the response in stimulatory, and the resulting graded potential is called an excitatory postsynaptic potential. EPSPs are important because the depolarization might reach threshold, thereby producing an action potential and a response from the cell. Neurons releasing neurotransmitter substances that cause EPSPs are excitatory neurons. In general, an EPSP occurs because the membrane has become more permeable to Na+ When the combination of a neurotransmitter with its receptor results in hyperpolarization of the postsynaptic membrane, the response is inhibitory because no action potentials are generated. This local hyperpolarization is called an inhibitory postsynaptic potential. IPSPs are important because they move the membrane potential farther from threshold, which decreases the likelihood of an action potential being generated. Neurons releasing neurotransmitter substances that cause IPSPs are called inhibitory neurons. The IPSP results from an increase in the permeability of the plasma to Cl- or K+, resulting in hyperpolarizaion of the postsynaptic cell.

What are connexons?

The connexons are groups of six tubular proteins, each called a connexin.

What is the depolarization phase characterized by?

The depolarization phase is characterized by the rapid increase in positive charge inside the neuron.

What are the depolarization and repolarization phases of an action potential?

The depolarization phase is characterized by the rapid increase in positive charge inside the neuron. When depolarizing graded potentials reach threshold, a large number of voltage-gated Na+ channels open rapidly. Sodium ions then diffuse into the cell, and the resulting depolarization causes additional voltage-gated Na+ channels to open. As a consequence, more Na+ diffuses into the cell, causing a greater depolarization of the membrane, which intern causes still more voltage-gated Na+ channels to open. This is an example of a positive-feedback cycle, and it continues until most of the voltage-gated Na+ channels in the plasma membrane are open. The repolarization phase is characterized by the rapid return of the membrane potential to the resting membrane potential. The inside of the cell returns to its negative state. Repolarization occurs as the membrane potential approaches its maximum depolarization. There, the inactivation gates of the voltage-gated Na+ channels are trigged to close by the specific membrane potential and Na+ entry stops. The voltage-gated K+ channels are now fully open and K+ exits the cell. The increased diffusion of K+ out fo the cell causes repolarization. At the end of repolarization, the return toward resting membrane potential causes the activation gates in the voltage-gated Na+ channels to close and the inactivation gates to open. Although this change does not affect the diffusion of Na+, it does return the voltage-gated Na+ channels to their resting state.

What is the development of myelin sheaths associated with?

The development of myelin sheaths is associated with the infant's continuing development of more rapid and better coordinated responses.

FIGURE 11.24 Neuronal Pathways and Circuits

The direction of action potential propagation is represented by the orange arrows. (a) General model of a convergent pathway; many neurons converge and synapse with a smaller number of neurons.

FIGURE 11.24 Neuronal Pathways and Circuits.

The direction of action potential propagation is represented by the orange arrows. (b) General model of a divergent pathway; a few neurons synapse with a larger number of neurons.

FIGURE 11.24 Neuronal Pathways and Circuits

The direction of action potential propagation is represented by the orange arrows. (c) Simple model of a reverberating circuit; input action potentials result in the production of larger number of output action potentials because neurons within the circuit are repeatedly stimulated to produce action potentials.

FIGURE 11.24 Neuronal Pathways and Circuits

The direction of action potential propagation is represented by the orange arrows. (d) Simple model of a parallel after-discharge circuit; several neurons in parallel that integrate complex processes stimulate a common output cell.

What does the duration of the absolute refractory period determine?

The duration of the absolute refractory period determines the maximum frequency of action potentials generated in an excitable cell.

What are action action potentials?

The electrical signals produced by the nervous system are called action potentials.

What does monoamine oxidase break down?

The enzyme monoamine oxidase breaks down some of the norepinephrine.

What do ependymal cells have?

The ependymal cells frequently have patches of cilia that help circulate cerebrospinal fluid through the brain cavities. Ependymal cells also have long processes at their basal surfaces that extend deep into the brain and the spinal cord and seem, in some case, to have astrocyte-like functions.

What are the essential components of a chemical synapse?

The essential components of a chemical synapse are the presynaptic terminal, the synaptic cleft, and the postsynaptic membrane.

What does the existence of the absolute refractory period guarantee?

