After Na+ channels close, which phase describes the membrane becoming more negative as K+ leaves the cell?

• A. Repolarizing phase
• B. Depolarizing phase
• C. Hyperpolarization
• D. Plateau phase

Answer: (A)

Repolarization is the phase where the membrane potential returns toward the negative resting value after depolarization. When Na+ channels close, the inward current drops, and K+ channels open, allowing positively charged K+ to leave the cell. This outflow makes the inside more negative, reversing the earlier positive shift and bringing the cell back toward its resting potential. Hyperpolarization would occur if K+ continued to leave past the resting level, and the plateau phase is a separate phase seen in heart muscle involving Ca2+ influx. Depolarization is the preceding phase driven by Na+ entry that makes the cell more positive.