Chapter 7: The Nervous System and Neuronal Excitability

The electrical current flow of ions depends on what two main factors?

1) voltage (the electrical potential difference between opposite charges that are separated from each other) (2) the type of substance through which the charges move.

What are the three ways that a neurotransmitter can be removed?

(1) diffusion (2) enzymatic degradation (3) uptake by cells.

Why is there little or no repair of a CNS axon after an injury?

(1) inhibitory proteins secreted by neuroglia, particularly oligodendrocytes, and (2) absence of growth-stimulating cues that were present during fetal development. Axons in the CNS are myelinated by oligodendrocytes rather than Schwann cells, and this CNS myelin is one of the factors inhibiting regeneration of neurons. Also, after axonal damage, nearby astrocytes proliferate rapidly, forming a type of scar tissue that acts as a physical barrier to regeneration

Substances that enter the neuron at the axon terminals are also moved to the cell body by retrograde transport. These substances include?

(1) trophic chemicals such as nerve growth factor and (2) harmful agents such as tetanus toxin and the viruses that cause rabies, herpes simplex, and polio.

The resting membrane potential is determined by what three factors?

(1) unequal distribution of ions in the ECF and cytosol, (2) differences in membrane permeability to various ions, and (3) action of the Na+/K+ ATPases.

What type of gated channel is activated by a touch on the arm?

A touch on the arm activates mechanically-gated channels.

If the total inhibitory effects are greater than the excitatory effects, the membrane hyperpolarizes (IPSP). The result is inhibition of the postsynaptic neuron and an inability to generate an action potential. A. EPSP B. Action potential C. IPSP

IPSP

Many inhibitory neurotransmitters bind to ionotropic receptors that contain chloride channels. In cells that actively transport Cl− ions into the ECF, _______s result from opening these Cl− channels. A. IPSP B. EPSP

IPSP

__________function as phagocytes. They remove cellular debris formed during normal development of the nervous system and phagocytize microbes and damaged nervous tissue.

Microglia

Which CNS neuroglia function as phagocytes?

Microglia function as phagocytes in the central nervous system.

True or False. A neurotransmitter has either an excitatory or inhibitory effect on the membrane of the postsynaptic cell.

True

True or False. A single postsynaptic neuron receives input from many presynaptic neurons, some of which release excitatory neurotransmitters and some of which release inhibitory neurotransmitters.

True

True or False. A synapse may be electrical or chemical. These two types of synapses differ both structurally and functionally.

True

True or False. Action potentials propagate more rapidly along myelinated axons than along unmyelinated axons

True

True or False. Although a single EPSP normally does not initiate an action potential, the postsynaptic cell does become more excitable. Because it is partially depolarized, it is more likely to reach threshold when the next EPSP occurs.

True

True or False. Although an individual graded potential undergoes decremental conduction, it can become stronger and last longer by summating with other graded potentials.

True

True or False. An action potential consists of a depolarizing phase and a repolarizing phase, which may be followed by an after-hyperpolarizing phase.

True

True or False. Are electrical synapse faster than a chemical synapse?

True

True or False. Axons in the CNS or PNS that have a myelin sheath are said to be myelinated, and those without it are said to be unmyelinated.

True

True or False. Because the plasma membrane is a good electrical insulator, the main paths for current to flow across the membrane are through ion channels

True

True or False. During the repolarizing phase, or falling phase, the membrane potential is restored to the resting state of −70 mV.

True

True or False. Graded potentials and action potentials are deviations in membrane potential(voltage), the difference in electrical charges that exists just across the plasma membrane.

True

True or False. IF: In a hypothetical neuron that is permeable only to K+ ions, an inside-negative membrane potential is generated if there is net movement of K+ ions from the cytosol into the ECF.

True

True or False. IF: In a hypothetical neuron that is permeable only to Na+ ions, an inside-positive membrane potential is generated if there is net movement of Na+ ions from the ECF into the cytosol.

True

True or False. If the net summation of EPSPs and IPSPs is a depolarization that reaches threshold, then an action potential occurs at the trigger zone of a postsynaptic neuron.

True

True or False. In a real neuron, which is more permeable to K+ ions than to Na+ ions, an inside-negative membrane potential is generated because the number of K+ ions that move out of the neuron is greater than the number of Na+ ions that move into the neuron.

True

True or False. In contrast to action potentials, graded potentials do not exhibit a refractory period

True

True or False. In most neurons, the tips of the axon terminals swell into synaptic end bulbs, which are so-named because these bulb-shaped structures can form synapses with other cells.

True

True or False. In neurons at rest, there are differences in membrane permeability to various ions because some ions are able to pass through the plasma membrane via specific leak channels, whereas other ions do not have transport mechanisms allowing them passage through the membrane.

True

True or False. In neurons, the resting membrane potential ranges from −40 to −90 mV. A typical value is −70 mV.

True

True or False. In the CNS, there is little or no repair of an axon after injury.

True

The peripheral nervous system is divided into what two divisions?

afferent and efferent divisions.

Examples of ________-__________ channels are those found in touch receptors and pressure receptors in the skin, in receptors that monitor stretching of internal organs, and in auditory receptors in the ears.

mechanically-gated

Synapses are essential for homeostasis because they allow information to be _________ and _________

filtered; integrated

A graded potential forms in response to the opening of _____________-________ channels or __________-___________ channels.

mechanically-gated; ligand-gated

During the depolarizing phase of the action potential, membrane permeability to Na+ ions ___________. Increases or Decreases. Once threshold is reached, voltage-gated _____ channels open rapidly. Because both the Na+ concentration and electrical gradients favor inward movement of Na+, there is a rush of Na+ ions into the neuron. However, the membrane potential _________ reaches ENa because, during the repolarizing phase, the voltage-gated Na+ channels close and Na+ membrane permeability __________. Increases or Decreases. This causes the membrane potential to peak at +30 mV at the end of the depolarizing phase. The total change in membrane potential from resting conditions to the end of the depolarizing phase is about 100 mV

increases Na+ never; decreases

The amplitude of a graded potential decreases as the distance from the graded potential's point of origin __________. Because they die out within a few millimeters of where they originate, graded potentials are useful for ________-distance communication only

increases; short

Chemical synapse: Action potentials cannot conduct across the synaptic cleft, so an alternate, _________ form of communication occurs. A. Direct B. Indirect

indirect

After it is generated, a graded potential spreads along the membrane in both directions away from the stimulus source. The spread of a graded potential is accomplished by?

local current flow.

The nervous system plays a major role in?

maintaining homeostasis: It keeps most controlled variables within limits that sustain life.

A __________-_________ channel opens or closes in response to mechanical stimulation in the form of touch, pressure, tissue stretching, or vibration (such as sound waves). The force distorts the channel from its resting position, opening the gate.

mechanically-gated

A graded potential occurs when a stimulus causes ________-___________ channels or ______-_______ channels to open or close in an excitable cell's plasma membrane

mechanically-gated; ligand-gated

The voltage that exists across the plasma membrane of a cell is called?

membrane potential (Vm).

A ______________ receptor is a type of neurotransmitter receptor that contains a neurotransmitter binding site and is coupled to a separate ion channel by a G protein.

metabotropic

Typically, plasma membranes have many ______ potassium ion (K+) leak channels than sodium ion (Na+) leak channels. And the K+ leak channels are ______ than the Na+ leak channels. Thus, the membrane's permeability to K+ is much _______ than its permeability to Na+.

more; leakier; higher

In most neurons, action potentials arise at the axon hillock, from which they travel along the axon to their destination. The axon hillock is also known as the? Because of its role in the generation of action potentials.

trigger zone

The absolute refractory period coincides with the period of Na+ channel activation and inactivation. Inactivated Na+ channels cannot reopen; they first must return to the _________ state.

resting

Leak channels are important for establishing the?

resting membrane potential.

When a cell is at rest (unstimulated), the voltage that exists across the plasma membrane is specifically termed the?

resting membrane potential.

