Questions: Voltage-Gated Sodium Channels

Question 1

A neuron's membrane depolarizes past threshold. Which sequence correctly describes what happens to the voltage-gated sodium channel?

• A. Inactivation gate closes first, then activation gate opens, allowing brief Na+ influx
• B. Activation gate opens rapidly, Na+ rushes in, then inactivation gate closes, terminating the current
• C. Both gates open simultaneously and stay open as long as the membrane remains depolarized
• D. The channel opens once and permanently inactivates, preventing any further Na+ entry

Answer: (B)

The activation (m) gate opens quickly at depolarization, allowing rapid Na+ influx. The inactivation (h) gate then closes slightly later, terminating current flow even while the membrane is still depolarized. This sequential mechanism makes the current transient rather than sustained.

Question 2

Voltage-gated sodium channels remain open and continue conducting Na+ for as long as the membrane is depolarized above threshold.

• T. True
• F. False

Answer: False

This is false. The inactivation gate closes within milliseconds of the activation gate opening, terminating Na+ influx even if the membrane is still depolarized. The channel enters an inactivated state and cannot reopen until the membrane repolarizes, which is the molecular basis of the absolute refractory period.

Question 3

Why can a second action potential not be triggered immediately after the first, even with a very strong stimulus?

Model answer: During the absolute refractory period, voltage-gated sodium channels are in the inactivated state — the inactivation gate is closed and the channel cannot reopen regardless of voltage. Only after repolarization allows the inactivation gate to reset can the channels respond to another stimulus.

The refractory period is not simply about membrane voltage — it is about the conformational state of the channel. Inactivation is voltage-dependent (requires repolarization to recover), so even driving the membrane back to threshold with an external current will not reopen inactivated channels.