Ch. 12 (2nd half)

Question 1

30. For an axon at resting membrane potential, the K+ leak channel is _______, the voltage-gated Na+ channel is _______, and the voltage-gated K+ channel is _______.
    a. open; inactivated; closed
    b. closed; inactivated; closed
    c. open; inactivated; open
    d. closed; closed; closed
    e. open; closed; closed

Answer 1

E

Question 2

31. During the falling phase of an action potential, the K+ leak channel on the axon is _______, the voltage-gated Na+ channel is _______, and the voltage-gated K+ channel is _______.
    a. open; inactivated; closed
    b. closed; inactivated; closed
    c. open; inactivated; open
    d. closed; closed; closed
    e. open; closed; closed

Answer 2

C

Question 3

38. Which of the following statements about a voltage clamp of a neuron to 0 mV is false?
    a. Once clamped, the voltage remains at 0 mV.
    b. Voltage-gated potassium channels open.
    c. Apart from the initial current shift from the clamp, no other current is produced.
    d. Voltage-gated sodium channels open
    e. An inward ionic current is produced during the opening of voltage-gated sodium channels.

Answer 3

C

Question 4

39. Which of the following statements regarding the structure of the voltage-gated Na+ channels is false?
    a. P loops mediate ion selectivity.
    b. Segment 4 of each domain is the voltage sensor.
    c. It has 4 domains with extensive sequence homology.
    d. The channel protein changes its primary structure in response to membrane depolarization.
    e. A cytoplasmic loop is thought to inactivate the channel by blocking the opening.

Answer 4

D

Question 5

40. Which of the following is a shared characteristic between a spiking neuron and a nonspiking neuron?
    a. High concentration of voltage-gated Na+ channels at the axon hillock
    b. The Hodgkin cycle
    c. A graded potential down the entire length of the axon
    d. An action potential down the entire length of the axon
    e. Neurotransmitter secretion based on a change in membrane potential

Answer 5

E

Question 6

41. How do nonspiking neurons function even though their depolarization signal significantly degrades with distance?
    a. Voltage-gated Na+ channels are replaced by ligand-gated Na+ channels.
    b. These neurons are very short, so there is no major signal decrement.
    c. There are sufficient numbers of voltage-gated Na+ channels to convey the signal without major decrement.
    d. These neurons do not release neurotransmitters, so signal degradation is not a problem.
    e. Voltage-gated K+ channels compensate for the lack of voltage-gated Na+ channels.

Answer 6

B

Question 7

45. Which of the following is the best explanation for the absolute refractory period of the action potential?
    a. Inactivated voltage-gated sodium channels
    b. Closed voltage-gated sodium channels
    c. Open slow calcium channels
    d. Inactivated voltage-gated potassium channels
    e. The passive properties of the axon membrane

Answer 7

A

Question 8

46. Which of the following statements about a local circuit in an axon is false?
    a. Na+ ions move into the cell through open Na+ channels.
    b. Ions flow in intracellular fluid, carrying current to more distant parts of the membrane.
    c. At the membrane, the ion movements change the distribution of charges on the membrane capacitance.
    d. An ionic current completes the local circuit as cations move toward the locus of the action potential and anions move away.
    e. Anions migrate into the membrane interior.

Answer 8

E

Question 9

47. _______ prevents bidirectional propagation of action potentials.
    a. The inactivation of Na+ channels
    b. The increased permeability to K+
    c. A decrease in membrane resistance
    d. Myelination
    e. The K+ channel

Answer 9

A

Question 10

48. Conduction velocity shows a(n) _______ axon diameter.
    a. proportional relationship to
    b. proportional relationship to the square root of
    c. exponential relationship to
    d. Either a or b, depending on the type of axon
    e. Either a or c, depending on the type of axon

Answer 10

D

Question 11

49. Which of the following is not one of the likely factors affecting the various velocities at which axons conduct action potentials?
    a. Myelination
    b. Temperature
    c. Length
    d. Diameter
    e. The number of voltage-gated Na+ channel per unit surface area

Answer 11

C

Question 12

50. Myelination by Schwann cells increases the velocity of action potential propagation by
    a. increasing the resistance and decreasing the capacitance, allowing the action potential to “jump” over the myelinated area.
    b. decreasing the resistance and increasing the capacitance, allowing the action potential to “jump” over the myelinated area.
    c. increasing the diameter of the neuron.
    d. increasing the number of voltage-gated sodium channels.
    e. increasing the resistance and increasing the capacitance, allowing the action potential to “jump” over the myelinated area.

Answer 12

A

Question 13

6. Explain in mechanistic terms how the action potential is an all-or-none phenomenon.

Answer 13

The action potential is initiated only when a threshold depolarization is reached near the axon hillock. That is, a certain critical number of voltage-gated Na+ channels have to open in order to cause a depolarization that is strong enough to initiate the Hodgkin cycle and, by definition, perpetuate the further opening of voltage-gated Na+ channels via their own depolarization. If the threshold is not reached, there will be no Hodgkin cycle or action potential.

Question 14

7. Explain in mechanistic terms why the action potential can travel a great distance along an axon without degrading.

Answer 14

The same mechanism that is responsible for the rising phase of the action potential also aids in its perpetuation along the axon without degradation. The action potential on one location on the axon can itself initiate an action potential at a neighboring location, and the induced action potential will have the same all-or-none amplitude as the original.