Some circuits are constructed so that, once the presynaptic cell is stimulated, it causes the postsynaptic cell to transmit a series of action potentials. One such circuit is called a __________________circuit. This arrangement sends action potentials back through the circuit again and again.

reverberating

Neurons of the __________ nervous system increase heart rate, and neurons of the ___________ nervous system slow it down. In general, the ___________ nervous system takes care of "rest-and-digest" activities. The ___________ nervous system helps support exercise or emergency actions—the so-called "fight-or-flight" responses A. Sympathetic B. Parasympthetic

sympathetic; parasympathetic parasympathetic; sympathetic

A ________ is a site of communication between a neuron and a target cell, which can be another neuron, a muscle fiber, or a gland cell.

synapse

A ______________ is the site of communication between two neurons or between a neuron and an effector cell.

synapse

True or False. At a chemical synapse, a presynaptic neuron converts an electrical signal (action potential) into a chemical signal (neurotransmitter release). The postsynaptic neuron then converts the chemical signal back into an electrical signal (postsynaptic potential).

true

Each gap junction at an electrical synapse, contains __________ __________, which function as tunnels that connect the cytosol of the two cells directly

tubular connexons

The Na+/K+ ATPases are transporters that expel ______ Na+ ions from the cytosol into the ECF and import ______ K+ ions from the ECF into the cytosol using the energy derived from the hydrolysis of ATP

three; two

An action potential does occur in response to a __________ stimulus, a stimulus that is just strong enough to depolarize the membrane to threshold

threshold

An action potential occurs only when the membrane potential reaches __________?

threshold

An action potential occurs in the membrane of the axon when depolarization reaches a certain level termed the __________?

threshold (about −55 mV in many neurons)

_________ is the process by which graded potentials add together

Summation

True or False The amplitude of a graded potential can vary from less than 1 mV to more than 50 mV

True

What are the four types of neural circuits?

1. Diverging circuit 2. Converging circuit 3. Reverberating circuit 4. Parallel after-discharge circuit

What are the to advantages of an electrical synapse?

1. Faster communication: Because 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 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.

Neurons and muscle fibers communicate with one another using two types of electrical signals?

1. Graded potential 2. Action potential

What are the two consequences of the flow of current across the membrane only at the nodes of Ranvier, in myelinated axon?

1. The action potential appears to "leap" from node to node as each nodal area depolarizes to threshold, thus the name saltatory. Because an action potential leaps across long segments of myelinated axon plasma membrane as current flows from one node to the next, it travels much faster than it would in an unmyelinated axon of the same diameter. 2. Opening a smaller number of channels only at the nodes rather than many channels in each adjacent segment of membrane, is a more energy-efficient mode of conduction. Because only small regions of the membrane depolarize and repolarize, minimal inflow of Na+ and outflow of K+ occurs each time an action potential passes by. Thus, less ATP is used by sodium-potassium pumps to maintain the low intracellular concentration of Na+ and the low extracellular concentration of K+.

What two factors affect the velocity (speed) of the action potential conduction?

1. The diameter of the axon: The larger the diameter of the axon, the faster the action potential is conducted; due to less resistance to local current flow, which allows adjacent regions of membrane to be brought to threshold more quickly. 2. The presence or absence of myelin: Conduction of action potentials is more rapid along myelinated axons than along unmyelinated axons

What are examples of voltage-gates channels?

1. Voltage-gated K+ channels, 2. Voltage-gated Na+ channels, and 3. Voltage-gated Ca2+ channels.

What are the different types of neuroglia are present in the CNS?

1. astrocytes, 2. oligodendrocytes, 3. microglia, and 4. ependymal cells

What are the two main categorized types of neurotransmitter receptors?

1. ionotropic receptors and 2. metabotropic receptors.

Neuron function relies on four types of ion channels?

1. leak channels, 2. ligand-gated channels, 3. mechanically-gated channels, and 4. voltage-gated channels.

What are the two types of cholinergic receptors of ACh?

1. nicotinic acetylcholine receptors and 2. muscarinic acetylcholine receptors

What are the three basic parts of a neuron?

A cell body, dendrites, and axon

Which type of graded potential describes a change in membrane potential from −70 to −60 mV? From −70 to −80 mV?

A change in membrane potential from −70 to −60 mV is a depolarizing graded potential because the membrane potential is inside less negative than at rest. A change in membrane potential from −70 to −80 mV is a hyperpolarizing graded potential because the membrane potential is inside more negative than at rest.

Large-diameter, myelinated axons called ___ fibers conduct action potentials at velocities ranging from 12-130 m/sec (27-280 mi/hr). A. A fibers B. B fibers C. C fibers D. D fibers

A fibers

Why does a stronger stimulus cause a larger graded potential than a weaker stimulus?

A stronger stimulus opens more mechanically-gated channels or ligand-gated channels than a weaker stimulus.

If the total excitatory effects are greater than the total inhibitory effects and threshold is reached, one or more action potentials will be triggered. Action potentials continue to be generated as long as the EPSP is at or above the threshold level. A. EPSP B. Action potential C. IPSP

Action Potential

True or False To understand how local current flow occurs: Consider a region of membrane that is depolarized. Local current flow occurs in the cytosol's positive charges move from the depolarized membrane region to the more negative adjacent membrane regions that are still at resting membrane potential. At the same time, local current flow occurs in the ECF as positive charges move from adjacent membrane regions to the more negative depolarized membrane region. As a result of local current flow, adjacent membrane regions become depolarized.

All True

True or False. The Na+/K+ ATPases help maintain the resting membrane potential by using energy derived from ATP hydrolysis to pump out Na+ as fast as it leaks in and to bring in K+ as fast as it leaks out. This means that there is no net charge movement across the membrane of a resting neuron because Na+ leakage into the cell and K+ leakage out of the cell are exactly balanced by the constant activity of the Na+/K+ ATPases. Because there is no net charge movement across the membrane and energy is required to maintain this constant condition, the resting neuron exists in a steady state

All True

True or False. Graded potentials have different names depending on which type of stimulus causes them and where they occur. For example: When a graded potential occurs in the dendrites or cell body of a neuron in response to a neurotransmitter, it is called a postsynaptic potential. The graded potentials that occur in sensory receptors are termed receptor potentials. When a graded potential occurs in the plasma membrane of a skeletal muscle fiber at the neuromuscular junction (NMJ), it is called an end plate potential (EPP)

All true

True or False. 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), they cancel each other out and the overall graded potential disappears.

All true

True or False. Some neurons, however, do have active transport mechanisms that move Cl− ions against their concentration gradient from the cytosol into the ECF. This generates a stronger Cl− concentration gradient compared to what would be the case without active Cl− transport. Consequently, the magnitude of the Cl− concentration gradient becomes greater than the magnitude of the Cl− electrical gradient, causing net movement of Cl− ions from the ECF into the cytosol of the neuron (via Cl− leak channels in the membrane) under resting conditions. This means that ECl is not equal to the resting membrane potential; in fact, ECl is more negative than the resting membrane potential in neurons that actively transport Cl− ions. Because there is net movement of Cl− ions across the membrane, Cl− ions make a significant contribution to the resting membrane potential in these cells.

All true

True or False. The membrane potential becomes increasingly negative with the continuous movement of more K+ out of the neuron and less Na+ into the neuron. Eventually, K+ movement out of the neuron slows down and Na+ entry into the neuron speeds up because the negative membrane potential creates an electrical gradient that favors movement of both Na+ and K+ ions into the neuron. There is still net movement of K+ ions out of the neuron, however, because the magnitude of the K+ concentration gradient is greater than the magnitude of the K+ electrical gradient. With the help of Na+/K+ ATPases, the resting membrane potential of the neuron stabilizes when it reaches about −70 mV. At this point, K+ movement out of the neuron is exactly balanced by Na+ movement into the neuron and there is no net movement of charge across the membrane

All true

True or False. 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.

