Final answer:
Voltage-gated Na+ channels inactivate shortly after the peak of depolarization during an action potential. The inactivation prevents further sodium entry, leading to repolarization. These channels return to a resting state once the membrane potential declines below -55 mV.
Step-by-step explanation:
When do the Na+ Channels Undergo Inactivation in an Action Potential?
The inactivation of voltage-gated Na+ channels is a crucial step in the action potential of a neuron. These channels have two gates: the activation gate and the inactivation gate. The activation gate opens at the threshold of approximately -55 mV, allowing Na+ to rush into the cell, which leads to the depolarization phase of the action potential. The inactivation gate, on the other hand, closes during the peak of depolarization.
Following depolarization, repolarization occurs as no more sodium can enter the cell due to the inactivation of Na+ channels. These channels remain in this state until the membrane potential reverts back below -55 mV during repolarization. The recovery phase sees the inactivation gates reopening and the activation gates closing, which returns the channels to their resting state and readies them for the next action potential. Throughout this process, potassium channels also play a role, opening as Na+ channels close to facilitate repolarization and subsequent restoration of the resting membrane potential.