Chapter 11

What is the primary role of the enzyme acetylcholinesterase (AChE) at a cholinergic synapse?

AChE degrades acetylcholine in the synaptic cleft.

Which of the following occurs first?

An action potential arrives at the axon terminal

How is an action potential propagated along an axon?

An influx of sodium ions from the current action potential depolarizes the adjacent area.

What type of conduction takes place in unmyelinated axons?

Continuous conduction

Where do most action potentials originate?

Initial segment

If an increase in extracellular potassium hyperpolarizes a neuron, which of the following would be correct?

It would change the membrane potential to a more negative value.

Sodium and potassium ions can diffuse across the plasma membranes of all cells because of the presence of what type of channel?

Leak channels

At threshold, axons will _______.

Likely generate an action potential if refractory periods have elapsed.

In which type of axon will velocity of action potential conduction be the fastest?

Myelinated axons with the largest diameter

What ion is responsible for the depolarization of the neuron during an action potential?

Na+ (sodium)

The concentrations of which two ions are highest outside the cell.

Na+ and Cl-

The concentrations of which two ions are highest outside the cell?

Na+ and Cl−

Sodium (Na+) and potassium (K+) move through the membrane of a neuron because of leak channels. Which of the following is the normal movement of these ions through the leak channels?

Na+ into the cell and K+ out of the cell

The Na+-K+ pump actively transports both sodium and potassium ions across the membrane to compensate for their constant leakage. In which direction is each ion pumped?

Na+ is pumped out of the cell and K+ is pumped into the cell

What prevents the Na+ and K+ gradients from dissipating?

Na+-K+ ATPase

What is the role of neurotransmitter at a chemical synapse?

Neurotransmitter binds to receptors on the postsynaptic cell membrane.

What characterizes repolarization, the second phase of the action potential?

Once the membrane depolarizes to a peak value of +30 mV, it repolarizes to its negative resting value of -70 mV.

Ions are unequally distributed across the plasma membrane of all cells. This ion distribution creates an electrical potential difference across the membrane. What is the name given to this potential difference?

Resting membrane potential (RMP)

Which of the following is described correctly?

Schwann cells provide the myelination in the peripheral nervous system.

Which of the following occurs during depolarization?

Sodium flows into the cell.

The sodium-potassium exchange pump transports potassium and sodium ions in which direction(s)?

Sodium ions are transported out of the cell. Potassium ions are transported into the cell.

Which of the following is FALSE regarding movement of ions across the membrane when a neuron is at resting potential?

The Na+ - K+ exchange pump moves Na+ into the neuron and K+ out of the neuron.

Why does the action potential only move away from the cell body?

The areas that have had the action potential are refractory to a new action potential.

Why does regeneration of the action potential occur in one direction, rather than in two directions?

The inactivation gates of voltage-gated Na+‎ channels close in the node, or segment, that has just fired an action potential.

What characterizes depolarization, the first phase of the action potential?

The membrane potential changes from a negative value to a positive value.

What event triggers the generation of an action potential?

The membrane potential must depolarize from the resting voltage of -70 mV to a threshold value of -55 mV.

What is the function of the myelin sheath?

The myelin sheath increases the speed of action potential conduction from the initial segment to the axon terminals

The resting membrane potential depends on two factors that influence the magnitude and direction of Na+ and K+ diffusion across the plasma membrane. Identify these two factors.

The presence of concentration gradients and leak channels

In an unmyelinated axon, why doesn't the action potential suddenly "double back" and start propagating in the opposite direction?

The previous axonal segment is refractory.

Which of the following statements about receptor potentials is FALSE?

The receptor potential is carried by neuroglia.

Multiple sclerosis (MS) is a disease that stops action potential propagation by destroying the myelin around (normally) myelinated axons. Which of the following best describes how MS stops action potential propagation?

Without myelin, the internode membrane resistance decreases, preventing local currents from reaching adjacent nodes.

A nerve is _______.

a bundle of axons

Which of the following is an example of a presynaptic cell?

a neuron

The membrane potential that occurs when neurotransmitters bind to their receptors is called _______.

a postsynaptic potential

Which of the following describes a change from the resting membrane potential?

a receptor potential, a synaptic potential or an action potential

A weak, subthreshold stimulus will result in _______.

a small depolarization at the receiving end of the neuron

The velocity of the action potential is fastest in which of the following axons?

a small myelinated axon

What is the magnitude (amplitude) of an action potential?

100 mV

What is the typical duration of a nerve action potential?

2 ms

The release of neurotransmitter occurs _______.

at the axon terminal

What mechanism releases neurotransmitter from presynaptic neurons?

exocytosis

In contrast to the internodes of a myelinated axon, the nodes __________.

have lower membrane resistance to ion movement

Compared to the electrical gradient for sodium at rest, the electrical gradient for potassium at rest is __________.

in the same direction and of the same magnitude

Increase in stimulus intensity _______.

increases the frequency of action potentials

The rate with which an action potential travels along an axon _______.

is called the conduction velocity and is measured in meters/sec

The channels that provide for the movement of potassium in the resting neuron are _______.

leakage

Which of the following allow the movement of potassium through the neuronal membrane?

leakage channels and voltage-gated potassium channels

The nodes of Ranvier are _______.

locations on the axon where the myelin sheath is absent

The typical concentration of sodium is _______.

lower than potassium intracellularly.

