When does the heart cell become relative refractory after an action potential?

The relative refractory period refers to a time during which a cardiac cell can potentially respond to a new stimulus, but the stimulus must be stronger than usual. After an action potential, the heart cell enters the absolute refractory period first, during which it cannot fire another action potential regardless of stimulus strength. This period corresponds to phases 0 and 1, where the sodium channels are inactive.

Once the cell completes the absolute refractory period—generally aligned with the end of phase 1—the cell enters the relative refractory period. During this phase, some of the ion channels have reset, and if a stronger-than-normal stimulus is applied, the cell can depolarize again, leading to another action potential. Understanding the timing of these phases is critical for grasping cardiac electrophysiology, as it directly influences arrhythmic potential and the overall functionality of the heart.