Final answer:
The sodium channel is inactivated during the action potential when the inactivation gate closes as the cell depolarizes, blocking further entry of Na+ ions.
Step-by-step explanation:
Inactivation of the sodium channel is possible because C) it is inactivated by the positive potential that develops during the action potential. The voltage-gated Na+ channel is composed of two gates: the activation gate and the inactivation gate. When a neuron is at rest, the activation gate is closed, and the inactivation gate is open. Upon depolarization, when the membrane potential reaches -55 mV, the activation gate opens allowing Na+ to enter the cell. As the action potential peaks at approximately +40 mV, the inactivation gate closes, preventing more Na+ from entering. This process is vital for the repolarization phase, as no additional sodium can enter the cell, aiding in the reset of the membrane potential. The inactivation gate re-opens at the end of the action potential, preparing the channel for a new cycle. In contrast to the Na+ channel, the voltage-gated K+ channel has only one gate, which opens later in the action potential cycle, contributing to repolarization.