CH 12

Generation of graded potentials in response to the opening of mechanically-gated channels or ligand-gated channels

(a) A mechanical stimulus (pressure) opens a mechanically-gated channel that allows passage of cations (mainly Na+ and Ca+) into the cell, causing a depolarizing graded potential. (b) The neurotransmitter acetylcholine (a ligand stimulus) opens a cation channel that allows passage of Na+, K+, and Ca+; Na+ inflow is greater than either Ca+ inflow or K+ outflow, causing a depolarizing graded potential. (c) The neurotransmitter glycine (a ligand stimulus) opens a Cl− channel that allows passage of Cl− ions into the cell, causing a hyperpolarizing graded potential.

Classification of Nerve Fibers Axons can be classified into three major groups based on the amount of myelination, their diameters, and their propagation speeds:

- A fibers - B fibers - C fibers

Explain why an initial threshold voltage necessitates a complete subsequent sequence of events that produces the set voltage changes of an action potential.

- A subthreshold stimulus does not cause an action potential. An action potential does occur in response to a threshold stimulus, which is just strong enough to depolarize the membrane to threshold. Several action potentials form in response to a suprathreshold stimulus. Each of the action potentials caused by the suprathreshold stimulus has the same amplitude (size) as the action potential caused by the threshold stimulus. For simplicity, the after-hyperpolarizing phase of the action potential is not shown. - An action potential will occur only once the membrane potential reaches threshold.

Factors That Affect the Speed of Propagation

- Amount of myelination- As you have just learned, action potentials propagate more rapidly along myelinated axons than along unmyelinated axons. - Axon diameter- Larger diameter axons propagate action potentials faster than smaller ones due to their larger surface areas. - Temperature- Axons propagate action potentials at lower speeds when cooled.

Identify the structures that make up the nervous system.

- Central Nervous System: consist of the brain and spinal cord. - Peripheral Nervous System: consists of tissue outside the CNS, which include nerves and sensory receptors; can be divided into three different division- Somatic, Automatic, and Enteric nervous system.

Neurotransmitter removal

- Diffusion- released neurotransmitter molecules diffuse away from the synaptic cleft. Once a neurotransmitter molecule is out of reach of its receptors, it can no longer exert an effect. - Enzymatic degradation Certain neurotransmitters are inactivated through enzymatic degradation. For example, the enzyme acetylcholinesterase breaks down acetylcholine in the synaptic cleft. - Uptake by cells (actively transported back into neuron that released them) - If a neurotransmitter could linger in the synaptic cleft, it would influence the postsynaptic neuron, muscle fiber, or gland cell indefinitely - removal of the neurotransmitter is essential for normal function.

A neurotransmitter causes either an excitatory or an inhibitory graded potential:

- Excitatory postsynaptic potential (EPSP) causes a depolarization of the postsynaptic cell, bringing it closer to threshold. Although a single EPSP normally does not initiate a nerve impulse, the postsynaptic cell does become more excitable. - Inhibitory postsynaptic potential (IPSP) hyperpolarizes the postsynaptic cell taking it farther from threshold.

Neurotransmitters bind to receptors on the postsynaptic neuron. Receptors are specific for particular neurotransmitters:

- Ionotropic Receptors contains a neurotransmitter binding site and an ion channel - type of ligand gated channel - Metabotropic Receptors - has a neurotransmitter binding site and is coupled to a separate ion channel by the G protein - binding of the neurotransmitter either causes the G protein to open or close the ion channel or activates a second messenger in the cytosol to cause the opening or closing of the ion channel

Active channels open in response to a stimulus (they are "gated"). There are 3 types of active, gated channels:

- Ligand-gated - Voltage-gated - Mechanically-gated

List and explain the three basic functions of the nervous system.

- Sensory function: detect internal stimuli, such as an increase in blood pressure, or external stimuli (ex, raindrop on your arm). This sensory information is then carried into the brain and spinal cord though cranial and spinal nerves. - Integrative function: processes sensory information by analyzing and making decisions for appropriate responses. - Motor function: Once sensory information is integrated, the nervous system may elicit an appropriate motor response by activating effectors (muscles and glands) through cranial and spinal nerves. Stimulation causes muscles to contract and glands to secrete.

Distinguish between spatial and temporal summation.

- Spatial summation occurs when postsynaptic potentials arrive at different locations at the same time. - Temporal summation occurs when postsynaptic potentials arrive at the same location at different times (very rapid). - Whether or not the postsynaptic cell reaches threshold depends on the net effect after Summation of all the postsynaptic potentials.

