The absolute refractory period is the time frame in which a neuron cannot fire another action potential. This is for one of two reasons. First, the voltage-gated sodium channels could already be opened. The channels are either opened or closed; there is no difference in magnitude during depolarization. Second, the voltage-gated sodium channels could be inactivated. This occurs during repolarization and means that no stimulation will cause the sodium channels to reopen. Towards the end of repolarization, once below the threshold potential, the sodium channels transition to a normal, closed conformation. Compare to: relative refractory period.
(“Refractory period”)