Question 15

8. Compare and contrast the techniques of patch clamping and voltage clamping.

Answer 15

Both patch clamping and voltage clamping provide experimental information about membrane currents, especially during an action potential. The patch-clamp technique uses a micropipette to record single channel currents, whereas the voltage-clamp technique shows whole cell ionic currents.

Question 16

9. What are the similarities and differences among the channels in the voltage-gated channel superfamily?

Answer 16

All the voltage-gated channels have principal subunits with extensive sequence homology and thus are evolutionarily related. Voltage-gated Na+ and Ca2+ channels have four domains, whereas the voltage-gated K+ channel has one domain that is homologous to one of the domains on the Na+ channel.

Question 17

10. Describe the significance of myelination.

Answer 17

Myelination greatly increases conduction velocity of an axon by increasing the membrane resistance while decreasing the membrane capacitance. In other words, conduction velocity is increased by increasing the length constant without increasing the time constant. Action potentials occur only at the nodes of Ranvier, in a process that is called saltatory conduction.

Question 18

5. A decrease in the absolute value of the membrane potential toward zero is called
   a. depolarization.
   b. an action potential.
   c. hyperpolarization.
   d. a membrane potential.
   e. voltage.

Answer 18

A

Question 19

6. Which of the following does not contribute to the cell’s membrane potential?
   a. Permeability to K+
   b. Permeability to Na+
   c. The overall resistance of the membrane
   d. Electrogenic ion pumps
   e. None of the above; all contribute to membrane potentials.

Answer 19

E

Question 20

7. The time constant (τ) depends on the
   a. resistance of the membrane.
   b. capacitance of the membrane.
   c. resistance and voltage of the membrane.
   d. resistance and capacitance of the membrane.
   e. resistance, capacitance, and voltage of the membrane.

Answer 20

D

Question 21

8. If a current pulse is generated on the membrane and creates a passive potential, which of the following will be true?
   a. The change in the membrane potential will increase as the distance from the current pulse increases.
   b. The change in the membrane potential will decrease as the distance from the current pulse increases.
   c. The change in the membrane potential will remain constant throughout the length of the membrane.
   d. The change in the membrane potential will fluctuate depending on the strength of the initial current pulse.
   e. There will be no change in membrane potential.

Answer 21

B

Question 22

9. The plasma membrane of a resting neuron is most permeable to which of the following ions?
   a. Na+
   b. K+
   c. Cl–
   d. Ca2+
   e. Mg2+

Answer 22

B

Question 23

11. In a typical neuron, which of the following ions is in passive equilibrium across the cell membrane?
    a. Na+
    b. K+
    c. Cl–
    d. Both Na+ and K+
    e. Both Na+ and Cl–

Answer 23

C

Question 24

12. A stimulating depolarizing current that depolarizes the axon hillock just slightly negative to the threshold will
    a. not change the overall membrane potential at all.
    b. produce an action potential.
    c. produce a very small action potential.
    d. produce a temporary graded potential.
    e. produce a small action potential that increases in amplitude as it travels along the axon.

Answer 24

D

Question 25

13. _______ channels govern the generation of an action potential.
    a. Ligand-gated Na+
    b. Ligand-gated K+
    c. Voltage-gated Na+
    d. Voltage-gated K+
    e. K+

Answer 25

C

Question 26

14. Considering the cycle of an action potential, when is the permeability to K+ at its greatest?
    a. During the resting membrane potential
    b. During the rising phase of the action potential
    c. At the peak of the action potential
    d. During the falling phase of the action potential
    e. The permeability of K+ remains the same throughout the action potential cycle.

Answer 26

D

Question 27

15. Which of the following statements regarding the action potential is false?
    a. In an extremely long axon, the action potential eventually will degrade.
    b. During the “falling” phase, K+ permeability increases.
    c. During the “rising” phase, Na+ moves into the neuron.
    d. The action potential lasts about 3 ms.
    e. In the recovery phase, Na+ channels are closed.

Answer 27

A

Question 28

16. What allows the action potential to return to a repolarized state?
    a. K+ leaks into cells.
    b. Voltage-gated Na+ channels become inactivated.
    c. Voltage-gated K+ channels become inactivated.
    d. Na+ reaches equilibrium across the neural membrane and stops leaking in.
    e. Voltage-gated Na+ channels close.

Answer 28

B

Question 29

17. Which of the following statements regarding cardiac pacemaker cells is false?
    a. They spontaneously generate action potentials.
    b. The frequency of action potential generation can be modified by neural input.
    c. The pacemaker cells are modified neural tissue.
    d. They are connected to myocardium via gap junctions.
    e. They have a current that is inward and activated by hyperpolarization.

Answer 29

C

Question 30

18. _______ are responsible for extending the time of the cardiac action potential relative to a neural action potential.
    a. Slow Ca2+ channels
    b. Slow Na+ channels
    c. Slow frequency of action potentials to the pacemaker
    d. Slow K+ channels
    e. Voltage-gated Na+ channels

Answer 30

A

Question 31

19. Which of the following is not likely to affect the conduction velocity of an action potential?
    a. Axon diameter
    b. Myelination
    c. Membrane surface area to volume ratio
    d. Temperature
    e. None of the above; all affect the velocity of an action potential.

Answer 31

E

Question 32

20. In myelinated axons, action potentials occur
    a. all along the axon.
    b. only at the internodes.
    c. only at the initial segment of the axon.
    d. at the internodes and nodes of Ranvier.
    e. only at the nodes of Ranvier.

Answer 32

E