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:

1. 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 (stimulus). This is because the sodium channels are inactivated and remain that way until hyperpolarisation occurs. In the cardiovascular mechanism, this refractory period is sometimes called effective refractory period (ERP).
2. 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 (stimulus) is large enough. This is because some of the sodium channels have been reactivated and have recovered but it is a difficult process due to the counter-acting potassium flow as some potassium ion channels are still open.

A fibre first enters the absolute refractory period directly after an action potential has been fired, then the relative refractory period. The absolute refractory period of a human muscle fibre is typically between 2.2 and 4.6 ms.

The refractory period causes 3 things to occur:

• The brain will be able to perceive nerve impulses as separated events since there is a time lag between them.
• The number of generated impulses or repetitive firing rate of a neuron will be limited in a given period of time.
• The nerve impulses will only travel in one direction. This is because, as the action potential is moving forwards along axons, the resting potential will be re-established behind it. However, not until this happens can a new action potential occur.