Deck 7: Ionic Basis of the Action Potential

Question 1

In a voltage clamp, which of the following is measured?

A) Magnitude of the ionic current
B) Direction of the ionic current
C) What ion is responsible for the current
D) a and b only
E) a, b and c

Answer 1

D

Question 2

How does a voltage clamp work?

A) It measures the voltage of a neuron that is placed in the preparation.
B) It changes the membrane potential of a neuron using two extracellular electrodes.
C) It holds the current across a neuronal membrane constant and measures the changes in voltage.
D) It holds the voltage of a neuron constant by injecting current equal to the ionic current passing across the cell membrane.
E) It inactivates ion channels so that the only current passing into the cell is delivered by the equipment.

Answer 2

D

Question 3

In a voltage clamp experiment, this current is observed immediately after a voltage step and varies linearly with the size of the voltage step.

A) Capacitative current
B) Leak current
C) Sodium current
D) Potassium current
E) Calcium current

Answer 3

B

Question 4

In a voltage clamp experiment, this current is observed immediately after a voltage step and lasts approximately 20 microseconds or less.

A) Capacitative current
B) Leak current
C) Sodium current
D) Potassium current
E) Calcium current

Question 5

Following the application of a depolarizing stimulus, which of the following occurs first in a neuron?

A) Afterdepolarizing potential
B) Afterhyperpolarizing potential
C) Activation of sodium channels
D) Activation of potassium channels
E) Inactivation of sodium channels

Question 6

An increase in the sodium conductance results in which of the following?

A) A depolarization of the neuronal membrane potential
B) A hyperpolarization of the neuronal membrane potential
C) A repolarization of the neuronal membrane potential
D) An action potential
E) An afterhyperpolarizing potential

Question 7

The late current observed in voltage clamp experiments contributes to which of these phases of the action potential?

A) Depolarization
B) Inactivation
C) Repolarization
D) Threshold
E) Refractory period

Question 8

The early current observed in voltage clamp experiments contributes to which of these phases of the action potential?

A) Depolarization
B) Inactivation
C) Repolarization
D) Threshold
E) Refractory period

Question 9

Which of the following ions is responsible for the late current following a voltage step in a voltage clamp experiment?

A) Sodium
B) Potassium
C) Chloride
D) Calcium
E) Magnesium

Question 10

In a voltage clamp experiment, you record from a neuron after a command voltage step to -10 mV. In a second experiment, you step the command voltage to +50 mV. What do you notice about the sodium current in the second experiment, compared to the first?

A) The sodium current is larger.
B) The sodium current is smaller.
C) The sodium current is the same magnitude.
D) The sodium current appears earlier in time.
E) The sodium current appears later in time.

Question 11

In a voltage clamp experiment, you observe that the magnitude of the sodium current increases when the membrane potential is stepped between -45 and +10 mV. Why?

A) The driving force on sodium increases as membrane potential increases.
B) The equilibrium potential for sodium increases as membrane potential increases.
C) More sodium channels open as membrane potential increases.
D) The resting potential of the neuron increases as membrane potential increases.
E) The leak conductance increases as membrane potential increases.

Question 12

In a voltage clamp experiment, you observe that the magnitude of the sodium current decreases when the membrane potential is stepped between +10 and +50 mV. Why?

A) The driving force on sodium decreases as membrane potential increases.
B) The equilibrium potential for sodium decreases as membrane potential increases.
C) The voltage difference opens fewer sodium channels as membrane potential increases.
D) The resting potential of the neuron decreases as membrane potential increases.
E) The leak conductance decreases as membrane potential increases.

Question 13

Which of the following can be attributed to the flow of sodium ions across the membrane?

A) Capacitative current
B) Gating current
C) Early current
D) Late current
E) Afterhyperpolarizing potential (AHP)

Question 14

Which of the following depends on the flow of potassium ions across the membrane?

A) Capacitative current
B) Gating current
C) Early current
D) Late current
E) Afterdepolarizing potential (ADP)

Question 15

What ion is responsible for the outward current seen in a voltage clamp experiment where the voltage is set to +25 mV?

A) Sodium
B) Potassium
C) Calcium
D) Magnesium
E) Chloride

Question 16

You are performing a voltage clamp experiment in which the neuron's resting potential is -70 mV. When you change the command voltage to + 40 mV, what is the first current that you observe?

A) Capacitative current
B) Inward potassium current
C) Inward sodium current
D) Outward potassium current
E) Outward sodium current

Question 17

A neuron bathed in a fluid containing Saxitoxin (STX) is held a in a voltage clamp experiment and the command voltage is stepped to -15 mV. Which of the following currents is observed?

A) Outward sodium current
B) Inward sodium current
C) Outward potassium current
D) Inward potassium current
E) No currents are observed

Question 18

The use of choline instead of sodium in the extracellular solution allowed Hodgkin and Huxley to eliminate which of the following from their experiments?