An Action Potential has two main phases:

- depolarizing phase - repolarizing phase

Describe the properties of an electrical synapse, the way impulses are transmitted

- electrical synapse, action potentials (impulses) conduct directly between the plasma membranes of adjacent neurons through structures called gap junctions. Each gap junction contains a hundred or so tubular connexons, which act like tunnels to connect the cytosol of the two cells directly. As ions flow from one cell to the next through the connexons, the action potential spreads from cell to cell.

Discuss the features of the graded potential including areas where generated, size, properties, and type.

- graded potential is a small deviation from the resting membrane potential that makes the membrane either more polarized (inside more negative) or less polarized (inside less negative) - occurs when a stimulus causes mechanically-gated or ligand-gated channels to open or close in an excitable cell's plasma membrane. Typically, mechanically-gated channels and ligand-gated channels can be present in the dendrites of sensory neurons, and ligand-gated channels are numerous in the dendrites and cell bodies of interneurons and motor neurons. Hence, graded potentials occur mainly in the dendrites and cell body of a neuron. - This mode of travel by which graded potentials die out as they spread along the membrane is known as decremental conduction. Because they die out within a few millimeters of their point of origin, graded potentials are useful for short-distance communication only.

Describe the effect the sum of the excitatory and inhibitory effects have on the postsynaptic neuron.

- neurotransmitter causes either an excitatory or an inhibitory graded potential. A neurotransmitter that causes depolarization of the postsynaptic membrane is excitatory because it brings the membrane closer to threshold. - excitatory postsynaptic potential (EPSP)- A depolarizing postsynaptic potential. Although a single EPSP normally does not initiate a nerve impulse, the postsynaptic cell does become more excitable. Because it is partially depolarized, it is more likely to reach threshold when the next EPSP occurs. - inhibitory postsynaptic potential (IPSP)- A hyperpolarizing postsynaptic potential. During hyperpolarization, generation of an action potential is more difficult than usual because the membrane potential becomes inside more negative and thus even farther from threshold than in its resting state.

Give examples of small molecule neurotransmitters and provide descriptions of how and where they work.

-Acetylcholine - one of the best studied neurotransmitters in many PNS and some CNS neurons and depending on location can be excitatory or inhibitory - Amino Acids - including glutamate and aspartate (excitatory) and GABA and glycine (inhibitory) - Biogenic Amines - norepinephrine, epinephrine, dopamine and serotonin - excitatory or inhibitory depending on the receptor - ATP and Other Purines - adenosine and derivatives Nitric Oxide - excitatory with widespread effects Carbon Monoxide - excitatory in the brain

Describe the pathway that the different potentials take from sensory stimulation to motor neuron to stimulated muscle fiber

1. As you touch the pen, a graded potential develops in a sensory receptor in the skin of the fingers. 2. The graded potential triggers the axon of the sensory neuron to form a nerve action potential, which travels along the axon into the CNS and ultimately causes the release of neurotransmitter at a synapse with an interneuron. 3. The neurotransmitter stimulates the interneuron to form a graded potential in its dendrites and cell body. 4. In response to the graded potential, the axon of the interneuron forms a nerve action potential. The nerve action potential travels along the axon, which results in neurotransmitter release at the next synapse with another interneuron. 5. This process of neurotransmitter release at a synapse followed by the formation of a graded potential and then a nerve action potential occurs over and over as interneurons in higher parts of the brain (such as the thalamus and cerebral cortex) are activated. Once interneurons in the cerebral cortex, the outer part of the brain, are activated, perception occurs and you are able to feel the smooth surface of the pen touch your fingers. 6. A stimulus in the brain causes a graded potential to form in the dendrites and cell body of an upper motor neuron, a type of motor neuron that synapses with a lower motor neuron farther down in the CNS in order to contract a skeletal muscle. The graded potential subsequently causes a nerve action potential to occur in the axon of the upper motor neuron, followed by neurotransmitter release. 7. The neurotransmitter generates a graded potential in a lower motor neuron, a type of motor neuron that directly supplies skeletal muscle fibers. The graded potential triggers the formation of a nerve action potential and then release of the neurotransmitter at neuromuscular junctions formed with skeletal muscle fibers that control movements of the fingers. 8. The neurotransmitter stimulates the muscle fibers that control finger movements to form muscle action potentials. The muscle action potentials cause these muscle fibers to contract, which allows you to write with the pen.