All true

True or Flase. At most chemical synapses, only one-way information transfer can occur—from a presynaptic neuron to a postsynaptic neuron or an effector, such as a muscle fiber or a gland cell. For example, synaptic transmission at an NMJ proceeds from a somatic motor neuron to a skeletal muscle fiber (but not in the opposite direction). Only synaptic end bulbs of presynaptic neurons can release neurotransmitter, and only the postsynaptic neuron's membrane has the receptor proteins that can recognize and bind that neurotransmitter. As a result, action potentials move in one direction.

All true

Why doesn't an action potential undergo decremental conduction?

An action potential does not undergo decremental conduction because it propagates (keeps its strength as it spreads along the membrane).

Will an action potential occur in response to a hyperpolarizing graded potential that spreads from the dendrites or cell body to the trigger zone of the axon? Why or why not?

An action potential will not occur in response to a hyperpolarizing graded potential because a hyperpolarizing graded potential causes the membrane potential to become inside more negative and therefore farther away from threshold (−55 mV).

Which type of synapse is typically associated with presynaptic inhibition or presynaptic facilitation?

An axoaxonic synapse: is typically associated with presynaptic inhibition or presynaptic facilitation.

________ transport moves organelles and synaptic vesicles from the cell body to the axon terminals.

Anterograde

Why does a graded potential gradually die out as it spreads along the plasma membrane?

As a graded potential spreads along the plasma membrane, it gradually dies out because its charges are lost across the membrane through leak channels.

________ are the most numerous of the neuroglia. They have processes that wrap around capillaries (the smallest blood vessels) in the CNS. The walls of brain capillaries consist of endothelial cells that are joined together by tight junctions. In effect, the tight junctions between the endothelial cells create a blood-brain barrier, which isolates neurons of the CNS from harmful agents and other substances in the blood.

Astrocytes

At a synapse between neurons, how does the function of the presynaptic neuron differ from the postsynaptic neuron?

At a synapse between neurons, the presynaptic neuron sends the signal and the postsynaptic neuron receives the message.

How can the neurotransmitter acetylcholine (ACh) be excitatory at some synapses and inhibitory at other synapses?

At some excitatory synapses, ACh binds to ionotropic receptors that contain cation channels that open and subsequently generate EPSPs in the postsynaptic cell. At some inhibitory synapses, ACh binds to metabotropic receptors coupled to G proteins that open K+ channels, resulting in the formation of IPSPs in the postsynaptic cell.

Why does the ionotropic receptor have faster response in the postsynaptic cell than the metabotropic receptor?

Because activation of an ionotropic receptor quickly opens or closes ion channels in the postsynaptic membrane and the subsequent postsynaptic potential that is generated usually lasts for only a few milliseconds or less. On the contrast, metabotropic receptor causes a slow responsein the postsynaptic cell because activation of a metabotropic receptor opens or closes ion channels more slowly (because G protein and second messengers are involved) and the subsequent postsynaptic potential that is generated typically lasts for hundreds of milliseconds to several minutes or even longer.

What would happen if summation of graded potentials in a neuron did not occur?

Because individual graded potentials undergo decremental conduction, they would die out as they spread through the dendrites and cell body if summation did not occur and an action potential would not be generated at the trigger zone of the axon.

K+ and Na+ ions help establish the resting membrane potential in what way?

Because the plasma membrane of a neuron has more K+ leak channels than Na+ leak channels, the number of K+ ions that diffuse down their concentration gradient out of the cell into the ECF is greater than the number of Na+ ions that diffuse down their concentration gradient from the ECF into the cell. As more and more K+ ions exit, the inside of the plasma membrane becomes increasingly negative, and the outside of the membrane becomes increasingly positive.

Neurons at rest are permeable to Cl− ions because their plasma membranes contain Cl− leak channels. Why are Cl- ions not significantly influential to the resting membrane potential?

Because there is no net movement of these ions across the plasma membrane through the Cl− leak channels. Because the plasma membranes of these cells lack active transport mechanisms for maintaining the Cl− concentration gradient.

Small-diameter, unmyelinated axons known as _____fibers conduct action potentials at velocities ranging from 0.5-2 m/sec (1-4 mi/hr). A. A fibers B. B fibers C. C fibers D. D fibers

C fibers

This nervous system processes many different kinds of incoming sensory information. It is also the source of thoughts, emotions, and memories. Most signals that stimulate muscles to contract and glands to secrete originate in the this nervous system A. Central Nervous System (CNS) B. Peripheral Nervous System (PNS)

Central Nervous System

______________ are substances that permit fast current flow because they have a low resistance. The extracellular and intracellular fluids of the body are good _______________ because they consist of ions that carry the current. _____________ are substances that decrease current flow because they have a high resistance. The plasma membrane is a good ___________ since membrane lipids have few charged groups and cannot carry current.

Conductors; conductors Insulators insulator

____________ conduction, which involves step-by-step depolarization, repolarization, and after-hyperpolarization of each adjacent segment of the plasma membrane; and ions flow through their voltage-gated channels in each adjacent segment of the membrane. A. Continuous conduction B. Saltatory conduction

Continuous conduction

____________, several presynaptic neurons synapse with a single postsynaptic neuron. This arrangement permits more effective stimulation or inhibition of the postsynaptic neuron.

Convergence

This is an example of which type of circuit? A single motor neuron that synapses with skeletal muscle fibers at neuromuscular junctions receives input from several pathways that originate in different brain regions.

Converging circuit

Which of the three basic parts of the neuron is known as the main input portions of the neuron?

Dendrites

________ are short, highly branched processes that extend from the cell body. Because they receive signals from other neurons or from stimuli in the environment.

Dendrites

What is the function of each of the three main parts of the neuron?

Dendrites receive input; the cell body functions as the control center, and it can receive input; the axon sends output in the form of action potentials to another neuron or effector cell by releasing a neurotransmitter at its synaptic end bulbs.

Some of the 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. Which type of way is the neurotransmitter being removed? A. Uptake by cell B. Enzyme degradation C. Diffusion

Diffusion

Metabotropic: The G protein, in turn, either ___________ opens (or closes) an ion channel or it activates a _________ messenger pathway that opens (or closes) an ion channel or causes another response in the cell, such as increasing the synthesis of new ________, modifying the activity of existing ________, or increasing the intracellular _______ levels.

Directly; second; proteins; proteins; Ca2+

A single presynaptic neuron may synapse with several postsynaptic neurons. Such an arrangement, called ________________, permits one presynaptic neuron to influence several postsynaptic neurons (or several muscle fibers or gland cells) at the same time.

Divergence

If the total excitatory effects are greater than the total inhibitory effects but less than the threshold level of stimulation, the result is an _______ that does not reach threshold. Following an EPSP, subsequent stimuli can more easily generate an action potential through summation because the neuron is partially depolarized. A. EPSP B. Action potential C. IPSP

EPSP

Certain neurotransmitters are inactivated through enzymatic degradation. Which type of way is the neurotransmitter being removed? A. Uptake by cell B. Enzyme degradation C. Diffusion

Enzyme degradation

For example, the enzyme acetylcholinesterase, which is located on the postsynaptic membrane, breaks down acetylcholine in the synaptic cleft. Which type of way is the neurotransmitter being removed? A. Uptake by cell B. Enzyme degradation C. Diffusion

Enzyme degradation

________ _______ line the ventricles of the brain and central canal of the spinal cord. They produce and assist in the circulation of cerebrospinal fluid. (Ventricles and the central canal are spaces filled with cerebrospinal fluid, which protects and nourishes the brain and spinal cord.)

Ependymal cells

Why may electrical synapses work in two directions, but chemical synapses can transmit a signal in only one direction?

In many electrical synapses (gap junctions), ions may flow equally well in either direction, so either neuron may function as the presynaptic neuron. At a chemical synapse, one neuron releases neurotransmitter and the other neuron has receptors that bind this chemical. Thus, the signal can proceed in only one direction.