The existence of the absolute refractory period guarantees that, once an action potential is begun, both the depolarization and the repolarization phases will be completed, or nearly completed, before another action potential can begin and that a strong stimulus cannot lead to prolonged depolarization of the plasma membrane.

What is the primary way neurons are hyper polarized after an action potential?

The exit of K+ ions is the primary way neurons are hyperpolarized after an action potential. When voltage-gated K+ channels open, K+ diffuses out of the cell down its concentration gradient. Likewise, opening of ligand-gated K+ channels would hyperpolarize a neuron. This is the mechanism employed by some inhibitory neurotransmitters. In addition, if levels of extracellular K+ decrease, the concentration gradient for K+ exit is now greater. The steeper concentration gradient encourages more K+ to diffuse out of the cell through leak channels. This causes the resting membrane potential to decrease, thus hyper polarizing the cell and making action potential generation more difficult.

What is the first part of the refractory period?

The first part of the refractory period, during which complete insensitivity exists to another stimulus, is called the absolute refractory period. In many cells, it occurs from the beginning of the action potential until near the end of repolarization.

What is the importance of myelinated axons illustrated in?

The importance of myelinated axons is dramatically illustrated in diseases that gradually destroy the myelin sheath, such as multiple sclerosis and some cases of diabetes mellitus. Action potential transmission is slowed, resulting in impaired control of skeletal and smooth muscles. In severe cases, action potential transmission can become completely blocked.

What happens to the inside of the cell during repolarization?

The inside of the cell returns to its negative state.

What does the interaction between a neurotransmitter and a receptor represent?

The interaction between neurotransmitter and a receptor represents an equilibrium: Neurotransmitter + Receptor = Neurotransmitter - Receptor complex

What are the major organelles within presynaptic terminals?

The major cytoplasmic organelles within presynaptic terminals are mitochondria and numerous membrane-bound synaptic vesicles, which contain neurotransmitters, such as acetylcholine.

What is the postsynaptic membrane?

The membrane of the postsynaptic cell associated with the presynaptic terminal is the postsynaptic membrane.

What is the membrane potential?

The membrane potential is a measure of the electrical properties of the cell membrane and is due to two major characteristics: 1. Ionic concentration differences across the plasma membrane 2. Permeability characteristics of the plasma membrane.

What is the motor division?

The motor division, as described in the bright light example, transmits electrical signals from the CNS to effector organs, such as muscles in the eyes and glands. Thus, division consists of two branches: (1) the somatic nervous system and (2) the autonomic nervous system.

Based on the structures they supply, what are the two subcategories of the motor division?

The motor division, as described in the bright light example, transmits electrical signals from the CNS to effector organs, such as muscles in the eyes and glands. Thus, the motor division is also called the efferent division (efferent = away). The motor division consists of two branches: (1) the somatic nervous system and (2) the autonomic nervous system. The somatic nervous system is the voluntary division fo the motor division. It allows you to decide to move your skeletal muscles, such as when raising your hand to ask a question, or to stand and walk across the room. The CNS generates electrical signals that are sent to the skeletal muscles by nerves of the somatic nervous system. The cell bodies of somatic motor neurons are located within the CNS, and their axons extend by way of nerves to control skeletal muscle cells. The autonomic nervous system is the involuntary division of the motor division. It regulates activities without our conscious control such as contractions of smooth muscle, cardiac muscle, and secretions by certain glands. For example, your heart rate increases when you hear an unexpected, loud noise, which startles you. The ANS has two sets of neurons between the CNS and the effector. Cell bodies of the first neuron are within the CNS. Their axons connects with the cell bodies of the second neuron in an autonomic ganglion. Axons of the second neuron extend to the effector.

What does movement of materials within the axon provide?

The movement of materials within the axon is necessary for its normal function, but, unfortunately, it also provides an entry way for infectious agents and harmful substances to the CNS. For example, rabies and herpes viruses can enter damaged axons in the skin and be transported within the axons to the CNS.

Is the myelin sheath of Schwann cells continuous?

The myelin sheath is not continuous but contains gaps every 0.3 - 1.5 mm. At these locations are slight constrictions where the myelin sheaths of adjacent cells dip toward the axon but do not cover it, leaving an area where the myelin sheath is much thinner and about 2 - 3 micrometers in length.

What does the myelin sheath do for each axon?

