Question 1

In a voltage-clamp experiment, what parameter does the experimenter set, what parameter is measured, and what parameter is calculated?

Accepted Answer

The experimenter sets the membrane potential, measures the ionic current, and calculates the conductance.

Question 2

What is the significance of the reversal potential measured in a voltage-clamp experiment?

Accepted Answer

It is the membrane potential at which the net ionic current is zero, indicating equilibrium for a specific ion.

Question 3

When the membrane potential (Vm) is hyperpolarized from -65 mV to -130 mV in a voltage-clamp experiment, what happens to the membrane current?

Accepted Answer

Only a brief capacitive current is observed, followed by little to no ionic current.

Question 4

What are the key differences between the early inward current and the late outward current observed by Hodgkin and Huxley in the squid giant axon?

Accepted Answer

The early inward current is transient and carried by Na+ ions, while the late outward current is sustained and carried by K+ ions.

Question 5

How can tetrodotoxin (TTX) and tetraethylammonium (TEA) be used to separate ionic currents in voltage-clamp experiments?

Accepted Answer

TTX blocks Na+ currents and TEA blocks K+ currents.

Question 6

Why is there no inward current observed when the membrane potential is stepped from -65 mV to +44 mV, despite a large Na+ conductance (gNa)?

Accepted Answer

Because the driving force for Na+ is zero at +44 mV, which is near the Na+ equilibrium potential.

Question 7

Given the following values: $$E_{Na} = +55\ 	ext{mV}$$, $$V_{m} = 0\ 	ext{mV}$$, and $$I_{Na} = -1\ 	ext{mA}$$, calculate the Na+ conductance ($$g_{Na}) $$using Ohm's Law.

Accepted Answer

$$g_{Na} = \frac{I_{Na}}{V_{m} - E_{Na}} = \frac{-1}{-55} = 0.0182\ 	ext{mS}$$

Question 8

Which of the following best describes the relationship between conductance and driving force in determining ionic current?

Accepted Answer

Ionic current is the product of conductance and the driving force: $$I_x = g_x (V_m - E_x)$$

Question 9

What is the main difference between the absolute and relative refractory periods following an action potential?

Accepted Answer

During the absolute refractory period, no action potential can be triggered, while during the relative refractory period, a stronger stimulus is required to trigger an action potential.

Question 10

Which ion has the highest intracellular concentration in the squid neuron according to the provided table?

Accepted Answer

Potassium (K+)

Question 11

In the context of the action potential, what is the effect of increasing Na+ conductance (gNa) at threshold?

Accepted Answer

It causes Na+ to enter the neuron, depolarizing the membrane toward $$E_{Na}.$$

Question 12

What is the primary effect of tetrodotoxin (TTX) on neuronal ionic currents?

Accepted Answer

TTX blocks Na+ channels, preventing inward current.

Question 13

Which equation is used to calculate the equilibrium potential for an ion using its concentration gradient?

Accepted Answer

The Nernst equation: $$E_x = 58 \log \frac{[X]_{out}}{[X]_{in}}$$

Question 14

What happens to the membrane current when extracellular Na+ is eliminated in a voltage-clamp experiment?

Accepted Answer

The early inward current disappears, but the late outward current remains.

Question 15

Which of the following best describes the time course of an action potential in the squid giant axon?

Accepted Answer

Rapid activation of gNa, followed by delayed activation of gK, then inactivation of gNa and repolarization by gK.

Question 16

Which of the following statements about K+ conductance (gK) in the squid giant axon is correct?

Accepted Answer

K+ conductance activates with a delay and reaches a steady-state value without inactivation.

Question 17

During the peak of the action potential, why does the driving force for Na+ become very low?

Accepted Answer

Because the membrane potential approaches the Na+ equilibrium potential ($$E_{Na}$$), reducing $$(V_m - E_{Na}).$$

Question 18

Which pharmacological agent is used to block K+ currents in voltage-clamp experiments?

Accepted Answer

Tetraethylammonium (TEA)

Ionic Basis of the Action Potential: Voltage-Clamp Analysis in the Squid Giant Axon

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