advantages of an electrical synapse

1.Faster communication- action potentials conduct directly through gap junctions, electrical synapses are faster than chemical synapses. At an electrical synapse, the action potential passes directly from the presynaptic cell to the postsynaptic cell. The events that occur at a chemical synapse take some time and delay communication slightly. 2. Synchronization- Electrical synapses can synchronize (coordinate) the activity of a group of neurons or muscle fibers. In other words, a large number of neurons or muscle fibers can produce action potentials in unison if they are connected by gap junctions. The value of synchronized action potentials in the heart or in visceral smooth muscle is coordinated contraction of these fibers to produce a heartbeat or move food through the gastrointestinal tract.

List the neuropeptides, give examples of their mechanism of action and the results of their action.

3-40 amino acids long Numerous and widespread in the CNS and PNS Enkephalins - analgesic effect Endorphins and dynorphins - "opioid peptides" that act as natural pain killers -also related to memory and learning, temperature control, hormone regulation and mental illness Substance P - released by neurons transmitting input from peripheral pain receptors to CNS

Compare the absolute and relative refractory periods and the relation of axon diameter to action potential generation frequency.

Absolute refractory period: even a very strong stimulus cannot initiate a second action potential. This period coincides with the period of Na+ channel activation and inactivation. Inactivated Na+ channels cannot reopen; they first must return to the resting state. Large-diameter axons have a larger surface area and have a brief absolute refractory period of about 0.4 msec. Because a second nerve impulse can arise very quickly, up to 1000 impulses per second are possible. Small-diameter axons have absolute refractory periods as long as 4 msec, enabling them to transmit a maximum of 250 impulses per second. relative refractory: the period of time during which a second action potential can be initiated, but only by a larger-than-normal stimulus. It coincides with the period when the voltage-gated K+ channels are still open after inactivated Na+ channels have returned to their resting state

List the sequence of events involved in generation of a nerve impulse. (action potential)

Action potential (AP) or impulse.. The action potential arises at the trigger zone (here, at the junction of the axon hillock and the initial segment) and then propagates along the axon to the axon terminals. The green-colored regions of the neuron indicate parts that typically have voltage-gated Na+ and K+ channels (axon plasma membrane and axon terminals).

Describe the different forms, locations, and purposes of the six types of neuroglia.

Astrocytes- star-shaped cells have many processes and are the largest and most numerous of the neuroglia.The processes of astrocytes make contact with blood capillaries, neurons, and the pia mater (a thin membrane around the brain and spinal cord). Oligodendrocytes- responsible for forming and maintaining the myelin sheath around CNS axons. Microglial cells or Microglia- small cells with slender processes that give off numerous spinelike projections. Microglial cells or microglia function as phagocytes. Like tissue macrophages, they remove cellular debris formed during normal development of the nervous system and phagocytize microbes and damaged nervous tissue. Ependymal cells-cuboidal to columnar cells arranged in a single layer that possess microvilli and cilia. These cells line the ventricles of the brain and central canal of the spinal cord (spaces filled with cerebrospinal fluid, which protects and nourishes the brain and spinal cord). Functionally, ependymal cells produce, possibly monitor, and assist in the circulation of cerebrospinal fluid. They also form the blood-cerebrospinal fluid barrier Schwann Cells- cells encircle PNS axons. Like oligodendrocytes, they form the myelin sheath around axons. A single oligodendrocyte myelinates several axons, but each Schwann cell (SCHVON or SCHWON) myelinates a single axon

Compare and contrast continuous and saltatory conduction. (propagation of AP)

By passive spread, the current proceeds by (a) continuous conduction in unmyelinated axons, or by the much faster process of (b) saltatory conduction in myelinated axons (as the AP jumps from one node to the next as shown in this graphic).

Distinguish between action potential and graded potentials.

Graded potentials (described shortly) are used for short-distance communication only. Action potentials (also described shortly) allow communication over long distances within the body.

Electrical Signals in Neurons- electrically excitable. They communicate with one another using two types of electrical signals:

Graded potentials are used for short-distance communication only. Action potentials allow communication over long distances within the body.

parallel after-discharge circuit

In this circuit, a single presynaptic cell stimulates a group of neurons, each of which synapses with a common postsynaptic cell. A differing number of synapses between the first and last neurons imposes varying synaptic delays, so that the last neuron exhibits multiple EPSPs or IPSPs. If the input is excitatory, the postsynaptic neuron then can send out a stream of impulses in quick succession. Parallel after-discharge circuits may be involved in precise activities such as mathematical calculations.