The myelin sheath protects and electrically insulates each axon. These characteristics help myelinated axons conduct electrical signals more rapidly than unmyelinated axons.

What does the nervous system consists of?

The nervous system consists of two major divisions. (1) the central nervous system and (2) the peripheral nervous system. These two systems communicate with each other and with the body to maintain homeostasis.

List the divisions of the nervous system and describe the characteristics of each.

The nervous system is divided into the CNS and PNS. CNS contains brain and spinal cord PNS contains nerves and ganglia The PNS is divided into two divisions.. Motor and Sensory Sensory division also known as the afferent division. Responds to stimuli and sends it to the brain and rest of the body. Motor division also known as the efferent division. Carries out functions such as moving limbs and contraction of muscles involuntary and voluntary. The motor division is further divided into the somatic nervous system which allows us to move our limbs and the autonomic nervous system which allows our organs to function and respond to stimuli. The ANS has two divisions known as sympathetic and parasympathetic. The sympathetic is the fight or flight while the parasympathetic is the rest and digest. The third division of the nervous system is the enteric nervous system, which is known as the brain of the digestive system.

What does the nervous system do?

The nervous system regulates and coordinates functions of the body required to maintain homeostasis.

1. Neuron Cell Body

The neuron cell body, or soma, performs the typical functions of any cell, such as protein synthesis and packaging of proteins into vesicles. Each neuron cell body contains a single, relatively large, and centrally located nucleus with a prominent nucleolus. Neurons have extensive rough endoplasmic reticulum (ER), called Nissl bodies. The abudance of Nissle bodies reflects the significant amount of protein synthesis neurons perform. The Golgi apparatuses are located near the nucleus, and mitochondria and other organelles are present. Large numbers of intermediate filaments (neurofilaments) and microtubules form bundles that organize the cytoplasm into different regions.

Explain the organizational patterns of neurons within the CNS?

The organizational patterns of neurons within the CNS vary from relatively simple to extremely complex.

Predict the effect of a decrease in the extracellular concentration of Ca2+ on the resting membrane potential.

The outside of the plasma membrane would become more negative causing the flow of K+ towards the outside of the cell. It would make the outside less positive and the inside less negative. It would make the resting membrane potential smaller.

What is the parasympathetic division?

The parasympathetic division regulates resting functions, such as digesting food or slowing the heart rate, and is called the rest-and-digest division.

What is the PNS?

The peripheral nervous system (PNS) is responsible for detecting stimuli in and around the body and sending that information to the CNS and then communicating messages from the CNS to the body.

Why is there a charge difference across the plasma membrane?

There is a difference in charge across the plasma membrane because of the uneven distribution of positive and negative ions across it. For simplicity, we can say that the inside of the cell is negative compared with the outside of the cell.

Why is the plasma membrane more permeable to K+?

What ion is the major influence on the resting membrane potential? Explain its role.

The plasma membrane is more permeable to K+ because of a higher proportion of K+ leak ion channels compared with leak channels for other ions. Positively charged K+ can therefore diffuse down its concentration gradient from inside to outside the cell. Negatively charged proteins and other molecules cannot diffuse through the plasma membrane with the K+. As K+ diffuses out of the cell, the loss of positive charges make the inside of the plasma membrane more negative. Because opposite charges attract, K+ is attracted back toward the cell. the accumulation of K+ outside the plasma membrane makes the outside of the plasma membrane positive relative to the inside. The resting membrane potential is an equilibrium. This equilibrium is established when the tendency for K+ to diffuse out fo the cell is equal to the tendency for K+ to move into the cell. To reiterate, the concentration gradient for K+ is toward the outside of the cell, but because of the negative charge inside the cell, K+ tends to be pulled back toward the interior of the cell.

What is still an area of intense research?

The precise molecular mechanism regulating synaptic transmission is still an area of intense research. However, most neurophysiologist agree that synaptic vesicle membranes have a Ca2+ sensor, such as synaptotagmin.

What does the presynaptic terminal consist of?

The presynaptic terminal consists of the end of an axon of the presynaptic cell.

If an axon has been severed, so that it is no longer connected to its neuron cell body, what will be the effect on the distal and proximal portions of the axon? Explain your prediction.

The proximal and distal ends of the neuron cell body will die because of the severed neuron cell body. It contains the nucleus, and without the nucleus the cell cannot function correctly.

