Absolute refractoriness of a neuron is due to?
(A) Closure of activated Na channels (B) Closure of inactivated Na channels (C) Hyperpolarization of CI channels (D) Opening of rectifier K + channels
(B) Absolute refractory period: The period during which 2nd action potential cannot be elicited, even with strong stimulus is called "ABSOLUTE REFRACTORY PERIOD". Reason for this restriction is that sholy after action potential is initiated, the sodium channels (or calcium channels / both) become inactivated & no amount of excitatory signal applied to these channels at this point will open inactivated gates. Only condition that will allow them to reopen is when membrane potential returns to or gets close to original resting membrane potential level. Then, within another small fraction of second, inactivated gates of channels open & new action potential can be initiated This period for large myelinated nerve fibers is about 1/2500 second thus, such fibers can transmit maximum of about 2500 impulses per second - Na have 2 gates, activated gate & an inactivated gate. At resting state - active gate - closed - inactive gate - open At stimulus - both gates are open At absolute refractory period - active gate - open; inactive gate - closed Next action potential is generated only after inactivated gate is open & both gates go back to resting state.