During the action potential, which ion movement causes repolarization?

• A. Potassium ions (K+) efflux through voltage-gated K+ channels
• B. Sodium ions (Na+) influx
• C. Calcium ions (Ca2+) influx
• D. Chloride ions (Cl−) efflux

Answer: (C)

Repolarization is driven by the outward movement of potassium ions through voltage-gated potassium channels. After the peak of the action potential, the sodium channels inactivate and potassium channels open in response to the depolarized membrane. The efflux of K+ reduces the positive charge inside the cell, returning the membrane potential toward its resting level and often causing a brief after-hyperpolarization.

Calcium influx has a different role in many tissues. In cardiac cells, it contributes to the plateau phase and sustained depolarization, not the rapid return to resting potential. Sodium influx initiates and propagates the rising phase of the action potential, while chloride movements are generally linked to inhibitory effects or stabilization rather than the rapid repolarization process.