Which of the following is NOT a functional region of a neuron?

medullary region

A suprathreshold stimulus results in _______.

more action potentials

Longer stimuli will allow for _______.

more action potentials to occur, the absolute refractory period to finish and the relative refractory period to finish

Saltatory propagation occurs in _________ axons, in which action potentials _________.

myelinated; move from one node of Ranvier to another

A molecule that carries information across a synaptic cleft is a

neurotransmitter.

A single action potential is described as _______.

not graded

The stimulus for graded potentials includes _______.

sensory stimuli and neurotransmitter

Hyperpolarization results from __________.

slow closing of voltage-gated K+ channels

The movement of what ion is responsible for the local currents that depolarize other regions of the axon to threshold?

sodium (Na+)

Establishing the resting membrane potential requires energy through the use of the _______.

sodium-potassium pump

What separates the presynaptic and postsynaptic cells at a chemical synapse?

synaptic cleft

The small space between the sending neuron and the receiving neuron is the

synaptic cleft.

The repolarization phase of an action potential results from __________.

the opening of voltage-gated K+ channels

During an action potential, the rapid decrease in sodium permeability and simultaneous increase in potassium permeability is responsible for __________.

the repolarization phase

During the relative refractory period, _______.

the stimulus must be above threshold to generate an action potential

During an action potential of a neuron, what directly causes the different channels to open and close?

the transmembrane potential (voltage)

The minimum voltage that is required to generate an action potential is called the _______.

threshold voltage

The region on the neuron where action potentials are generated is called the ______.

trigger zone

In this simulation, ___________________ will be used to stimulate the axon.

voltage

The generation of an action potential in a neuron requires the presence what type of membrane channels?

voltage-gated channels

What is primarily responsible for the brief hyperpolarization near the end of the action potential?

voltage-gated potassium channels taking some time to close in response to the negative membrane potential

An action potential requires _______.

voltage-gated sodium channels to open and sodium to flow with its electrochemical gradient

To reach threshold, the amount of sodium _______.

entering the cell must overcome the potassium exiting

In a typical neuron, what is the equilibrium potential for sodium

+66 mV

Around what transmembrane potential does threshold commonly occur?

-60 mV

What is the value for the resting membrane potential for most neurons?

-70 mV

In a typical neuron, what is the equilibrium potential for potassium?

-90 mV

Approximately how fast do action potentials propagate in unmyelinated axons in humans?

1 meter per second

On average, the resting membrane potential is -70 mV. What does the sign and magnitude of this value tell you?

The inside surface of the plasma membrane is much more negatively charged than the outside surface.

An electrochemical gradient causes the movement of Na+ and K+ through the membrane. Which choice best represents the actual forces causing ion movements through the membrane of an undisturbed or resting neuron?

Both the concentration gradient and the electrical gradient move Na+ into the cell.

Which fibers generate the smallest value for conduction velocity?

C fibers

Which of the following best describes the role of calcium in synaptic activity?

Calcium influx into the synaptic terminal causes vesicle fusion with the plasma membrane and the release of neurotransmitter.

What changes occur to voltage-gated Na+ and K+ channels at the peak of depolarization?

Inactivation gates of voltage-gated Na+‎ channels close, while activation gates of voltage-gated K+‎ channels open.

What ion causes repolarization of the neuron during an action potential?

K+ (potassium)

The membranes of neurons at rest are very permeable to _____ but only slightly permeable to _____

K+; Na+

A depolarizing synaptic potential is also known as _______.

an excitatory postsynaptic potential

How do action potential propagation speeds in myelinated and unmyelinated axons compare?

Propagation is faster in myelinated axons.

Which of the following occurs first in the generation of an action potential?

The membrane depolarizes.

At rest, why is the transmembrane potential of a neuron (-70 mV) closer to the potassium equilibrium potential (-90 mV) than it is to the sodium equilibrium potential (+66 mV)?

The membrane is much more permeable to potassium ions than to sodium ions.

The plasma membrane is much more permeable to K+ than to Na+. Why?

There are many more K+ leak channels than Na+ leak channels in the plasma membrane.

What opens first in response to a threshold stimulus?

Voltage-gated Na+ channels

What is the first change to occur in response to a threshold stimulus?

Voltage-gated Na+ channels change shape, and their activation gates open.

Which of the following occurs during repolarization?

Voltage-gated potassium channels open and some voltage-gated sodium channels inactivate. Potassium flows out of the cell.

During an action potential, after the membrane potential reaches +30 mV, which event(s) primarily affect(s) the membrane potential?

Voltage-gated sodium channels begin to inactivate (close) and voltage-gated potassium channels begin to open.