What is repolarization phase characterized by?

The repolarization phase is characterized by the rapid return of the membrane potential to the resting membrane potential.

How is the plasma membranes equilibrium established?

The resting membrane potential is an equilibrium. This equilibrium is established when the tendency for K+ to diffuse out of the cell is equal to the tendency for K+ to move into the cell. To reiterate, the concentration gradient for K+ is toward the outside of the cell, but because of the negative charge inside the cell, K+ tends to be pulled back toward the interior of the cell.

How does the resting membrane establish an equilibrium?

The resting membrane potential is an equilibrium. This equilibrium is established when the tendency for K+ to diffuse out of the dell is equal to the tendency for K+ to move into the cell. Te reiterate, the concentration gradient for K+ is toward the outside of the cell, but because of the negative charge inside the cell, K+ tends to be pulled back toward the interior of the cell.

What is the resting membrane potential proportional too?

The resting membrane potential is proportional to the tendency for K+ to diffuse out of the cell, not to the actual rate of flow for K+.

What is the resting membrane potential of neurons and skeletal muscle fibers?

The resting membrane potential of neurons is approximately -70 mV, and that of skeletal muscle fibers is approximately -90mV.

What does the resting membrane potential result from?

The resting membrane potential results from two characteristics of neurons: 1. The permeability characteristics of the resting plasma membrane. 2. Differences in concentration of ions between the intracellular and the extracellular fluids.

Is the resting plasma membrane permeable to Ca2+?

The resting plasma membrane is not very permeable to Ca2+, either. The plasma membrane is relatively permeable to Cl-. but these negatively charged ions are repelled by the negative charge inside the cell.

Is the resting plasma membrane permeable to Na+?

The resting plasma membrane is not very permeable to Na+. In fact, because the resting plasma membrane is 50-100 times less permeable to Na+ than to K+, very little Na+ can diffuse into the resting cell.

What is the relative refractory period?

The second part of the refractory period, called the relative refractory period, follows the absolute refractory period.

FIGURE 11.2 Information Flow in the Nervous System

The sensory division of the peripheral nervous system (PNS) detects stimuli and conducts action potentials to the central nervous system (CNS). The CNS interprets incoming action potentials and initiates action potentials that are conducted through the motor division to produce a response. The motor division is divided into the somatic nervous system and the autonomic nervous system. The enteric nervous system is an independent branch of the PNS and is not illustrated in this figure.

What is the sensory division?

The sensory division transmits electrical signals from specialized receptors in the body toward the CNS. For this reason, the sensory division is also called the afferent division.

Where are the cell bodies of sensory, somatic motor, and autonomic neurons located? What is a synapse?

The sensory division transmits electrical signals from specialized receptors in the body toward the CNS. For this reason, the sensory division is also called the afferent division. Sensory receptors detect stimuli and then send input along nerves, which extend from the receptor to the brain or spinal cord. The cell bodies of somatic motor neurons are located within the CNS, and their axons extend by way of nerves to control skeletal muscle cells. The ANS has two sets of neurons between the CNS and the effector. Cell bodies of the first neuron are within the CNS. Their axons connect with the cell bodies of the second neuron in an autonomic ganglion. Axons of the second neuron extend to the effector. Synapse- a junction between two nerve cells, consisting of a minute gap across which impulses pass by diffusion of a neurotransmitter.

What is the simplest organizational pattern?

The simplest organization is a serial pathway, where the input travels along only one pathway.

What restores the resting membrane potential?

The sodium-potassium pump restores normal resting ion concentrations by transporting these ions in the opposite direction of their movement during the action potential. That is, Na+ is pumped out fo the cell and K+ is pumped into the cell. The sodium-potassium pump is too slow to have an effect on either the depolarization or the repolarization phase of individual action potentials. As long as the Na+ and K+ concentrations remain unchanged across the plasma membrane, all the action potentials produced by a cell are identical. They all take the same amount of time, and they all exhibit the same magnitude.

What is the somatic nervous system?

The somatic nervous system is the voluntary division of the motor division. It allows you to decide to move your skeletal muscles, such as when raising your hand to ask a question. or to stand and walk across the room. The CNS generates electrical signals that are sent to the skeletal muscles by nerves of the somatic nervous system. The cell bodies of somatic motor neurons are located within the CNS, and their axons extend by way of nerves to control skeletal muscle cells.

