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. 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.

Answer 1

A stronger stimulus is needed to initiate another action potential during the relative refractory period because some Na+ channels have reset but K+ outflow counters 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 some Na+ channels have returned to their resting position and K+ channels are still open, so Na+ entry is offset by K+ loss. Consequently, a relatively stronger stimulus is needed to overcome the outflow of K+ ions and bring the membrane potential up to the threshold required to open enough Na+ channels to initiate another action potential.