The CNS processes sensory information by analyzing it and making decisions for appropriate responses—an activity known as integration. A. Sensory receptors B. Integrative C. Motor

Integrative

__________ are responsible for integration—they process incoming sensory information from sensory neurons and then may elicit a motor response by activating the appropriate motor neurons. A. Sensory neurons B. Motor neurons C. Interneurons

Interneurons

Which functional class of neurons is responsible for integration?

Interneurons are responsible for integration.

________ or ___________ neurons are located entirely within the CNS between sensory and motor neurons.

Interneurons or association

The main ions that influence the resting membrane potential of a neuron are?

K+ and Na+ ions (K+ and Na+ ions do not have equal roles, however, in establishing the resting membrane potential.)

Which ion has the greatest influence on the resting membrane potential of a neuron? Why?

K+ ions have the greatest influence on the resting membrane potential of a neuron because neurons at rest are most permeable to K+ ions.

In general, the more permeable the plasma membrane is to a particular ion, the greater the influence that ion has on the resting membrane potential. Neurons at rest are permeable to _____ and ______ ions because there are specific leak channels for these ions in their plasma membranes

K+; Na+

What is an example of an ionotropic receptor?

Ligand-gated channels

Which parts of a neuron contain mechanically-gated channels? Ligand-gated channels?

Ligand-gated channels and mechanically-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.

_________ ________ _______ refers to the passive movement of charges from one region of membrane to adjacent regions of membrane due to differences in membrane potential in these areas

Local current flow

Na+/K+ ATPases remove ________ positive charges from the cell than they bring into the cell (three Na+ ions exported for every two 2 K+ ions imported), These transporters are __________, which means they contribute to the negativity of the resting membrane potential. However, their total contribution is very small, only a few millivolts of the total −70 mV resting membrane potential in a typical neuron.

More; electrogenic

Once sensory information is integrated, the CNS may elicit an appropriate motor response. For this to occur, motor information is conveyed from the CNS through cranial and spinal nerves of the PNS to effectors (muscles and glands). Stimulation of the effectors causes muscles to contract and glands to secrete. A. Sensory receptors B. Integrative C. Motor

Motor

______ or ________ _______ convey action potentials away from the CNS to effectors in the periphery. They comprise the efferent division of the PNS.

Motor or efferent neurons

What is responsible for the white appearance of white matter?

Myelin makes white matter appear white.

Many excitatory neurotransmitters bind to ionotropic receptors that contain cation channels. Name the cation ions?

Na+, K+, and Ca2+

The _______-________ pump also contribute to the generation of the resting membrane potential.

Na+/K+ ATPases (sodium-potassium pumps)

The first channels that open, the voltage-gated ____ channels, allow _____ to rush into the cell, which causes the depolarizing phase. Then voltage-gated _____ channels open, allowing _______ to flow out, which produces the repolarizing phase.

Na+; Na+ K+; K+

Extracellular fluid (ECF) is rich in ____ and ______. In cytosol, however, the main cation is ______, and the two dominant __________ are proteins and the ________ ions attached to molecules such as the three phosphates in ATP.

Na+; chloride ions (Cl−) K+; anions; phosphate

A _______ is a bundle of axons that lies outside the brain and spinal cord

Nerve

What are the components of the peripheral nervous system?

Nervous and sensory receptors

The CNS contains billions of neurons organized into complicated networks called __________ circuits, functional groups of neurons that process specific types of information

Neural

________ or _____ make up about half the volume of the CNS.

Neuroglia or glia

_________, also known as nerve cells, are the basic functional units of the nervous system. (A functional unit is the smallest component of a system that can carry out the functions of that system.)

Neurons

What are types of ligands that can open or close ligand-gated channels?

Neurotransmitters, hormones, and chemicals in food or odor For example, the neurotransmitter acetylcholine opens cation channels that allow Na+ and Ca2+ to diffuse inward and K+ to diffuse outward.

______________ the chemical substances that neurons use to communicate with other neurons, muscle fibers, and gland cells.

Neurotransmittersare

Does it take a large number of charges to be separated across the plasma membrane in order to establish the normal resting membrane potential? Explain your answer.

No. Only a tiny fraction of all of the charges in the ECF and cytosol must be separated across the plasma membrane in order to establish the normal resting membrane potential.

________________ are responsible for forming and maintaining the myelin sheath around axons of neurons in the CNS. The myelin sheath is a multilayered lipid and protein covering.

Oligodendrocytes

During the after-hyperpolarizing phase, the voltage-gated K+ channels remain ________?

Open

_________ ______-___________ circuits may be involved in precise activities such as mathematical calculations

Parallel after-discharge

In which region of the brain does perception primarily occur?

Perception occurs primarily in the cerebral cortex.

This nervous system consists of all nervous tissue outside the CNS. A. Central Nervous System B. Peripheral Nervous System

Peripheral Nervous System

_______ transport moves membrane vesicles and other cellular materials from the axon terminals to the cell body to be degraded or recycled.

Retrograde

Many neurotransmitters are actively transported back into the neuron that released them (_________). Others are transported into neighboring neuroglia (_______). Which type of way is the neurotransmitter being removed? A. Uptake by cell B. Enzyme degradation C. Diffusion

Reuptake uptake Uptake by cell

What is the difference between reuptake and uptake of a neurotransmitter?

Reuptake is transport of the neurotransmitter back into the neuron that originally released it; uptake is transport of the neurotransmitter into another cell and not into the neuron that originally released it.

________ conduction, the special mode of action potential propagation that occurs along myelinated axons, occurs because of the uneven distribution of voltage-gated channels. Saltatory or Continuous conduction?

Saltatory

___________ cells form the myelin sheath around axons of neurons in the PNS.

Schwann

_________ ______ are a type of neuroglia found only in the PNS. They form the myelin sheath around axons of PNS neurons. They also participate in axon regeneration, which is more easily accomplished in the PNS than in the CNS.

Schwann cells

Two types of neuroglia produce myelin sheaths: _________ _______ (in the PNS) and ___________ (in the CNS)

Schwann cells; oligodendrocytes

What are examples of functions performed by neurons?

Sensing, thinking, remembering, controlling muscle activity, and regulating glandular secretions

_________ or ________ ________ convey action potentials into the CNS. They constitute the afferent division of the PNS.

Sensory neurons or afferent neurons

________ ______detect external or internal stimuli, such as a raindrop landing on your arm or an increase in blood acidity. This sensory information is then conveyed through cranial and spinal nerves of the PNS into the brain and spinal cord of the CNS. A. Sensory receptors B. Integrative C. Motor

Sensory receptors

In a _________ series circuit, a presynaptic neuron stimulates a single postsynaptic neuron.

Simple

__________ summation is summation of postsynaptic potentials in response to stimuli that occur at different locations in the membrane of a postsynaptic cell at the same time. A. Spatial B. Temporal

Spatial

____________ summation results from the buildup of neurotransmitter released simultaneously by several presynaptic end bulbs; _____________ summation results from the buildup of neurotransmitter released by a single presynaptic end bulb two or more times in rapid succession.

Spatial; Temporal

True or False As you know, neurons are permeable to bothK+ and Na+ ions. Because the plasma membrane of a neuron is more permeable to K+ ions than to Na+ ions, the number of K+ ions that move out of the neuron through K+ leak channels is greater than the number of Na+ ions that move into the neuron through Na+ leak channels. In other words, there is net movement of positive charge out of the neuron, which generates an inside-negative membrane potential

TRUE; TRUE

This is an example of which summation results from buildup of neurotransmitter released by a single presynaptic end bulb two or more times in rapid succession

Temporal summation

___________ summation is summation of postsynaptic potentials in response to stimuli that occur at the same location in the membrane of the postsynaptic cell but at different times.

Temporal summation

What are the two main subdivisions of the nervous system?

The central nervous system (CNS) and the peripheral nervous system (PNS)

What is the function of the efferent division of the PNS?

The efferent division of the PNS conveys motor output from the CNS to effectors (muscles and glands).

