Chapter 11 Homework

Which of the neuroglial cell types shown form myelin sheaths within the CNS?

A

Cl− is a common, negatively charged extracellular ion. Predict the effect on the RMP if many Cl− gated channels are suddenly opened.

A more negative RMP would result.

Predict the possible effect of a drug that totally blocks the neurotransmitter receptor on the postsynaptic membrane. For example, curare is a neurotoxin used by several South American cultures. The primary effect of curare is that acetylcholine, a major neuromuscular neurotransmitter, cannot bind at its receptor because curare is blocking it. Predict the possible effects of curare on the postsynaptic membrane and muscle.

Local graded potential and action potential transmission is blocked, and there is no response by the postsynaptic cell, the muscle.

Which statement best describes exocytosis?

Membrane organelles fuse with the membrane and release contents out of the cell.

The repolarization phase of the action potential, where voltage becomes more negative after the +30mV peak, is caused primarily by __________.

K+ ions leaving the cell through voltage-gated channels

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

K+; Na+

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

Leak channels

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

During action potential propagation 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 in the refractory period.

Suppose that both stimuli seen in these graphs happened equally at the same time on a postsynaptic membrane as a result of two different synapses. Which of the following best describes the result?

There would be little or no graded potential.

Which choice best characterizes K+ leakage channels?

transmembrane protein channels that are always open to allow K+ to cross the membrane without the additional input of energy

Which of these materials or structures would be found in greatest amounts or numbers at E?

vesicles containing neurotransmitter

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

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

voltage-gated channels

In the animation's analogy of "Fire!" and "Don't Fire!" of the neuron, what physiological response is implied?

"Don't Fire!" implies the membrane potential becomes more negative.

Which of the following statements is true of both membrane potential responses shown in the graphs?

Both responses are examples of graded potentials.

Events that occur during synaptic activity are listed here, but they are arranged in an incorrect order. Choose the correct order of these events below. (a) Voltage-gated calcium channels open (b) Neurotransmitter binds to receptors (c) Action potential arrives at axon terminal (d) Neurotransmitter is removed from the synaptic cleft (e) Neurotransmitter released into synaptic cleft (f) Graded potential generated in postsynaptic cell

(c) Action potential arrives at axon terminal (a) Voltage-gated calcium channels open (e) Neurotransmitter released into synaptic cleft (b) Neurotransmitter binds to receptors (f) Graded potential generated in postsynaptic cell (d) Neurotransmitter is removed from the synaptic cleft

At which membrane potential, given the graphs shown, do a sufficient number of the voltage-gated Na+ channels start to open?

-60 mV

Assume you have a membrane with only potassium leakage channels. The RMP is -90mV. Predict the RMP if we add Na+ leakage channels. The most likely RMP value of Na+ is __________.

-70 mV

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

-70 mV

What is the resting membrane potential of the neuron used in the experiment?

-80 mV

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

100 mV

How is an action potential propagated along an axon?

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

At which point of the illustrated action potential would voltage-gated Na+ channels be mostly open but voltage-gated K+ channels be mostly closed?

B

Which of the cell types shown helps determine capillary permeability in the CNS?

B

In which area of the neuron is an action potential initially generated?

C

What ion is entering the axon terminal at A, and what effect does it have?

Ca2+, which then causes release of neurotransmitter from the axon terminal

What is the role of calcium in synaptic activity?

Calcium influx into the synaptic terminal causes vesicle fusion.

Choose the correct response to this statement: Presynaptic neurons synapse on the ________ of the postsynaptic neuron.

Cell body or dendrites

What type of conduction takes place in unmyelinated axons?

Continuous conduction

At which of the points along the illustrated action potential can a second action potential be produced, but only with a stimulus significantly greater than the one that produced the first?

D

Which of the cell types shown is most associated with the production and flow of cerebrospinal fluid (CSF)?

D

Which of the neuroglial cell types shown are found in the peripheral nervous system (PNS)?

E

During an action potential, hyperpolarization beyond (more negative to) the resting membrane potential is primarily due to __________.

K+ ions diffusing through voltage-gated channels

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.

