Absolute refractory period is due to

Ans. (d) Closure of inactive gates of sodium channel(Ref: BRS physiology, Costanzo, 5th ed/p.11)Absolute refractory period:Corresponding to the period from the time the firing level is reached until repolarization is about one-third complete.No stimulus, no matter how strong, will not excite the nerve during absolute refractory periodCause - inactivation gates of the Na+ channel are closed when the membrane potential is depolarized. They remain closed until repolarization occurs. No action potential can occur, until the inactivation gates open.Relative refractory period:Begins from the remaining part of repolarization to the end of action potentialStronger than normal stimulus(suprathreshold stimulus) produces action potential in relative refractory periodCause: The K+ conductance is higher than at rest, and the membrane potential is closer to the K+ equilibrium potential and farther from threshold; therefore, more inward current is required to bring the membrane to threshold