True or False. A cell that exhibits a membrane potential is said to be polarized. Most body cells are polarized; the membrane potential varies from +5 mV to -100 mV in different types of cells.

True

What is the functional significance of the myelin sheath?

The myelin sheath electrically insulates the axon of a neuron and increases the speed of conduction of action potentials.

The autonomic nervous system is comprised of what two main branches?

The parasympathetic nervous system and the sympathetic nervous system.

The resting membrane potential of a cell exists because?

There's an excess of negative ions in the cytosol along the inside surface of the membrane and an equal excess of positive ions in the extracellular fluid (ECF) along the outside surface of the membrane. The excess charges are located only very close to the membrane; the rest of the extracellular fluid or cytosol contains equal numbers of positive and negative charges and is electrically neutral.

Neuroglia support neurons in a variety of other ways such as?

They nourish and protect neurons and maintain homeostasis in the interstitial fluid that bathes them

What are examples of sensory receptors?

Touch receptors in the skin, olfactory (smell) receptors in the nose, and stretch receptors in the stomach wall.

Each phase of the action potential sequentially regenerates along the axon of a neuron from the ________ zone to the axon __________.

Trigger; terminals.

True or False As more and more K+ ions exit, the inside of the plasma membrane becomes increasingly negative, and the outside of the membrane becomes increasingly positive.

True

True or False During a depolarizing graded potential, the membrane potential is inside less negative than the resting level; During a hyperpolarizing graded potential, the membrane potential is inside more negative than the resting level.

True

True or False. In the absence of neurotransmitter (the ligand), the ion channel component of the ionotropic receptor is closed. When the correct neurotransmitter binds to the ionotropic receptor, the ion channel opens, and an EPSP or IPSP occurs in the postsynaptic cell.

True

True or False. Ionotropic receptor: When Cl− channels open, there is net movement of chloride ions into the cell because the chloride equilibrium potential (ECl) is not equal to the resting membrane potential. The inward flow of Cl− ions causes the inside of the postsynaptic cell to become more negative (hyperpolarized). In cells that do not actively transport Cl− ions, opening Cl− channels does not change the membrane potential because ECl is equal to the resting membrane potential and there is no net movement of Cl− ions into or out of the cell. However, opening these Cl− channels does stabilize the membrane potential and makes it more difficult for excitatory neurotransmitters to depolarize the cell.

True

True or False. It is important to point out that only a tiny fraction of all of the charges in the ECF and cytosol must be separated across the plasma membrane in order to establish the normal resting membrane potential.

True

True or False. It is the wave that travels around the stadium, not the actual fans. Because they can travel along a membrane without dying out, action potentials function in communication over long distances.

True

True or False. Like motor neurons, interneurons usually have numerous dendrites and one main axon extending from their cell bodies. About 99% of all neurons in the body are interneurons.

True

True or False. Most neural synapses are either axodendritic or axosomatic.

True

True or False. Most of the time, spatial and temporal summations are acting together to influence the chance that a neuron fires an action potential.

True

True or False. Neurons at rest are also permeable to Cl− ions because there are Cl− leak channels in their plasma membranes. However, Cl− ions do not make a significant contribution to the resting membrane potential in most neurons

True

True or False. Regeneration of the action potential along an unmyelinated axon of a neuron is similar to the "wave" that is performed by fans at a football game.

True

True or False. Resting neurons are essentially impermeable to the anionic proteins and phosphates (A− ions) of the cytosol because their plasma membranes do not have transport mechanisms for these ions. This means that A− ions do not have a direct impact on the resting membrane potential.

True

True or False. Synapses are also important because some diseases and neurological disorders result from disruptions of synaptic communication, and many therapeutic and addictive chemicals affect the body at these junctions.

True

True or False. The ENS helps regulate the activity of the smooth muscle and glands of the GI tract. Although the ENS can function independently, it communicates with and is regulated by the other branches of the ANS.

True

True or False. The amount of Na+ that enters the postsynaptic cell is greater than the amount of K+ that leaves it because the resting membrane potential of the postsynaptic cell is closer to the potassium equilibrium potential (EK) than to the sodium equilibrium potential (ENa).

True

True or False. The arrival of an action potential at the synaptic end bulb ultimately causes the release of neurotransmitters from the synaptic vesicles. The released neurotransmitter molecules, in turn, excite or inhibit the target cell.

True

True or False. The duration of the action potential in most neurons is about 1-2 msec.

True

True or False. The resting membrane potential is an electrical potential difference (voltage) that exists across the plasma membrane of an excitable cell under resting conditions.

True

True or False. The sum of all of the excitatory and inhibitory effects at any given time determines the effect on the postsynaptic neuron, which may respond.

True

True or False. The symbol A− is used to refer collectively to the negatively charged (anionic) proteins and phosphate ions of the cytosol

True

True or False. The voltage-gated Na+ channels of a neuron are sensitive to the extracellular Ca2+ concentration. This is because Ca2+ ions in the ECF bind to the extracellular surfaces of voltage-gated Na+ channels and increase the voltage that these channels require to open. This decreases the excitability of the neuron because the voltage-gated Na+ channels now require a higher voltage than normal to open.

True

True or False. This flow of charged particles is called current. In living cells, the flow of ions (rather than electrons) constitutes the electrical current

True

True or False. To say that these electrical signals are graded means that they vary in amplitude (size), depending on the strength of the stimulus

True

Each voltage-gated Na+ channel has two separate gates, an ______________ gate and an ____________ gate.

activation gate and an inactivation gate.

True or False. When an action potential propagates along a myelinated axon, local current flow occurs through the extracellular fluid surrounding the myelin sheath and through the cytosol from one node of Ranvier to the next.

True

True or False Cl− ions passively distribute across the membrane until the chloride equilibrium potential (ECl) becomes the same as the resting membrane potential. In other words, at −70 mV, the Cl− concentration gradient (from ECF to cytosol) is equal in magnitude to the opposing Cl− electrical gradient (from cytosol to ECF) and there is no net Cl− movement across the membrane.

True; True

True or False. A typical neuron in the CNS receives input from 1,000 to 10,000 synapses. Integration of these inputs involves summation of the postsynaptic potentials that form in the postsynaptic neuron. Summation is the process by which graded potential add together.

True; True

True or False. Few voltage-gated channels are present in regions where a myelin sheath covers the axon plasma membrane. By contrast, at the nodes of Ranvier (where there is no myelin sheath), the axon plasma membrane has many voltage-gated channels. Hence, current carried by Na+ and K+ flows across the membrane mainly at the nodes.

True; True

True or False. Graded potentials are larger or smaller depending on how many ligand-gated or mechanically-gated channels have opened (or closed) and how long each remains open. The opening or closing of these ion channels alters the number of specific ions that move across the plasma membrane to cause the graded potential

True; True

True or False. Some inhibitory neurotransmitters bind to metabotropic receptors that cause K+ channels to open. When the K+ channels open, a larger number of potassium ions diffuses outward. The outward flow of K+ ions causes the inside of the postsynaptic cell to become more negative (hyperpolarized), resulting in the formation of an IPSP.

True; True

True or False. The resting membrane potential (−70 mV) is closer to the K+ equilibrium potential (−90 mV) than to the Na+ equilibrium potential (+60 mV). This is because a resting neuron is more permeable to K+ ions than it is to Na+ ions, which allows K+ ions to have a greater influence on the resting membrane potential than Na+ ions

True; True

True or False. Another aspect of Na+/K+ ATPase function that is relevant to resting membrane potential is..... That the Na+/K+ ATPases expel Na+ ions out of the neuron as fast as they leak in and import K+ ions into the neuron as fast as they leak out. This action of the Na+/K+ ATPases stabilizes the resting membrane potential, keeping it at −70 mV. This means that at −70 mV, there is ___ net movement of Na+ or K+ ions across the membrane of a resting neuron

True; True; True; NO net movement

True or False Action potential propagates only a relatively short distance in a few milliseconds. Continuous conduction occurs in unmyelinated axons and in muscle fibers.

