During the relative refractory period, what must occur for depolarization to happen?

During the relative refractory period, a stronger-than-normal stimulus is required to initiate depolarization. This period follows the absolute refractory period, during which a neuron cannot be stimulated to fire another action potential, regardless of the strength of the stimulus.

In the relative refractory phase, the neuron is in a state of partial recovery from the previous action potential; some sodium channels have reset, but others remain inactivated, and the overall membrane potential may still be hyperpolarized (more negative than the resting membrane potential). As a result, to reach the threshold necessary for depolarization and to generate another action potential, a significantly stronger stimulus is necessary—one that exceeds the usual threshold. This is why a larger-than-normal stimulus can successfully trigger depolarization in this phase. Understanding this concept is crucial for grasping how action potentials are generated and how nerve communication is regulated during different phases of repolarization.