Describe the ions, channels, integral-protein pumps, and electrochemical gradient, that contribute to generation of a resting membrane potential.

Ion Channels- When ion channels are open, they allow specific ions to move across the plasma membrane, down their electrochemical gradient—a concentration (chemical) difference plus an electrical difference. Recall that ions move from areas of higher concentration to areas of lower concentration (the chemical part of the gradient). Also, positively charged cations move toward a negatively charged area, and negatively charged anions move toward a positively charged area (the electrical aspect of the gradient). As ions move, they create a flow of electrical current that can change the membrane potential.

What are axodendric, Axosomatic and axiomatic

Most synapses between neurons are axodendritic (from axon to dendrite), while others are axosomatic (from axon to cell body) or axoaxonic (from axon to axon). In addition, synapses may be electrical or chemical, and they differ both structurally and functionally.

Identify neurons on the basis of their structural and functional classifications.

Multipolar neurons- usually have several dendrites and one axon. Most neurons in the brain and spinal cord are of this type, as well as all motor neurons Bipolar neurons- have one main dendrite and one axon. They are found in the retina of the eye, the inner ear, and the olfactory area of the brain. Unipolar neurons- have dendrites and one axon that are fused together to form a continuous process that emerges from the cell body. These neurons are more appropriately called pseudounipolar neurons because they begin in the embryo as bipolar neurons. During development, the dendrites and axon fuse together and become a single process. The dendrites of most unipolar neurons function as sensory receptors that detect a sensory stimulus such as touch, pressure, pain, or thermal stimuli. The trigger zone for nerve impulses in a unipolar neuron is at the junction of the dendrites and axon. The impulses then propagate toward the synaptic end bulbs. The cell bodies of most unipolar neurons are located in the ganglia of spinal and cranial nerves.

Describe the repolarization of the nerve cell membrane.

Na+ channel inactivation gates close and K+ channels open. The membrane starts to become depolarized as some K+ ions leave the neuron and a few negative charges begin to build up along the inside surface of the membrane.

membrane potential

The voltage across a cell's plasma membrane.

Microglial cells or Microglia

These neuroglia are small cells with slender processes that give off numerous spinelike projections. Microglial cells or microglia function as phagocytes. Like tissue macrophages, they remove cellular debris formed during normal development of the nervous system and phagocytize microbes and damaged nervous tissue.

Define the anatomic, chemical, enzymatic, and receptor components of a chemical synapse.

They are separated by the synaptic cleft, a space of 20-50 nm* that is filled with interstitial fluid. Nerve impulses cannot conduct across the synaptic cleft, so an alternative, indirect form of communication occurs. In response to a nerve impulse, the presynaptic neuron releases a neurotransmitter that diffuses through the fluid in the synaptic cleft and binds to receptors in the plasma membrane of the postsynaptic neuron. The postsynaptic neuron receives the chemical signal and in turn produces a postsynaptic potential, a type of graded potential. Thus, the presynaptic neuron converts an electrical signal (nerve impulse) into a chemical signal (released neurotransmitter). The postsynaptic neuron receives the chemical signal and in turn generates an electrical signal (postsynaptic potential). The time required for these processes at a chemical synapse, a synaptic delay of about 0.5 msec, is the reason that chemical synapses relay signals more slowly than electrical synapses.

What is the axon hillock?

This is a part of the neuron, between the cell body and the axon, that controls traffic down the axon through summation of excitatory and inhibitory postsynaptic potentials.

propagation of the action potential

action potential keeps its strength as it spreads along the membrane. This mode of conduction is called propagation, and it depends on positive feedback. As you have already learned, when sodium ions flow in, they cause voltage-gated Na+ channels in adjacent segments of the membrane to open. Thus, the action potential travels along the membrane rather like the activity of that long row of dominoes. In actuality, it is not the same action potential that propagates along the entire axon. Instead, the action potential regenerates over and over at adjacent regions of membrane from the trigger zone to the axon terminals. In a neuron, an action potential can propagate in this direction only—it cannot propagate back toward the cell body because any region of membrane that has just undergone an action potential is temporarily in the absolute refractory period and cannot generate another action potential. Because they can travel along a membrane without dying out, action potentials function in communication over long distances.

