15.1.8—Refractory period

After an action potential, the same axon region enters a refractory recovery period. Na⁺ voltage-gated channels close, K⁺ channels open and K⁺ leaves, repolarising the membrane; once the resting state is nearly restored, the channels become responsive again.

• During recovery, the membrane cannot immediately generate another normal action potential, so impulses remain separate rather than merging.
• A new action potential is generated in the membrane ahead of the original event, not behind it, because the region behind is refractory.
• The minimum interval between impulses limits the maximum impulse frequency.

The essential boundary is temporary unresponsiveness followed by restored responsiveness. The refractory period therefore links Na⁺/K⁺ channel recovery to discrete impulses, one-way propagation and a ceiling on firing frequency, without requiring a larger action-potential amplitude.

The refractory period is not a synapse and does not make an impulse travel faster. It is the recovery state of an axon membrane after an action potential; detailed synaptic transmission and muscle responses are covered separately.