True; true

True or False. Different neurons may have different thresholds for action potential generation, but the threshold in a particular neuron is usually constant. The generation of an action potential depends on whether a particular stimulus is able to bring the membrane potential to threshold.

True; true

True or False. During the few ten-thousandths of a second that the voltage-gated Na+ channel is open, about 10,000 Na+ ions flow across the membrane and change the membrane potential considerably, but the concentration of Na+ hardly changes because of the millions of Na+ present in the ECF. The sodium-potassium pumps easily bail out the 10,000 or so Na+ ions that enter the cell during a single action potential and maintain the low concentration of Na+ inside the cell.

True; true

True or False. In a neuron, an action potential can propagate along the axon away from the cell body 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.

True; true

True or False. Removal of the neurotransmitter from the synaptic cleft is essential for normal synaptic function. If a neurotransmitter could linger in the synaptic cleft, it would influence the postsynaptic neuron, muscle fiber, or gland cell indefinitely.

True; true

True or False. The greater the voltage, the greater the current (assuming that resistance remains constant). Furthermore, the greater the resistance, the smaller the current (as long as voltage remains constant).

True; true

True or False. The plasma membrane of a neuron has more K+ leak channels than Na+ leak channels, which means that resting neurons are much more permeable to K+ ions than to Na+ ions. In fact, resting neurons are more permeable to K+ ions than to any other type of ion in the ECF or cytosol because K+ leak channels are the most abundant type of leak channel in their plasma membranes. As a result of the high permeability to K+ and relatively lower permeability to Na+, the resting membrane potential of most neurons is influenced primarily by K+ ions and to a lesser extent by Na+ ions.

True; true ; true

True or False. In contrast to neurons, glia do not generate or propagate action potentials, and they can multiply and divide in the mature nervous system. Mature neurons are unable to divide because they remain in the G0 phase of interphase. 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.

True; true; true

True or False. Sensory receptors are either the peripheral endings (dendrites) of sensory neurons or separate cells located close to sensory neurons. When the peripheral endings of sensory neurons serve as sensory receptors, they may be encapsulated (surrounded by a connective tissue capsule) or free (not encapsulated). The trigger zone for action potentials is at the junction of the dendrites and the axon of the sensory neuron. Once an action potential is generated, it propagates along the axon into the CNS.

True; true; true

True or False. The same neurotransmitter can be excitatory at some synapses and inhibitory at others, depending on the structure of the neurotransmitter receptor to which it binds. For example, at some excitatory synapses, acetylcholine (ACh) binds to ionotropic receptors containing cation channels that open and subsequently generate EPSPs in the postsynaptic cell. By contrast, at some inhibitory synapses ACh binds to metabotropic receptors coupled to G proteins that open K+ channels, resulting in the formation of IPSPs in the postsynaptic cel

True; true; true

True or False An action potential that occurs in a neuron is called a nerve action potential or nerve impulse. In most neurons, an action potential causes the release of neurotransmitters, which allow the neuron to communicate with another neuron, a muscle fiber, or a gland cell. An action potential that occurs in a muscle fiber is called a muscle action potential or muscle impulse. When an action potential occurs in a muscle fiber, the muscle fiber contracts.

True; true; true; true

The neurons that release norepinephrine, for example, rapidly take up the norepinephrine and recycle it into new synaptic vesicles. Theses membrane proteins are called neurotransmitter transporters. Which type of way is the neurotransmitter being removed? A. Uptake by cell B. Enzyme degradation C. Diffusion

Uptake by cell;

Which channels are open during the depolarizing phase? During the repolarizing phase?

Voltage-gated Na+ channels are open during the depolarizing phase, and voltage-gated K+ channels are open during the repolarizing phase.

IF: Membrane potential generation in a neuron that is permeable only to K+ ions. Why does the K+ electrical gradient favor movement of K+ ions from the ECF into the cytosol when an inside-negative membrane potential exists?

When an inside-negative membrane potential exists, the K+ electrical gradient favors movement of K+ ions from the ECF into the cytosol because the K+ ions are attracted to the negative charges along the inside surface of the plasma membrane and are repelled by the positive charges along the outside surface of the plasma membrane.

IF: Membrane potential generation in a neuron that is permeable only to Na+ ions. Why does the Na+ electrical gradient favor movement of Na+ ions from the cytosol into the ECF when an inside-positive membrane potential exists?

When an inside-positive membrane potential exists, the Na+ electrical gradient favors movement of Na+ ions from the cytosol into the ECF because the Na+ ions are attracted to the negative charges along the outside surface of the plasma membrane and are repelled by the positive charges along the inside surface of the plasma membrane.

Given the existence of leak channels for both K+ and Na+, could the membrane repolarize if the voltage-gated K+ channels did not exist? Explain your answer.

Yes. The leak channels would still allow K+ to exit more rapidly than Na+ could enter the axon

What are the two main phases of action potential?

a depolarizing phase and a repolarizing phase

During the __________ __________ period, even a very strong stimulus cannot initiate a second action potential.

absolute refractory

Voltage-gated channels are responsible for the generation and conduction of _________ potentials.

action

An _______ potential or ________ is a sequence of rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state.

action potential (AP) or impulse

The _______ ________ of the PNS conveys input into the CNS from sensory receptors in the body. This division provides the CNS with sensory information about the somatic senses (tactile, thermal, pain, and proprioceptive sensations) and special senses (smell, taste, vision, hearing, and equilibrium). A. Afferent division B. Efferent division

afferent division

The ______-_________ phase occurs when the voltage-gated K+ channels remain open after the repolarizing phase ends.

after-hyperpolarizing

While the voltage-gated K+ channels are open, outflow of K+ may be large enough to cause an _____-____________ phase of the action potential

after-hyperpolarizing

Following the repolarizing phase, there may be an ______-____________ phase, also called the _____________, during which the membrane potential temporarily becomes more negative than the resting level.

after-hyperpolarizing; undershoot

So an action potential is generated in response to a threshold stimulus, but it does not form when there is a subthreshold stimulus. In other words, an action potential either completely occurs or it does not occur at all. This characteristic of an action potential is known as the ____-____-_______ __________?

all‐or‐none principle.

The _________ 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 A. Somatic B. Autonomic

autonomic

___________ synapses (from axon to dendrite); ____________ synapses (from axon to cell body); ____________ synapses (from axon to axon)

axodendritic; axosomatic; axoaxonic

The ______ is a single long, thin process that extends from the cell body. It functions as the output portion of the neuron by generating action potentials and then propagating them toward another neuron, a muscle fiber, or a gland cell.

axon

The axon usually connects to the cell body at a cone-shaped region called the?

axon hillock

Along the length of an axon, side branches called axon collaterals may extend off. The axon and its collaterals end by dividing into smaller processes called?

axon terminals.

For an axon to function, materials must move between the cell body and axon terminals, a process known as?

axonal transport.