What is the synaptic cleft?

The space separating the axon ending and the cell with which it synapses is the synaptic cleft.

Myelinated axons can be describe as "functionally shorter" axons compared to unmyelinated axons. Explain what is meant by "functionally shorter."

The speed of action potential conduction along an axon depends on the myelination of the axon. Action potentials are conducted more rapidly in myelinated than unmyelinated axons because they are formed quickly at each successive node of Ranvier, instead of being propagated more slowly through every part of the axon's membrane, as in unmyelinated axons.

What does the speed of action potential conduction along an axon depend on?

What is the speed of action potential conduction affected by?

The speed of action potential conduction is also affected by the thickness of the myelin sheath, which is determined by how many times oligodendrocytes or Schwann cells wrap around the axon. Heavily myelinated axons have a thicker myelin sheath and conduct action potentials more rapidly than lightly myelinated axons.

Compare the speed of action potential conduction in (a) heavily myelinated, lightly myelinated, and unmyelinated axons and (b) large-diameter and small-diameter axons.

The speed of action potential conduction is also affected by the thickness of the myelin sheet, which is determined by how many times oligodendrocytes or Schwan cells wrap around the axon. Heavily myelinated axons have a thicker myelin sheath and conduct action potentials more rapidly than lightly myelinated axons. Loss of the myelin sheath surrounding neurons in the CNS is characteristic of multiple sclerosis. The accompanying reduction in action potential speed accounts for a reduced ability to regulate skeletal muscle movements. In addition to myelination, the diameter of an axon affects the speed of action potential conduction. Large-diameter axons conduct action potentials more rapidly than small-diameter axons because large-diameter axons have a greater surface area. Consequently, at a given site on an axon, more voltage-gated Na+ channels open during depolarization, resulting in a greater local current flow, which more rapidly stimulates adjacent membrane areas. Nerve fibers (axons) are classified according to their size and degree of myelination. It is not surprising that the structure of nerve fibers reflects their functions. There are three nerve fiber types: (1) type A, (2) type B, and (3) type C. Type A fibers are large-diameter, myelinated axons that conduct action potentials at 15-120 m/s (34-268 mi/h). Motor neurons supplying skeletal muscle and most sensory neurons have type A fibers. Rapid response to the external environment is possible because of the rapid input of sensory information to the CNS and the rapid output of action potentials to skeletal muscles. Type B fibers are medium-diameter, lightly myelinated axons that conduct action potentials at 3- 15 m/s (7-34 mi/h), and type C fibers are small-diameter, unmyelinated axons that conduct action potentials at 2 m/s or less (4.5 mi/h). Types B the stomach, intestines, and heart. The responses necessary to maintain internal homeostasis, such as digestion, need not be as rapid as responses to the external environment.

What is the sympathetic division?

The sympathetic division readies the body for physical activity, and is called the fight-or-flight division.

How is the synapse an essential structure?

The synapse is an essential structure for the integration carried out by the CNS. For example, action potentials propagated along axons from sensory organs to the CNS perception of a sensation, action potentials must be transmitted across synapses as they travel through the CNS to the cerebral cortex, where information is interpreted.

What is the postsynaptic cell?

The target cell receiving the signal is called the postsynaptic cell.

What does the tightly wrapped membranes of Schwann cells constitute?

The tightly wrapped membranes of the Schwann cells constitute the myelin sheath and give myelinated axons a white appearance because of the high lipid concentration.

1. Maintaining homeostasis

The trillions of cells in the human body do not function independently of each other but must work together to maintain homeostasis. For example, heart cells must contract at a rate that ensures adequate delivery of blood to all tissues of the body. The nervous system can stimulate or inhibit these activities to help maintain homeostasis.

What are the two cell types that make up the nervous system?

The two cell types that make up the nervous system, neurons and glial cells, work together to monitor the body's environment and make changes when needed. There are an estimated 100 billion neurons in our body, yet glial cells account for over half of the brain's weight, and there can be 10 to 50 times more glial cells than neurons in various parts of the brain. In order to understand how the nervous system functions, a knowledge of neuron structure and glial cell types is important.

What is the most common way neurons become depolarized?

