Question

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) The statement is incorrect; a threshold-level depolarization always triggers an action potential.
C) Only a few K+ channels have returned to their resting position.
D) Only K+ channels are still open, so Na+ entry is offset by K+ loss.
E) Some Na+ channels have returned to their resting position and K+ channels are still open, so Na+ entry is offset by K+ loss.

Answer 1

During the relative refractory period, an initial threshold-level depolarization usually doesn't trigger an action potential because not all Na+ channels are reset and open K+ channels counteract Na+ entry, requiring a stronger stimulus for action potential initiation.

Step-by-step explanation:

During the relative refractory period, an initial threshold-level depolarization is usually not sufficient to trigger an action potential because E) Some Na+ channels have returned to their resting position and K+ channels are still open, so Na+ entry is offset by K+ loss. After the peak of an action potential, voltage-gated Na+ channels close and become inactivated, preventing further Na+ entry. As the resting potential is approached, these channels begin to revert to their resting state, but not all at once, which means only some are ready to open again. During this phase, voltage-gated K+ channels that opened at the peak of the action potential are still open, allowing K+ to leave the cell, which counteracts the effect of Na+ ions entering. Therefore, a stronger-than-normal stimulus is required to achieve the net depolarization needed to reach the threshold and generate a new action potential during the relative refractory period.