Where do most action potentials originate?

Initial segment

During the hyperpolarization phase of the action potential, when the membrane potential is more negative than the resting membrane potential, what happens to voltage-gated ion channels?

K+ channels close. Na+ channels go from an inactivated state to a closed state.

Which of the following is the clearest example of a neuronal membrane's selective permeability?

K+ ions can diffuse across the membrane more easily than Na+ ions.

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

Myelinated axons with the largest diameter

The diffusion of what ion, across the neuronal membrane, is responsible for the local currents that depolarize regions of the axon to threshold?

Na+ (sodium)

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

Na+ and Cl-

What is happening to voltage-gated channels at this point in the action potential?

Na+ channels are inactivating, and K+ channels are opening.

Which of the following correctly states the direction followed by the specified ions when their voltage-gated channels open?

Na+ ions move into the axon; K+ ions move out.

During the action potential of a neuron, which ion is primarily crossing the membrane during the depolarization phase, and in which direction is the ion moving?

Na+ is entering 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

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

Na+Na+ and Cl−

What is the direct role of neurotransmitter at a chemical synapse?

Neurotransmitter binds to receptors on the postsynaptic cell membrane and allows ions to diffuse across the 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.

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

Propagation is faster in myelinated axons.

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)

Tetraethylammonium (TEA) blocks voltage-gated K+ channels such that K+ cannot pass even when the channels are open. However, TEA leaves K+ leakage channels largely unaffected. How would you expect the action potential to change if you treated a neuron with TEA?

The action potential would depolarize as usual, but the repolarization phase would take longer, causing the action potential to be more broad in time.

Which of the stimuli in this experiment would be considered a threshold stimulus?

Stimulus 2

Which of the following best describes the Na+ and K+ concentrations across a neuron's plasma membrane?

The Na+ concentration is higher outside the cell compared to inside. The K+ concentration is higher inside the cell compared to outside.

Imagine that the cell membrane from the previous problem becomes more permeable to Na+. Predict how this will affect the RMP.

The RMP will be more positive.

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.

You are going to record RMP from a cell using an electrode. You place your electrode and record a resting membrane potential every millisecond. You record an initial value of -70mV; however, over time you notice that your recordings become more and more positive until the RMP reaches 0mV. Assuming that Na+Na+ and K+K+ are the major determinants of RMP in this cell, which of the following could best explain your results?

The cell's Na+-K+ ATPase pumps have stopped functioning.

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.

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.

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.

Which of the following statements most accurately describes the effect caused by binding of the neurotransmitter (green dots) to the structure labeled C?

The membrane potential of the postsynaptic membrane changes.

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.

In a myelinated axon, how do the nodes of Ranvier differ from other segments of the same axon?

The nodes are more permeable to ions.

Hypothetically, what would be the most immediate effect of doubling the number of Na+ leakage channels in the plasma membrane?

The resting membrane potential would become less negative (more positive).

Suppose a drug is developed that blocks K+ leakage channels. The drug prevents ions from passing through those channels. If this drug was applied to a neuron, what would be the most immediate effect on that neuron?

The resting membrane potential would become less negative (more positive).

The myelin on myelinated neurons can be degraded or destroyed in diseases such as multiple sclerosis-a process called demyelination. If a myelinated neuron was affected by demyelination, how would this affect action potentials in that neuron?

The speed of action potential propagation would be slower.

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.

Which of the following best characterizes depolarization?

small, consecutive steps of Na+ penetration into the axon along its length

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.

When an action potential arrives at the end of the axon terminal, a series of events take place that result in the release of neurotransmitter from the presynaptic axon. Select the answer that correctly describes the primary stimulus for vesicles to move toward the cell membrane and eventually release their contents.

Voltage-gated membrane channels open, and Ca+2 enters the cytoplasm, increasing intracellular calcium.