There are several ways neurons become depolarized, but Na+ entry is the most common. Because there are few Na+ leak channels, entry of Na+ into the cells is typically a regulated process. If either ligand-gated Na+ or voltage-gated Na+ channels open, Na+ diffuses into the cell down its concentration gradient. As Na+ diffuses into the cell, the inside of the membrane becomes more positive, or is depolarized. This is the principle way most neurons respond to excitatory stimuli.

What are the two major ways to hyperpolarize neurons?

There are two major ways to hyperpolarize neurons: (1) K+ exit and (2) Cl- entry.

What are the two types of changes to the resting membrane potential?

There are two types of changes to the resting membrane potential: (1) depolarization and (2) hyper polarization.

Describe the concentration differences for Na+ and K+ that exist across the plasma membrane.

There is a higher concentration of Na+ and Cl- outside the cell than inside the cell, while there is higher concentration of K+ on the inside of the cell. Recall from chapter 3 that this distribution of ions is called a concentration gradient. For Na+, there is a steep concentration gradient from the outside of the cell to the inside of the cell. For K+

What do the branches of the nervous system do?

These branches of the nervous system are highly integrated and work very efficiently to tightly regulate the internal environment of our body.

Purines

These chemicals are nitrogen-containing compounds that are derived from nucleic acids. The most well understood of these are adenosine and ATP.

What are nodes of Ranvier?

These gaps in the myelin sheath are the nodes of Ranvier.

What areas does the organization of nervous tissue give?

These groupings give nervous tissue distinctive areas, called gray matter and white matter.

Biogenic Amines

These neurotransmitters fall into two categories: (1) catecholamines and (2) indoleamines. The catecholamine neurotransmitters are derived from the amino acid tyrosine and include dopamine, norepinephrine, and epinephrine. The indoleamine neurotransmitters are derived from eh amino acids histidine and tryptophan and include histamine and serotonin, respectively.

What is the electrical charge difference across the plasma membrane called?

This electrical charge difference across the plasma membrane is called a potential difference.

What is hypokalemia?

This lower-than-normal concentration of blood levels of K+ is called hypokalemia.

What is threshold?

Threshold is the membrane potential at which an action potential is generated. An action potential is generated when voltage-gated Na+ channels open.

What do axoaxonic synapses allow?

Through axoaxonic synapses, one neuron can release a neuromodulator that influences the release of a neurotransmitter for the presynaptic terminal of another neuron.

What can action potentials do?

Thus, we see that action potentials can propagate, or spread, across the plasma membrane. An action potential produce at one point on the plasma membrane stimulates the production of an action potential at the adjacent point of the same plasma membrane.

What is tight regulation of voltage-gated Na+ channels important for?

Tight regulation of voltage-gated Na+ channels is important for the synchronization of membrane permeability of Na+. It seems that closed voltage-gated Na+ channels are stabilized by Ca2+ and thus are sensitive to changes in the extracellular concentration of Ca2+. Positively charged Ca2+ in the extracellular fluid is attracted to the negatively charged groups of proteins within the voltage-gated Na+ channels. If the extracellular concentration of Ca2+ decreases, these ions diffuse away form the voltage-gated Na+ channels, causing the channels to open. If the extracellular concentration of Ca2+ increases, it binds to voltage-gated Na+ channels, causing them to close. Therefore, normal levels of Ca2+ in the extracellular fluid are crucial to keeping the voltage-gated Na+ channels closed until he neuron generates an action potential.

Given that tissue A has significantly more K+ leak ion channels than tissue B, which tissue has the larger resting membrane potential.

Tissue B would have a larger membrane potential because it has a more difficult time regulating the charge difference due to the lack of K+ leak ion channels.

Distinguish between spatial summation and temporal summation. In what part of the neuron does summation take place?

Two types of summation are possible: spatial summation and temporal summation. Spatial summation occurs when multiple action potentials from separate neurons arrive simultaneously at the same postsynaptic neuron. In the postsynaptic neuron, each actin potential causes a depolarizing graded potential that undergoes summation at the trigger zone. if the summated depolarization reaches threshold, an action potential is produced. Temporal summation results when two or more action potentials arrive very close together at the postsynaptic cell from the presynaptic terminal of a particular neuron. The first action potential causes a depolarizing graded potential in the postsynaptic membrane that remains for a few milliseconds before it disappears, although its magnitude decreases through time. Temporal summation in the postysnaptic cell results when the second action potential from the presynaptic neuron initiates a second graded depolarization before the postsynaptic cell's membrane potential returns to its resting value.