Action potentials

allow communication over long distances within the body

B fibers

are axons with diameters of 2-3 μm. Like A fibers, B fibers are myelinated and exhibit saltatory conduction at speeds up to 15 m/sec (34 mi/hr). B fibers have a somewhat longer absolute refractory period than A fibers. B fibers conduct sensory nerve impulses from the viscera to the brain and spinal cord. They also constitute all of the axons of the autonomic motor neurons that extend from the brain and spinal cord to the ANS relay stations called autonomic ganglia.

ependymal cells

are cuboidal to columnar cells arranged in a single layer that possess microvilli and cilia. These cells line the ventricles of the brain and central canal of the spinal cord (spaces filled with cerebrospinal fluid, which protects and nourishes the brain and spinal cord). Functionally, ependymal cells produce, possibly monitor, and assist in the circulation of cerebrospinal fluid. They also form the blood-cerebrospinal fluid barrier

A fibers

are the largest diameter axons (5-20 μm) and are myelinated. A fibers have a brief absolute refractory period and conduct nerve impulses (action potentials) at speeds of 12 to 130 m/sec (27-290 mi/hr). The axons of sensory neurons that propagate impulses associated with touch, pressure, position of joints, and some thermal and pain sensations are A fibers, as are the axons of motor neurons that conduct impulses to skeletal muscles.

C fibers

are the smallest diameter axons (0.5-1.5 μm) and all are unmyelinated. Nerve impulse propagation along a C fiber ranges from 0.5 to 2 m/sec (1-4 mi/hr). C fibers exhibit the longest absolute refractory periods. These unmyelinated axons conduct some sensory impulses for pain, touch, pressure, heat, and cold from the skin, and pain impulses from the viscera. Autonomic motor fibers that extend from autonomic ganglia to stimulate the heart, smooth muscle, and glands are C fibers. Examples of motor functions of B and C fibers are constricting and dilating the pupils, increasing and decreasing the heart rate, and contracting and relaxing the urinary bladder.

Graded potentials

are used for short-distance communication only.

Describe the various types of neuronal circuits in the nervous system.

billions of neurons organized into complicated networks called neural circuits, functional groups of neurons that process specific types of information. In a simple series circuit, a presynaptic neuron stimulates a single postsynaptic neuron. The second neuron then stimulates another, and so on. However, most neural circuits are more complex. Types of circuits include diverging, converging, reverberating, and parallel after-discharge.

Describe the internal and external structures of the neuron

cell body dendrites axon and axon terminals Neurofibrils Axoplasm Axolemma Axon terminal Synaptic end bulbs Myelin Nodes of Ranvier

Describe the internal and external functions of the neuron

cell body- contains a nucleus surrounded by cytoplasm that includes typical cellular organelles such as lysosomes, mitochondria, and a Golgi complex. Neuronal cell bodies also contain free ribosomes and prominent clusters of rough endoplasmic reticulum, termed Nissl bodies. The ribosomes are the sites of protein synthesis. Newly synthesized proteins produced by Nissl bodies are used to replace cellular components, as material for growth of neurons, and to regenerate damaged axons in the PNS. The cytoskeleton includes both neurofibrils composed of bundles of intermediate filaments that provide the cell shape and support, and microtubules which assist in moving materials between the cell body and axon. Aging neurons also contain lipofuscin a pigment that occurs as clumps of yellowish brown granules in the cytoplasm. Lipofuscin is a product of neuronal lysosomes that accumulates as the neuron ages, but does not seem to harm the neuron. A collection of neuron cell bodies outside the CNS is called a ganglion Dendrites- are the receiving or input portions of a neuron. The plasma membranes of dendrites (and cell bodies) contain numerous receptor sites for binding chemical messengers from other cells. Dendrites usually are short, tapering, and highly branched. In many neurons the dendrites form a tree-shaped array of processes extending from the cell body. Their cytoplasm contains Nissl bodies, mitochondria, and other organelles. axon- neuron propagates nerve impulses toward another neuron, a muscle fiber, or a gland cell. An axon is a long, thin, cylindrical projection that often joins to the cell body at a cone-shaped elevation called the axon hillock. The part of the axon closest to the axon hillock is the initial segment. In most neurons, nerve impulses arise at the junction of the axon hillock and the initial segment, an area called the trigger zone, from which they travel along the axon to their destination. An axon contains mitochondria, microtubules, and neurofibrils. Because rough endoplasmic reticulum is not present, protein synthesis does not occur in the axon. The cytoplasm of an axon, called axoplasm, is surrounded by a plasma membrane known as the axolemma. Along the length of an axon, side branches called axon collaterals may branch off, typically at a right angle to the axon. The axon and its collaterals end by dividing into many fine processes called axon terminals

Subdivisions of the Nervous System

central nervous system and peripheral nervous system

Ligand-gated

channels respond to a neurotransmitter or hormone and are mainly concentrated at the synapse.