The electrochemical gradient for potassium ions when the transmembrane potential is at the resting potential (-70 mV) is caused by what?

a chemical gradient going out of the cell and an electrical gradient going into the cell

What type of channel in the postsynaptic membrane binds neurotransmitter?

a chemically gated channel

A synapticcleft, or synaptic gap, can be found between a neuron and ______.

a musclecell. a gland. All of these. another neuron.

When neurotransmitter molecules bind to receptors in the plasma membrane of the receiving neuron,

ion channels in the plasma membrane of the receiving neuron open.

The mechanism by which the neurotransmitter is returned to a presynaptic neuron's axon terminal is specific for each neurotransmitter. Which of the following neurotransmitters is broken down by an enzyme before being returned?

acetylcholine

Where are action potentials regenerated as they propagate along an unmyelinated axon?

at every segment of the axon

Where are action potentials regenerated as they propagate along a myelinated axon?

at the nodes

An excitatory postsynaptic potential occurs _______.

at the receiving end of the interneuron

The conducting region of the neuron is the _______.

axon

Where in the neuron is an action potential initially generated?

axon hillock

During depolarization, which gradient(s) move(s) Na+ into the cell?

both the electrical and chemical gradients

Events that occur at a cholinergic synapse are listed here, but they are arranged in an incorrect order. Choose the correct order of these events below. (a) Calcium influx triggers exocytosis of ACh. (b) An action potential depolarizes the synaptic terminal. (c) ACh is removed by AChE. (d) ACh binds to receptors on the postsynaptic membrane.

b→ a→ d→ c

Leak channels allow the movement of potassium and sodium ions by what type of membrane transport?

channel-mediated diffusion

The electrochemical gradient for sodium ions in a neuron when the transmembrane potential is at the resting potential is caused by what?

chemical and electrical gradients both going into the cell

Binding of a neurotransmitter to its receptors opens __________ channels on the __________ membrane.

chemically gated; postsynaptic

We describe the regeneration of the action potential down the membrane of the axon of the neuron as _______.

conduction or propagation

Increasing the amount of myelination _______.

decreases the time between action potentials

An action potential is self-regenerating because __________.

depolarizing currents established by the influx of Na+‎ flow down the axon and trigger an action potential at the next segment

Calcium and magnesium are both _______.

divalent cations

During the action potential, when does sodium permeability initially DECREASE?

during the peak of depolarization

Binding of the neurotransmitter to its receptor causes the membrane to __________.

either depolarize or hyperpolarize

What type of membrane transport causes the depolarization phase of the action potential in neurons?

facilitated diffusion

An axon that is more negative than the resting membrane potential is said to be _______.

hyperpolarized

Action potential propagation begins (is first generated at) what region of a neuron?

initial segment

In what part of the neuron does the action potential typically initiate?

initial segment of the axon

The time interval between action potentials is called the _______.

interspike interval

Which of the following is used to block pain?

lidocaine

Which of the following describes a B fiber?

medium diameter, lightly myelinated

Increasing the voltage resulted in which of the following?

no change to the action potential

Let's consider a scenario in which the resting membrane potential changes from −70 mV to +70 mV, but the concentrations of all ions in the intracellular and extracellular fluids are unchanged. Predict how this change in membrane potential affects the movement of Na+. The electrical gradient for Na+ would tend to move Na+ __________ while the chemical gradient for Na+ would tend to move Na+ __________.

out; in

What causes repolarization of the membrane potential during the action potential of a neuron?

potassium efflux (leaving the cell)

In a synapse, neurotransmitters are stored in vesicles located in the __________.

presynaptic neuron

The node-to-node "jumping" regeneration of an action potential along a myelinated axon is called __________.

saltatory propagation

Which of the following blocks voltage-gated sodium channels?

tetrodotoxin and lidocaine

Increasing the strength of the stimulus applied to the sensory receptor increased _______.

the frequency of action potentials in the sensory neuron, the amount of neurotransmitter released at the axon terminal of the sensory neuron and the frequency of action potentials in the interneuron

The time interval for conduction would be shortest with

the largest and most heavily myelinated axons

Which stimulus was at or above threshold?

the moderate and strong stimuli

When the stimulus intensity increases, _______.

the number of action potentials increases

If a signal from a sending neuron makes the receiving neuron more negative inside,

the receiving neuron is less likely to generate an action potential.

The frequency of action potentials is _______.

the reciprocal of the interspike interval, and measured in hertz

In this activity, the stimulus voltage used was _______.

the same for all of the axons and suprathreshold for all of the axons

In a neuron, sodium and potassium concentrations are maintained by the sodium-potassium exchange pump such that __________.

the sodium concentration is higher outside the cell than inside the cell and the potassium concentration is higher inside the cell than outside the cell.

What is the electrochemical gradient of an ion?

the sum of the electrical and chemical gradients for that ion

When calcium ions enter the synaptic terminal,

they cause vesicles containing neurotransmitter molecules to fuse to the plasma membrane of the sending neuron.

An action potential releases neurotransmitter from a neuron by opening which of the following channels?

voltage-gated Ca2+ channels

The depolarization phase of an action potential results from the opening of which channels?

voltage-gated Na+ channels