My question is: What actually CAUSES a neurone to hyperpolarise during a nervous impulse? I understand the reasoning behind why hyperpolarisation is necessary - to maintain the unidirectional propagation - but I can't find ANYWHERE any explanation for why the neurone should "overshoot " while restoring resting potential.

Can anyone shed any light?



I did not know the answer but one of the senior electrophysiologists in our department has provided the following:-

The repolarization of the action potential results from inactivation of sodium channels and activation of delayed rectifier potassium channels. Delayed rectifier channels are so named because there is a delay before they open, which allows the upstroke of the action potential to occur. These potassium channels are also quite slow to close, so the 'overshoot' is caused by the persistent opening of delayed rectifier potassium channels after the action potential has been repolarized.  This period of 'overshoot' is essential to speed up the switch of sodium channels from inactivated to closed, to allow them to reopen for a subsequent action potential.  Therefore, the amplitude and duration of the 'overshoot' will determine the refractory period, the time after an action potential when another cannot be evoked.