A) Driving force on sodium
B) Driving force on potassium
C) Membrane potential fluctuation
D) Leak currents
E) Capacitative currents

Question 19

Hodgkin and Huxley found a delayed potassium conductance that occurred following membrane depolarization but not hyperpolarization. For this reason, this conductance was called the

A) depolarizing conductance.
B) rightward shift.
C) s-curve.
D) onward conductance.
E) delayed rectifier.

Question 20

Hodgkin and Huxley found that the delayed potassium conductance could be described mathematically by the product of four exponential equations. This suggests that

A) there are four potassium channels present in the neuron.
B) the opening of potassium channels depends on four independent events.
C) the potassium channel is made up of a single monomeric protein.
D) the potassium current will increase exponentially with membrane potential.
E) the opening of potassium channels is delayed.

Question 21

By subtracting the potassium current from the total ion current in their experiments, Hodgkin and Huxley were able to determine the

A) magnitude of the calcium current.
B) duration of the calcium current.
C) magnitude of the sodium current.
D) magnitude and duration of the sodium current.
E) contribution of potassium to the resting membrane potential.

Question 22

By calculating the ionic conductances in the neuronal membrane, Hodgkin and Huxley were able to

A) describe the time course of the action potential.
B) predict the molecular makeup of the voltage-gated ion channels.
C) explain the basis of the refractory period.
D) explain the threshold membrane potential.
E) do all of the above.

Question 23

When Hodgkin and Huxley recorded from a neuron under voltage clamp at +52 mV, which of the following describes what they observed?

A) An inward early current followed by an outward late current
B) No early current and an outward late current
C) An outward early current followed by an inward late current
D) An outward early current followed by an outward late current
E) An inward early current followed by no late current

Question 24

Which of the following is true of sodium and potassium conductances in neurons?

A) They both increase with an increase in the membrane potential.
B) They both decrease with an increase in the membrane potential.
C) Sodium conductance increases, while potassium conductance decreases, with an increase in the membrane potential.
D) Sodium conductance decreases, while potassium conductance increases, with an increase in the membrane potential.
E) None of these describe the relationship between ion conductance and membrane potential.

Question 25

When triethanolamine (TEA) is applied to a neuron in a voltage clamp experiment, what results are observed?

A) An inward early current followed by an outward late current
B) No early current and an outward late current
C) An outward early current followed by an inward late current
D) An outward early current followed by an outward late current
E) An inward early current followed by no late current

Question 26

During the late phase of an action potential, the threshold for activating a subsequent action potential is larger than when the neuron is at rest. This relative refractory period is due to a very large

A) increase in sodium conductance.
B) increase in potassium conductance.
C) decrease in potassium conductance.
D) increase in chloride conductance.
E) decrease in calcium conductance.

Question 27

What is the source of the gating currents?

A) Movement of ions across the cell membrane
B) Ion movement associated with the capacitance ("charging") of the cell membrane
C) Changes in the conductance of ion channels
D) Translocation of charged residues in ion channel proteins
E) Influx of calcium from the extracellular space

Question 28

What is thought to be the functional significance of charged amino acids along the S4 helix of the voltage-gated sodium channel?

A) They form the basis for sodium selectivity of the channel.
B) They provide a mechanism for inactivation.
C) They attract the negative charges of nearby channel subunits, forming a functional protein.
D) They form the pore of the channel.
E) They provide the gating mechanism for channel opening.

Question 29

In the rotational model of sodium channel activation, what happens when the membrane is depolarized?

A) An activation "gate" on the N-terminus of the protein swings open, allowing ions to move.
B) A short loop between domains III and IV swings open to allow sodium into the cell.
C) The S4 helices rotate, shifting the position of the S6 helices to open the ion pore.
D) A group of negatively-charged amino acids along the C-terminus rotate to open the ion pore.
E) A short loop between S4 and S5 swings open to allow sodium into the cell.

Question 30

Which of the following is thought to play a role in the inactivation of potassium channels?

A) Charged residues on the S4 helix
B) An inactivation gate between domains III and IV
C) A "ball" on the N-terminus of each of the channel subunits
D) A group of negatively-charged amino acids along the C-terminus
E) A short loop between the S4 and S5 helices

Question 31

The time constant for the decay of whole-cell sodium current is different than the time course of single sodium channel inactivation. Why?

A) The sodium current depends on the probability of sodium channel activation.
B) The time course for sodium channel inactivation is longer than the sodium current.
C) The sodium current decays more slowly because of leak channels.
D) The sodium current decay depends on the time course of sodium channel inactivation.
E) The sodium current decays more rapidly as more sodium channels inactivate.