What are type A fibers?

Compare the function of type A nerve fibers with that of types B and C nerve fibers.

Type A fibers are large-diameter, myelinated axons that conduct action potentials at 15-120 m/s (34-268 mi/h). Motor neurons supplying skeletal muscle and most sensory neurons have type A fibers. Rapid response to the external environment is possible because of the rapid input of sensory information to the CNS and the rapid output of action potentials to skeletal muscles. Type B fibers are medium-diameter, lightly myelinated axons that conduct action potentials at 3- 15 m/s (7-34 mi/h), and type C fibers are small-diameter, unmyelinated axons that conduct action potentials at 2 m/s or less (4.5 mi/h). Types B the stomach, intestines, and heart. The responses necessary to maintain internal homeostasis, such as digestion, need not be as rapid as responses to the external environment.

What are type B and type C fibers?

Type B fibers are medium-diameter, lightly myelinated axons that conduct action potentials at 3-15 m/s (7-34 mi/h), and Type C fibers are small-diameter, unmyelinated axons that conduct action potentials at 2 m/s or less (4.5 mi/h). Types B and C fibers are primarily part of the ANS, which stimulates internal organs, such as the stomach, intestines, and heart. The responses necessary to maintain internal homeostasis, such as digestion, need not be as rapid as responses to external environment.

Describe unmyelinated axons

Unmyelinated axons are not devoid of myelin, as their name suggests. Instead, the axons rest in invaginations of the Schwann cells or oligodendrocytes. The glial cell's plasma membrane surrounds each axon but does not wrap around it many times. Thus, each axon is surrounded why a series of Schwann cells, and each Schwann cell can simultaneously surround more than one unmyelinated axon.

What do Unmyelinated axons use?

Unmyelinated axons use continuous conduction in which each section of membrane along the length of the axon generates an action potential.

What is a local current? How do local currents cause the propagation of action potentials in unmyelinated axons?

Unmyelinated axons use continuous conduction in which each section of membrane along the length of the axon generates an action potential. When an action potential is produced, the inside of the membrane becomes more positive than the outside. On the outside of the membrane, Na+ from the adjacent area is attracted to the negative charges at the site of the action potential. Once inside the cell, Na+ diffuses away from its entry point. This diffusion of Na+ is called a local current. As a result of the local current, the part of the membrane immediately adjacent to the action potential depolarizes.

What is the presynaptic terminal?

Usually, axons branch many times and contact multiple effector cells. The axon ending at the synapse is called the presynaptic terminal.

What are synaptic vesicles?

Usually, axons branch many times and contact multiple effector cells. The axon ending at the synapse is called the presynaptic terminal. There, the axon endings have many synaptic vesicles, which store the signal molecules produced by the neuron.

What are neurontransmitters?

2. Voltage-gated ion channels.

Voltage-gated ion channels open and close in response to a specific, small voltage change across the plasma membrane. In an unstimulated cell, the inside of the cell is negatively charged relative to the outside. This charge difference can be measured in units called millivolts (mV; 1 mV = 1/1000 V). For reference, a "double-A" battery generates 1.5 V of electricity. When a cell is stimulated, the permeability of the plasma membrane changes because gated ion channels open or close. The movement of ions into or out of the cell changes the charge difference across the plasma membrane, which, in turn, can cause voltage gated ion channels to open or close. Voltage-gated channels specific for Na+ and K+ are most numerous in electrically excitable tissues, but voltage-gated Ca2+ channels are also important, especially in smooth muscle and cardiac muscle fibers.

What do we also say about depolarization?

We also say that depolarization is movement of the membrane potential closer to zero. Because depolarization moves the membrane potential closer to the point of action potential generation, it is always excitatory to the cell. In other words, depolarization makes a neuron more likely to generate an action potential.

What creates graded potentials in the neuron?

We have now established that the resting membrane potential can be altered in such a way that the cell is either more likely or less likely to generate an action potential. These changes in the resting membrane potential are initiated by the opening of certain ion channels. In fact, the opening of particular ion channels is a highly regulated process that is either excitatory or inhibitory to a neuron. Regulated opening o ion channels creates graded potentials in the neuron, usually in the dendrites or the cell body.

