Refractory Period

Refractory periods are a short phase in time following an action potential where another action potential cannot be generated. There are two types of refractory periods:
The absolute refractory period is a period where it is completely impossible for another action potential to be activated, regardless of the size of the trigger. This is because the sodium channels are inactivated and remain that way until hyperpolarization occurs.
The relative refractory period is the period, that occurs during the undershoot phase, where an action potential can be activated but only if the trigger is large enough. This is because the sodium channels have been reactivated but it is a difficult process due to the counter-acting potassium flow.