Question 32

When is a sodium channel most likely to open following a depolarizing stimulus in the neuron?

A) 100 milliseconds
B) 10 milliseconds
C) 4-6 milliseconds
D) 1-2 milliseconds
E) <1 millisecond

Question 33

When is a potassium channel most likely to open following a depolarizing stimulus in the neuron?

A) 100 milliseconds
B) 10 milliseconds
C) 4-6 milliseconds
D) 1-2 milliseconds
E) <1 millisecond

Question 34

What is the approximate mean open time for a voltage-gated sodium channel?

A) 100 milliseconds
B) 10 milliseconds
C) 5 milliseconds
D) 2 milliseconds
E) <1 millisecond

Question 35

Which of the following ion conductances is responsible for the afterhyperpolarizing potential?

A) Sodium conductance
B) Potassium conductance
C) Chloride conductance
D) Cadmium conductance
E) Magnesium conductance

Question 36

What is the role of Calcium-dependent potassium channels in action potential dynamics?

A) They allow potassium to leave the cell during the falling phase of the action potential.
B) They allow potassium to enter the cell during the falling phase of the action potential.
C) They decrease potassium conductance, causing a return to the resting potential.
D) They increase potassium conductance, causing a slow afterhyperpolarizing potential.
E) They decrease the length of the relative refractory period following an action potential.

Question 37

Spike frequency adaptation is mediated by a(n)

A) increase in sodium conductance with repeated action potentials.
B) decrease in sodium conductance with repeated action potentials.
C) increase in potassium conductance with repeated action potentials.
D) decrease in potassium conductance with repeated action potentials.
E) decrease in the availability of sodium channels with repeated action potentials.

Question 38

The transport rate of the sodium-potassium cotransporter increases as

A) intracellular sodium concentration increases.
B) extracellular sodium concentration increases.
C) intracellular potassium concentration increases.
D) extracellular potassium concentration increases.
E) the transport rate does not change with ion concentration.

Question 39

Which of the following is true regarding the role of calcium on action potentials?

A) The magnitude of the action potential depends on calcium concentration.
B) Extracellular calcium concentrations set the time constant for action potential decay.
C) The falling phase of the action potential is calcium-dependent in some cells.
D) The afterhyperpolarizing potential is shorter with increased extracellular calcium.
E) The rising phase of the action potential is calcium-dependent in some cells.

Question 40

You perform a series of voltage-clamp experiments with 2 mM extracellular calcium, 0.2 mM extracellular calcium and 0 mM extracellular calcium. You notice that the potassium current increases with each step. What is the origin of this increase?

A) Decreased membrane potential
B) Increased threshold potential
C) Activation of calcium-activated sodium channels
D) Activation of calcium-activated potassium channels
E) Activation of calcium channels

Question 41

Both the afterhyperpolarizing current (AHP) and the afterdepolarizing current (ADP) depend on calcium levels altering

A) sodium conductance.
B) potassium conductance.
C) chloride conductance.
D) calcium conductance.
E) magnesium conductance.

Question 42

What does the voltage clamp experiment measure?

Question 43

Briefly describe the voltage clamp experimental setup and how it works.

Question 44

How do changes in sodium and potassium conductance following membrane depolarization lead to an action potential?

Question 45

You are recording from a neuron under voltage clamp to evaluate whether a new chemical "achematoxin" has any effect on the nervous system. You step the command voltage to +15 mV and find that there is a capacitative current and a robust, delayed late current. What effect, if any, do you suspect achematoxin has on neurons? Explain your answer.

Question 46

Name and describe the three distinct currents that occur following the depolarization of the membrane in a voltage clamp experiment.

Question 47

Briefly, explain the significance of Hodgkin and Huxley's finding that the time-course of potassium conductance could be modelled mathematically by an exponential function raised to the 4^th power.

Question 48

Describe the "gating currents" and their physiological source.

Question 49

Identify and describe the molecular mechanism responsible for sodium channel activation.

Question 50

Describe the molecular mechanism of sodium channel inactivation, including experimental evidence supporting this mechanism.

Question 51

What is the molecular mechanism responsible for potassium channel inactivation? Support your answer with experimental evidence.

Question 52

Compare the time course of individual sodium channel opening and sodium current in the whole cell following membrane depolarization, and explain any differences between them.

Question 53

What is the afterhyperpolarizing potential (AHP), and what is the ionic basis for it?

Question 54

What is frequency adaptation, and what is the mechanism by which it occurs?

Question 55

In an experiment, Hodgkin and Huxley demonstrated that a hyperpolarizing prepulse potential increased sodium conductance by up to 70%, but a depolarizing prepulse potential decreased sodium conductance, with a depolarization to -25 mV resulting in virtually zero sodium conductance. What does this experiment demonstrate?

Question 56

How does extracellular calcium affect neuronal excitability?