What type of channel on the postsynaptic membrane binds neurotransmitter?

a chemically gated channel

A postsynaptic cell can be a neuron, a muscle cell, or a secretory cell. What is an example of a presynaptic cell?

a neuron

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

a small myelinated axon

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

either depolarize or hyperpolarize

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

Which of the following mechanisms is most significant in returning the ion concentrations to the resting state (from point D to point E)?

active transport by the Na+-K+ pump

What condition will increase the diffusion of molecules, such as neurotransmitters?

an increase in the amount of neurotransmitter exocytized by the presynaptic axon

During what part of the action potential do voltage-gated Na+ channels begin to inactivate (their inactivation gates close)?

at the end of the depolarization phase, as the membrane potential approaches its peak value

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

at the nodes of Ranvier

Where in the neuron is an action potential initially generated?

axon hillock

What change in a neuron is being measured in the graph?

the voltage measured across the axon membrane at a specific point as an action potential travels past

Which areas of this neuron would be classified as receptive regions?

both A and B

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

both the electrical and chemical gradients

If the membrane of a postsynaptic dendrite is setting up a graded potential, what must have happened after neurotransmitter was released by the presynaptic terminal? The neurotransmitter __________.

bound at postsynaptic receptors to open postsynaptic ion channels

How would the receptors at C best be classified?

chemically gated

Which component has a role in the postsynaptic cell during synaptic activity?

chemically gated channels

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

chemically gated; postsynaptic

What is the structure at A?

dendrite

Which of the following events would be directly affected if a neuron had a mutation that prevented the production of voltage-gated Na+ channels?

depolarization leading to 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

By which method does the structure at B release neurotransmitter?

exocytosis

Neurotransmitter is released from presynaptic neurons through what mechanism?

exocytosis

What is the major role of the Na+-K+ pump in maintaining the resting membrane potential?

maintaining the concentration gradients for Na+ and K+ across the cell membrane

Which of the following would increase the membrane permeability to K+?

more K+ leakage channels

Complete the following sentence. The operation of the Na+−K+ ATPase pump __________.

moves 3 Na+ to the ECF and 2 K+ to the cytoplasm

What structural classification describes this neuron?

multipolar

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

myelinated; move from one node of Ranvier to another

Which of the following stimuli caused the reaction in the graph on the left?

opening of gated Na+ channels

Which of the following is expected to occur first if the membrane potential increase shown in the graph on the left were to reach the threshold value indicated at −55 mV?

opening of voltage-gated Na+ channels

Hyperpolarization results from __________.

slow closing of voltage-gated K+ channels

Let's consider a scenario in which the resting membrane potential changes from −−70 mVmV to +70 mVmV, 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+Na+. The electrical gradient for Na+Na+ would tend to move Na+Na+ __________ while the chemical gradient for Na+Na+ would tend to move Na+Na+ __________.

out; in

Which of the following choices best represents synaptic transmission?

presynaptic axon to synapse to dendrite or postsynaptic cell body

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

presynaptic neuron

You are working on a new medication to selectively kill cancer cells. You test the drug on a culture of normal neurons to check for any side effects. Despite the presence of the drug, a normal resting membrane potential of −70 mV is maintained. Look at each of the figures below to analyze how the drug might be affecting the neuron. Figure A shows neuron activity under normal, or control, conditions (without the drug). Figure B shows neuron activity that has been altered due to exposure to the drug. Based on these results, the drug is most likely __________.

preventing Na+Na+-gated channels from opening

In addition to diffusion, what are two other mechanisms that terminate neurotransmitter activity?

reuptake and degradation

The box labeled D illustrates three mechanisms by which the effects of a neurotransmitter may be terminated. Which of the following mechanisms is NOT included in the figure?

reuptake of the neurotransmitter by transport into the postsynaptic cell

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

saltatory conduction

Imagine you changed the concentration of K+ outside a neuron such that the resting membrane potential changed to -80 mV (from the normal resting value of -70 mV). What have you changed?

the electrical gradient for K+ and the concentration gradient for K+

An action potential in one segment of axon causes adjacent sections of axon membrane to reach threshold through what mechanism?

the generation of local currents

The repolarization phase of an action potential results from __________.

the opening of voltage-gated K+ channels

What is the electrochemical gradient of an ion?

the sum of the electrical and concentration gradients for that ion