Electrical synapse: As ions flow from one cell to the next through the tubular connexons, the action potential spreads from cell to cell. The flow of ions through gap junctions is ________________; at other electrical synapses, ions flow through gap junctions in _____ direction only. Gap junctions are common in _______ muscle and __________ smooth muscle. They also occur in the CNS.

bidirectional; one cardiac; visceral

The relative refractory period is the period of time during which a second action potential _______ be initiated, but only by a larger-than-normal stimulus. It coincides with the period when the voltage-gated K+ channels are still _______ after inactivated Na+ channels have returned to their ________ state

can open; resting

The ______ _______ contains most of the organelles, including the nucleus. Because of its ability to direct protein synthesis and other cellular activities, it functions as the control center of the neuron. Like dendrites, it also serves as an input portion of the neuron because it can receive signals from other neurons.

cell body (soma)

Astrocyte processes surrounding brain capillaries secrete _________ that maintain the __________ of these tight junctions. Astrocytes also help to maintain the appropriate chemical environment for the generation of ________ potentials. (For example, they regulate the concentration of important ions such as K+, take up excess neurotransmitters, and serve as a conduit for the passage of nutrients and other substances between capillaries and neurons) In the embryo, astrocytes secrete chemicals that appear to regulate the _______, __________, and _________________ among neurons in the brain. Astrocytes may also play a role in the formation of _________ synapses.

chemicals; "tightness" action; growth, migration, and interconnections; neural

The repolarizing phase begins when the inactivation gates of the voltage-gated Na+ channels _______. Now the voltage-gated Na+ channel is in an ____________state. The threshold-level depolarization that opened the voltage-gated Na+ channels also opens voltage-gated K+ channels Because the voltage-gated K+ channels open more ________, their opening occurs at about the same time that the voltage-gated Na+ channels are _______ The slower opening of voltage-gated K+ channels and the closing of previously open voltage-gated Na+ channels produce the ______________ phase of the action potential. As the Na+ channels are inactivated, Na+ inflow _______. At the same time, the K+ channels are opening, accelerating K+ outflow. Slowing of Na+ inflow and acceleration of K+ outflow causes the membrane potential to change from +30 mV to −70 mV. As the membrane potential approaches −70 mV, the inactivated Na+ channels revert to their _________ state.

close inactivated state slowly; closing repolarizing slows resting state

What are the two types of propagation?

continuous conduction and saltatory conduction

Unmyelinated axons exhibit _____________ conduction; myelinated axons exhibit ___________ conduction.

continuous; saltatory

In a __________ circuit, the postsynaptic neuron receives action potentials from several different sources

converging

During the reploarizing phase of the action potential, membrane permeability to Na+ ___________ and membrane permeability to K+ ____________

decreases; increases

As a graded potential spreads to adjacent regions of membrane by local current flow, it gradually dies out because its charges are lost across the membrane through open leak channels. This mode of travel by which graded potentials die out as they spread along the membrane is known as _________ _________?

decremental conduction

Most graded potentials occur in the __________ and _______ ________ of a neuron.

dendrites; cell body

When the response makes the membrane less polarized (inside less negative), it is termed a _____________ graded potential

depolarizing

Inflow of sodium ions (Na+) causes the __________ phase of the action potential, and outflow of potassium ions (K+) causes the ____________ phase.

depolarizing; repolarizing

Without the Na+/K+ ATPases, the Na+ and K+ concentration gradients would eventually ________; K+ would no longer be able to leave the cell and Na+ would no longer be able to enter the cell, and a normal resting membrane potential would no longer be established.

dissipate

In a _____________ circuit, the action potential from a single presynaptic neuron causes the stimulation of increasing numbers of cells along the circuit

diverging

Sensory signals are also arranged in __________ circuits, allowing sensory input to be relayed to several regions of the brain. This arrangement amplifies the signal.

diverging

Axonal transport that occurs in a retrograde (backward) direction involves?

dyneins.

The _______ ________ of the PNS conveys output from the CNS to effectors (muscles and glands). A. Afferent division B. Efferent division

efferent division

At an ______________ synapse, action potentials conduct directly between adjacent cells through gap junctions. A. Chemical B. Electrical

electrical

The ability to produce action potentials in response to stimuli is referred to as? Once generated, neurons use action potentials to communicate with other neurons, muscle fibers, or gland cells in one or more organs of the body. This, in turn, regulates the organ's activity and ultimately restores homeostatic conditions.

electrical excitability.

When ion channels are open, they allow specific ions to move across the plasma membrane, down their _____________ gradient—a concentration (chemical) difference plus an electrical difference. As ions move, they create a flow of electrical current that can change the membrane potential.

electrochemical

Third branch of the autonomic nervous system is the _________ nervous system (ENS), an extensive network of neurons confined to the wall of the gastrointestinal (GI) tract.

enteric

In general, an ________ potential is the membrane potential at which the concentration gradient and electrical gradient for a particular ion are equal in magnitude but opposite in direction and there is no net movement of that ion across the plasma membrane.

equilibrium

Neurons and muscle fibers are considered to be ________ ______ because they exhibit electrical excitability, the ability to respond to a stimulus and convert it into an action potential.

excitable cells

A neurotransmitter that depolarizes the postsynaptic membrane is excitatory because it brings the membrane closer to threshold. A depolarizing postsynaptic potential is called an ________ postsynaptic potential (EPSP).

excitatory

Throughout the nervous system, the cell bodies of adjacent neurons are often clustered together. A cluster of neuronal cell bodies in the PNS is called a? A similar arrangement of neuronal cell bodies in the CNS is known as a?

ganglion (plural is ganglia); nucleus (plural is nuclei)

A ________ potential is a small deviation from the membrane potential that makes the membrane either less polarized (inside less negative) or more polarized (inside more negative)

graded

Ligand-gated channels participate in the generation of __________ potentials. A. Action potential B. Graded potential

graded

Like ligand-gated channels, mechanically-gated channels are involved in the formation of _________ potentials.

graded

(1) ________ potentials, which are used for short-distance communication only, and (2) ________ potentials, which allow communication over long distances in the body

graded; action

The changes in membrane (voltage) potential give rise to ________ potentials and ______ potentials, allowing neurons to communicate with other neurons, muscle fibers, or gland cells

graded; action

The __________ the summation of EPSPs, the greater the chance that threshold will be reached. At threshold, one or more action potentials arise.

greater

The net effect of opening cation channels in the postsynaptic cell is that Na+ inflow is ________ than either K+ outflow or Ca2+ inflow, and the inside of the postsynaptic cell becomes less negative (depolarized).

greater

More Na+ ions than Ca2+ ions enter the postsynaptic cell because the concentration of Na+ ions in the ECF is ________ than the concentration of Ca2+ ions in the ECF.

higher

When the response makes the membrane more polarized (inside more negative), it is termed a _________________ graded potential

hyperpolarizing

During after-hyperpolarizing phase: Unlike voltage-gated Na+ channels, most voltage-gated K+ channels do not enter an _________ state. Instead, they alternate between ________ and _______ states.

inactivated state; closed (resting) and open (activated)

The propagation of an action potential is accomplished by?

local current flow

During presynaptic _____________, there is a decrease in the amount of neurotransmitter released from the presynaptic neuron. During presynaptic ____________, there is an increase in the amount of neurotransmitter released from the presynaptic neuron.

inhibition facilitation

A neurotransmitter that causes hyperpolarization of the postsynaptic membrane is inhibitory. 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. A hyperpolarizing postsynaptic potential is termed an ___________ postsynaptic potential (IPSP).

inhibitory

Ligand-gated channels are numerous in the dendrites and cell bodies of _______ and ________ neurons

interneurons; motor

An ____________ receptor is a type of neurotransmitter receptor that contains both a neurotransmitter binding site and an ion channel as part of its structure. A. Ionotropic receptor B. Metabotropic receptor

ionotropic

The function of the Na+/K+ ATPase is relevant to resting membrane potential because?

it maintains the Na+ and K+ concentration gradients. Na+ and K+ ions in turn use these concentration gradients to help generate the resting membrane potential.

