Sodium channel inactivation occurs during which phases of the action potential?

• A. Phase 1 and Phase 2
• B. Phase 2 and Phase 3
• C. Phase 3 and Phase 4
• D. Phase 1 and Phase 4

Answer: (B)

Sodium channel inactivation is the process that turns off the fast Na+ current shortly after the channels open, shaping the early part of the action potential. When depolarization kicks in, Na+ channels open to produce the rapid upstroke (Phase 0). They then quickly enter an inactivated state, which stops further Na+ influx and helps drive the membrane toward the plateau.

Because the inactivation gates close almost immediately after opening and remain closed through the plateau, the Na+ current is largely absent during Phase 2. As repolarization begins in Phase 3, the channels begin to recover from inactivation and return to the resting state, becoming activatable again for the next cycle. Therefore, the inactivated state is present through Phase 2 and into the early part of Phase 3, with recovery occurring as Phase 3 progresses into Phase 4.

That’s why the inactivation is associated with Phase 2 and Phase 3.