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.

Question

Quizplus · Accepted Answer

During the relative refractory period, some Na⁺ channels have returned to their resting position, but K⁺ channels are still open. This means that when a threshold-level depolarization occurs, some Na⁺ entry is offset by K⁺ loss, making it more difficult for the cell to depolarize enough to reach the threshold for an action potential. Therefore, 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