Axonal transport that occurs in an anterograde (forward) direction involves?

kinesins

Axonal transport uses proteins called ______ and _______ as "motors" to transport materials along the surfaces of microtubules of the neuron's cytoskeleton. Each of these motor proteins has a region that binds to the particle to be transported and a region that binds to a microtubule. The bound particle is carried by the motor protein as the motor protein uses energy from ATP hydrolysis to "walk" along the surface of the microtubule. Axonal transport moves materials in both directions—away from and toward the cell body.

kinesins and dyneins

The gates of _______ _______ randomly alternate between open and closed positions.

leak channels

C fibers carry information that is?

less critical, such as motor signals that cause contraction of smooth muscle in digestive organs

During the depolarizing phase, or rising phase, the negative membrane potential becomes _______ negative, reaches zero, and then becomes ________ The depolarizing phase reaches its peak at +30 mV. The part of the depolarizing phase between 0 mV and +30 mV is called the _________.

less; positive. overshoot

A ______-______ channel opens or closes in response to a specific ligand (chemical) stimulus.

ligand-gated

The axons of many neurons are surrounded by a ________ ________, a multilayered covering composed of lipids and proteins. Like insulation covering an electrical wire, the myelin sheath _______ the axon of a neuron and ________ the speed of conduction of action potentials.

myelin sheath insulates; increases

Within the brain and spinal cord are regions that look white, known as white matter, and regions that appear gray, called gray matter. White matter is composed primarily of ___________ axons. The whitish color of myelin gives white matter its name. The gray matter of the nervous system contains? It appears grayish rather than white because of the absence of ________ in these areas.

myelinated; neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia; myelin

The membrane potential during the after-hyperpolarizing phase becomes even more ___________ as it approaches the K+ equilibrium potential (EK) of about −90 mV. And once voltage-gated K+ channels ________, the membrane potential returns to the resting level of −70 mV.

negative close

A ______ is a bundle of axons in the PNS, whereas a tract is a ______ of axons in the CNS.

nerve; bundle

The _________ are larger in size; they consist of many amino acids linked together by peptide bonds.

neuropeptides

The membrane proteins that accomplish such uptake are called neurotransmitter _______________.

neurotransmitter transporters

Chemical synapse: In response to an action potential, the presynaptic neuron releases a ______________ that diffuses through the fluid in the synaptic cleft and binds to __________ in the plasma membrane of the postsynaptic neuron. The postsynaptic neuron receives the __________ signal and, in turn, produces a postsynaptic potential, a type of _________ potential. Thus, the presynaptic neuron converts an ___________ signal (action potential) into a ___________ signal (released neurotransmitter). The postsynaptic neuron receives the chemical signal and, in turn, generates an ____________ 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 __________ than electrical synapses.

neurotransmitter; receptors chemical; graded; electrical; chemical electrical signal slowly

Gaps in the myelin sheath that appear at intervals along the axon, known as?

nodes of Ranvier

In the resting state of a voltage-gated Na+ channel, the inactivation gate is _______, but the activation gate is _________. As a result, Na+ _________ move into the cell through these channels. At threshold, voltage-gated Na+ channels are _________. In the activated state of a voltage-gated Na+ channel, both the activation and inactivation gates in the channel are _____ and Na+ inflow begins. As more channels open, Na+ inflow _______, the membrane depolarizes further, and more Na+ channels open. This is an example of a __________ feedback mechanism

open; closed cannot activated open increases positive feedback

In this circuit, a single presynaptic cell stimulates a group of neurons, each of which synapses with a common postsynaptic cell?

parallel after-discharge circuit

Throughout your life, your nervous system exhibits __________, the capability to change based on experience. At the level of individual neurons, the changes that can occur include the sprouting of new dendrites, synthesis of new proteins, and changes in synaptic contacts with other neurons. Undoubtedly, both chemical and electrical signals drive the changes that occur.

plasticity

the neuron receiving the message is called the _____________ neuron

postsynaptic

At a synapse between two neurons, the neuron sending the signal is called the ___________ neuron

presynaptic

What are the types of presynaptic modulation?

presynaptic inhibition and presynaptic facilitation.

The alteration in the amount of neurotransmitter released by a presynaptic neuron is referred to as _________ _________?

presynaptic modulation (decrease or increase the neurotransmitter)

To communicate information from one part of the body to another, action potentials in a neuron must travel from the trigger zone of the axon (their point of origin) to the axon terminals. In contrast to a graded potential, an action potential is not decremental (it does not die out). Instead, an action potential maintains its strength as its spreads along the membrane. This mode of conduction, called ________________, depends on positive feedback.

propagation

The period of time after an action potential begins during which an excitable cell cannot generate another action potential in response to a normal threshold stimulus is called the ____________ period

refractory period

Despite this plasticity, mammalian neurons have very limited powers of ___________, the capability to replicate or repair themselves.

regeneration

In the PNS, an axon may undergo repair if the cell body is intact and if the Schwann cells that produce myelination remain active. Schwann cells aid the repair process by forming a ___________ __________ that guides and stimulates regrowth of the axon. Therefore, a person who injures axons of a nerve in an upper limb, for example, has a good chance of regaining nerve function.

regeneration tube

Each of the action potentials caused by a suprathreshold stimulus has the ______ amplitude (size) as an action potential caused by a threshold stimulus. Therefore, once an action potential is generated, the amplitude of an action potential is always the _____ and does not depend on stimulus __________. Instead, the greater the stimulus _______ above threshold, the greater the frequency of the action potentials until a maximum frequency is reached as determined by the absolute refractory period

same; same; intensity strength

Besides helping to maintain homeostasis, the nervous system is also responsible for your?

sensations, behaviors, and memories, and initiates all voluntary movements.

Mechanically-gated channels and ligand-gated channels can be present in the dendrites of ________ neurons

sensory

Most _______ neurons have only one process that extends from their cell bodies. This single process is an axon that has dendrites at its peripheral end. Theses neurons are associated with sensory receptors that detect a sensory stimulus such as touch, pressure, light, or sound.

sensory

Neurons are divided into what three functional classes? Based on the direction in which the action potential is conveyed relative to the CNS

sensory neurons, interneurons, and motor neurons.

A __________ __________ is a structure that monitors changes in the external or internal environment.

sensory receptor

A fibers carry urgent information such as?

sensory signals associated with touch, pressure, and position of joints, and motor signals that cause contraction of skeletal muscles

What are the diverse activities of the nervous system that can be grouped into three basic functions?

sensory, integrative, and motor.

Somatic motor neurons are part of the somatic nervous system; they convey action potentials to _______ muscles. Autonomic motor neurons, which are components of the autonomic nervous system, convey action potentials to?

skeletal; cardiac muscle, smooth muscle, or glands.

The _______-_________ neurotransmitters include acetylcholine, amino acids, biogenic amines, purines, gases, and endocannabinoids.

small-molecule

Neurotransmittersare are divided into what two classes?

small-molecule neurotransmitters and neuropeptides

The ________ nervous system 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. A. Somatic B. Autonomic

somatic

Most motor neurons have numerous dendrites and one main axon extending from their cell bodies. Depending on the branch of the efferent division of the PNS to which they belong, motor neurons are further classified into what two groups?

somatic motor neurons and autonomic motor neurons.

The efferent division within the peripheral nervous system is subdivided into what two type of nervous system?

somatic nervous system and an autonomic nervous system

This an example of which summation results from the buildup of neurotransmitter released simultaneously by several presynaptic end bulbs?

spatial summation

What are two types of summations?

spatial summation and temporal summation.

The amplitude of a graded potential depends on the stimulus _________. The ___________ the stimulus strength, the ________ the amplitude of the graded potential.

strength; greater; larger

An action potential does not occur in response to a ___________ stimulus, a stimulus that is a weak depolarization that cannot bring the membrane potential to threshold

subthreshold

Several action potentials form in response to a ____________ stimulus, a stimulus that is strong enough to depolarize the membrane above threshold

suprathreshold

Although the plasma membranes of presynaptic and postsynaptic neurons in a chemical synapse are close, they do not touch. They are separated by the ________ __________, a space of 20-50 nm that is filled with interstitial fluid

synaptic cleft

Within the synaptic end bulbs are many tiny membrane-enclosed sacs called ______ ______ that store chemical neurotransmitters.

synaptic vesicles

The central nervous system (CNS) consists of?

the brain and spinal cord

The production of electrical signals depends on two basic features of the plasma membrane of excitable cells?

the presence of specific types of ion channels and the existence of a resting membrane potential.

Electrical forces exist between these charged particles. Like charges repel each other, and opposite charges attract each other. In some cases, a partition may separate opposite charges. Such a separation of positive and negative charges is a form of potential energy, which can do work. The electrical potential difference between opposite charges that are separated from each other is termed as ___________?

voltage

A _______-________ channel opens in response to a change in membrane potential (voltage).

voltage-gated