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) 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) A and B

Answer 1

During the relative refractory period, a stronger stimulus is needed to trigger an action potential due to some Na+ channels being restored to their resting state and K+ channels remaining open, which offsets Na+ entry.

Step-by-step explanation:

During the relative refractory period, an initial threshold-level depolarization is usually not sufficient to trigger an action potential because only some Na+ channels have returned to their resting position and K+ channels are still open. This means that the Na+ entry is offset by K+ leaving the cell; hence, more stimulus is required to reach the threshold to start a new action potential. During this period, an initial threshold-level depolarization is usually not sufficient to trigger an action potential due to the presence of open potassium (K+) channels. The open K+ channels allow K+ ions to leave the cell, which offsets the influx of sodium (Na+) ions and makes it more difficult for the cell to reach the threshold and generate an action potential. Therefore, the correct answer is E) A and B.