Voltage-gated

channels respond to changes in the transmembrane electrical potential and are mainly located along the neuronal axon.

Mechanically-gated

channels respond to mechanical deformation (applying pressure to a receptor due to vibration, touch, or stretching).

postsynaptic cell

is the cell that receives a signal.

Describe the process of summation.

is the process by which graded potentials add together. If two depolarizing graded potentials summate, the net result is a larger depolarizing graded potential. If two hyperpolarizing graded potentials summate, the net result is a larger hyperpolarizing graded potential. If two equal but opposite graded potentials summate (one depolarizing and the other hyperpolarizing), then they cancel each other out and the overall graded potential disappears.

Describe the functions and relative number of neuroglia compared to neurons.

make up about half the volume of the CNS. Their name derives from the idea of early histologists that they were the "glue" that held nervous tissue together. We now know that neuroglia are not merely passive bystanders but rather actively participate in the activities of nervous tissue. Generally, neuroglia are smaller than neurons, and they are 5 to 25 times more numerous.

After Hyperpolarization of Action Potential

makes the cell more negative than its typical resting membrane potential. As the action potential passes through, potassium channels stay open a little bit longer, and continue to let positive ions exit the neuron. This means that the cell temporarily hyperpolarizes, or gets even more negative than its resting state. As the potassium channels close, the sodium-potassium pump works to reestablish the resting state.

Contrast the general functions of neuroglia and neurons.

neurons- possess electrical excitability, the ability to respond to a stimulus and convert it into an action potential. A stimulus is any change in the environment that is strong enough to initiate an action potential. An action potential (nerve impulse)is an electrical signal that propagates (travels) along the surface of the membrane of a neuron. It begins and travels due to the movement of ions (such as sodium and potassium) between interstitial fluid and the inside of a neuron through specific ion channels in its plasma membrane. Once begun, a nerve impulse travels rapidly and at a constant strength. Neuroglia- neuroglia are smaller than neurons, and they are 5 to 25 times more numerous. In contrast to neurons, glia do not generate or propagate action potentials, and they can multiply and divide in the mature nervous system. In cases of injury or disease, neuroglia multiply to fill in the spaces formerly occupied by neurons. Brain tumors derived from glia, called gliomas, tend to be highly malignant and to grow rapidly. Of the six types of neuroglia, four—astrocytes, oligodendrocytes, microglia, and ependymal cells—are found only in the CNS. The remaining two types—Schwann cells and satellite cells—are present in the PNS.

Go through the sequence of events that allow an action potential on an axon to be transmitted into a graded potential on a postsynaptic membrane.

postsynaptic membrane is the membrane that receives a signal (binds neurotransmitter) from the presynaptic cell and responds via depolarisation or hyperpolarisation. The postsynaptic membrane is separated from the presynaptic membrane by the synaptic cleft.

presynaptic neuron

refers to a nerve cell that carries a nerve impulse toward a synapse. It is the cell that sends a signal.

Oligodendrocytes

resemble astrocytes but are smaller and contain fewer processes. Processes of oligodendrocytes are responsible for forming and maintaining the myelin sheath around CNS axons. As you will see shortly, the myelin sheath is a multilayered lipid and protein covering around some axons that insulates them and increases the speed of nerve impulse conduction. Such axons are said to be myelinated

reverberating circuit

the incoming impulse stimulates the first neuron, which stimulates the second, which stimulates the third, and so on. Branches from later neurons synapse with earlier ones. This arrangement sends impulses back through the circuit again and again. The output signal may last from a few seconds to many hours, depending on the number of synapses and the arrangement of neurons in the circuit. Inhibitory neurons may turn off a reverberating circuit after a period of time. Among the body responses thought to be the result of output signals from reverberating circuits are breathing, coordinated muscular activities, waking up, and short-term memory.

converging circuit

the postsynaptic neuron receives nerve impulses from several different sources.

Astrocytes

Provide structural and metabolic support for neurons.

