Answer:
Inactivation of voltage-gated Na⁺ channels.
Explanation:
The refractory period occurs mainly due to the inactivation of voltage-dependent sodium (Na⁺) channels, which occurs during the peak of action potential and persists for most of the hyperpolarization period.
These inactivated sodium channels cannot be opened even if the membrane potential crosses the threshold. Slow closure of voltage-dependent potassium channels, which results in hyperpolarization, also contributes to the refractory period by making membrane depolarization difficult (even after voltage-dependent Na⁺ channels have returned to their closed, activatable state).
