Question

Explain how repolarization happens. Give characteristics of the absolute and relative refractory periods in terms of voltage-gated sodium channel conformation. At what point can a new action potential be generated?

Answer 1

Repolarization occurs due to decrease in further Na influx; and K efflux through the voltage gated K channels which open later than Na channels but remain activated for prolonged period.

Absolute refractory period: It is a short period following action potential during which second stimulus, no matter how strong it may be, cannot evoke any response. Nerve fibre completely loses its excitability. During depolarization, the m gates of sodium channels in the membrane of nerve are opened rapidly . During early repolarization, the channels are closed by closure of inactivation (h) gates of the sodium channels and slow potassium channels are not opened yet. These voltage gated Na channels do not open unless potential comes back to the resting level. Therefore during this period, the nerve fibre is not stimulated at all.

Relative refractory period: It is a short period during which the nerve fibre shows response, if the strength of stimulus is more than normal. It extends from the end of absolute refractory period to the start of after depolarisation of the action potential. During this stage, Na channels are coming out of the inactivated state and voltage gated K channels are still opened. The stronger stimulus at this stage is able to open more Na channels through m gates and thus excites response. A new action potential can be generated at the start of relative refractory period i.e. at point 1/3 of repolarization.