Sodium channel resting state occurs during which phase?

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

Answer: (A)

Sodium channels cycle through closed (resting), open, and inactivated states. When the cell is not excited and sits at its most negative voltage, the channels are in the closed, activatable (resting) state. This occurs during the phase of the action potential where the membrane potential is at its resting level, between impulses.

On arrival of a stimulus, the channels rapidly open, causing the sharp depolarization. They quickly inactivate after opening, and recovery from inactivation returns them to the resting closed state as the membrane potential returns to the resting level.

So, the resting state of the sodium channel corresponds to the resting membrane potential phase, the period when the cell is polarized and not actively depolarizing.