Somatic motor (efferent)

neurons that conduct impulses away from the CNS towards the skeletal muscles under voluntary control in the periphery.

Somatic sensory (afferent)

neurons that convey information from sensory receptors in the head, body wall and limbs towards the CNS.

Describe the organs in the CNS and their general functions

the brain and the spinal cord. The brain is the center of our thoughts, the interpreter of our external environment, and the origin of control over body movement.

sympathetic nervous system

the division of the autonomic nervous system that arouses the body, mobilizing its energy in stressful situations "fight-or-flight" responses

Schwann Cells

Supporting cells of the peripheral nervous system responsible for the formation of myelin.

Classify the organs of the nervous system into central and peripheral divisions and their subdivisions.

The anatomical divisions are the central and peripheral nervous systems. The CNS is the brain and spinal cord. The PNS is everything else and includes afferent and efferent branches with further subdivisions for somatic, visceral and autonomic function.

Compare the nervous system and endocrine system in maintaining homeostasis.

The portion of the brain that maintains the body's internal balance (homeostasis). The hypothalamus is the link between the endocrine and nervous systems. The hypothalamus produces releasing and inhibiting hormones, which stop and start the production of other hormones throughout the body.

Describe the events involved in depolarization of the nerve cell membrane.

Voltage-gated Na+ channels open during the steep depolarization phase allowing Na+ to rush into the cell and making the inside of the cell progressively more positive

depolarizing graded potential

a stimulus that causes the cell to be less negatively charged with respect to the extracellular fluid

hyperpolarizing graded potential

a stimulus that causes the cell to be more negatively charged

somatic nervous system (SNS)

conveys output from the CNS to skeletal muscles only. Because its motor responses can be consciously controlled, the action of this part of the PNS is voluntary.

autonomic nervous system (ANS)

conveys output from the CNS to smooth muscle, cardiac muscle, and glands. Because its motor responses are not normally under conscious control, the action of the ANS is involuntary. The ANS is comprised of two main branches, the sympathetic nervous system and the parasympathetic nervous system

Identify the basic types of gated ion channels and the stimuli that operate them.

Ligand-gated channels respond to a neurotransmitter or hormone and are mainly concentrated at the synapse. Voltage-gated channels respond to changes in the transmembrane electrical potential and are mainly located along the neuronal axon. Mechanically-gated channels respond to mechanical deformation (applying pressure to a receptor due to vibration, touch, or stretching).

resting membrane potential

the electrical charge of a neuron when it is not active

postsynaptic neuron

carries a nerve impulse away from a synapse or an effector cell that responds to the impulse at the synapse.

Discuss the role of neurotransmitters.

is a molecule released from a synaptic vesicle that excites or inhibits another neuron, muscle fiber, or gland cell. Many neurons contain two or even three types of neurotransmitters, each with different effects on the postsynaptic cell.

Signal transmission at the synapse- Explain the events of synaptic transmission.

is a one-way transfer from a presynaptic neuron to a postsynaptic neuron. When an AP reaches the end bulb of axon terminals, voltage-gated Ca2+ channels open and Ca2+ flows inward from the extracellular fluid, triggering release of the neurotransmitter. The neurotransmitter crosses the synaptic cleft and binds to ligand-gated receptors on the postsynaptic membrane.

Characterize the two means of axonal transport and their use.

slow axonal transport. - It conveys axoplasm in one direction only—from the cell body toward the axon terminals. Slow axonal transport supplies new axoplasm to developing or regenerating axons and replenishes axoplasm in growing and mature axons. Fast axonal transport, - uses proteins that function as "motors" to move materials along the surfaces of microtubules of the neuron's cytoskeleton; moves materials in both directions—away from and toward the cell body; occurs in an anterograde (forward) direction moves organelles and synaptic vesicles from the cell body to the axon terminals; can also occurs in a retrograde (backward) direction moves membrane vesicles and other cellular materials from the axon terminals to the cell body to be degraded or recycled. Substances that enter the neuron at the axon terminals are also moved to the cell body by fast retrograde transport. These substances include trophic chemicals such as nerve growth factor and harmful agents such as tetanus toxin and the viruses that cause rabies, herpes simplex, and polio.

parasympathetic nervous system.

the division of the autonomic nervous system that calms the body, conserving its energy. "rest-and-digest" activities

diverging circuit

the nerve impulse from a single presynaptic neuron causes the stimulation of increasing numbers of cells along the circuit