Why does an action potential cause local currents in the adjacent part of the cell membrane?

When action potential propagate they cause the charge inside the cell to go from negative to positive and this causes a local current.

What happens when action potential is produced?

When an action potential is produced, the inside of the membrane becomes more positive than the outside.

What is an excitatory postsynaptic potential?

When depolarization of the post synaptic cell occurs, the response is stimulatory, and the resulting graded potential is called an excitatory postsynaptic potential.

What happens when depolarizing graded potentials reach threshold?

When depolarizing graded potentials reach threshold, a large number of voltage-gated Na+ channels open rapidly. Sodium ions then diffuse into the cell, and the resulting depolarization causes additional voltage-gated Na+ channels to open. As a consequence, more Na+ diffuses into the cell, causing a greater depolarization of the membrane, which in turn causes still more voltage-gated Na+ channels to open. This is an example of a positive-feedback cycle, and it continues until most of the voltage-gated Na+ channels in the plasma membrane are open.

What happens when norepinephrine is secreted into the blood?

When norepinephrine is secreted into the blood form the adrenal medulla, it functions as a hormone. In the circulation norepinephrine is taken up primarily by liver and kidney cells, where the enzymes monoamine oxidase and catechol-O-methyltransferase convert it into inactive metabolites.

What are inhibitory postsynaptic potentials?

When the combination of a neurotransmitter with its receptor results in hyperpolarization of the postsynaptic membrane, the response is inhibitory because no action potentials are generated. This local hyperpolarization is called an inhibitory postsynaptic potential.

What happens to voltage-gated K+ channels when the graded potentials reaches threshold?

When the graded potential reaches threshold, the voltage-gated K+ channels start to open at the same time as the voltage-gated Na+ channels, but they open significantly more slowly than the voltage-gated Na+ channels. At this stage, because the voltage-gated K+ channels open so slowly, only a small number of them are open, compared with the number of voltage-gated Na+ channels. Depolarization occurs because much more Na+ diffuses into the cell than K+ diffuses out of it.

What does the neurotransmitter concentration reflect in the synaptic cleft?

When the neurotransmitter concentration in the synaptic cleft is high, many of the receptor molecules have neurotransmitter molecules bound to them; when the neurotransmitter concentration declines, the neurotransmitter molecules diffuse away from the receptor molecules.

What happens when the neurotransmitter norepinephrine is released?

When the neurotransmitter norepinephrine is released into the synaptic cleft, most of it is transported back into the presynaptic terminal, where it is repackaged into synaptic vesicles for later use.

What happens when the activation gates in the voltage-gated Na+ channels open and the inactivation gates close?

When the plasma membrane is at rest, the activation gates of the voltage-gated Na+ channel are closed, and the inactivation gates are open. Because the activation gates are close, Na+ cannot diffuse through the channels. When the graded potential reaches threshold, the change in the membrane potential causes many of the activation gates to open, and Na+ can diffuse through the Na+ channels into the cell. When the plasma membrane is at rest, voltage-gated

What happens when the plasma membrane is at rest?

When the plasma membrane is at rest, the activation gates of the voltage-gated Na+ channel are closed, and the inactivation gates are open. Because the activation gates are closed, Na+ cannot diffuse through the channels. When the graded potential reaches threshold, the change in the membrane potential causes many of the activation gates to open, and Na+ can diffuse through the Na+ channels into the cell.

What happens to voltage-gated K+ channels when the plasma membrane is at rest?

When the plasma membrane is at rest, voltage-gated K+ channels, which have on gate, are closed.

FIGURE 11.23 Summation

Whether a neuron fires an action potential depends upon the input from other neurons. If several EPSPs depolarize the neuron to threshold, it will generate an action potential. IF IPSPs prevent the cell from reaching threshold, it will not generate an action potential.

What do white matter of the CNS form?

White matter of the CNS forms nerve tracts, which propagate action potentials from one area of the CNS to another. In contrast, in the PNS, bundles of axons and their connective tissue sheaths are simply called nerves.

Explain how the sodium-potassium pump works to move ions.

With the breakdown of ATP, the sodium potassium pump can exchange 3 sodium ions for 2 potassium ions. This is a mechanism essential to maintaining the membrane potential.

Chapter 11 Anatomy and Physiology

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