During the relative refractory period,an initial threshold-level depolarization is usually not sufficient to trigger an action potential.Why? A) Only some Na⁺ channels have returned to their resting position. B) Only K⁺ channels are still open, so Na⁺ entry is offset by K⁺ loss. C) Only a few K⁺ channels have returned to their resting position. D) The statement is incorrect; a threshold-level depolarization always triggers an action potential. E) Some Na⁺ channels have returned to their resting position and K⁺ channels are still open, so Na⁺ entry is offset by K⁺ loss.

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Quizplus · Accepted Answer

During the relative refractory period, some Na+ channels have returned to their resting position, but K+ channels are still open after the repolarization phase of the preceding action potential. Therefore, a threshold-level depolarization is offset by K+ efflux, which means that more depolarization is required to reach the threshold for generating an action potential. This is why an initial threshold-level depolarization is usually not sufficient to trigger an action potential during the relative refractory period.

During the Relative Refractory Period,an Initial Threshold-Level Depolarization Is Usually