An action potential phase in which voltage-gated Na+ channels open and Na+ influx causes depolarization to around +30 mV is the

• A. Depolarizing phase
• B. Repolarizing phase
• C. Hyperpolarizing phase
• D. Resting phase

Answer: (A)

During the depolarizing phase, voltage-gated Na+ channels open in response to a stimulus, allowing Na+ to rush into the cell. This inward Na+ current makes the inside of the membrane more positive, so the membrane potential rises from the resting level (around -70 mV) toward the positive side, reaching about +30 mV. This rapid influx is what creates the sharp upstroke of the action potential. After this peak, Na+ channels inactivate and K+ channels open, which switches the process to repolarization. Hyperpolarization and the return to resting potential follow as ion gradients are restored. So the key idea is Na+ entry driving the rapid rise to a positive membrane potential—the depolarizing phase.