Describe the sequence of events during an action potential, including ion movements and key voltage-gated channels.

• A. Resting potential; depolarization via Na+ influx through voltage-gated Na+ channels; reach threshold; peak; repolarization via K+ efflux through voltage-gated K+ channels; hyperpolarization; Na+/K+ ATPase restores resting potential.
• B. Action potential begins with Cl- influx; hyperpolarization first then depolarization.
• C. Depolarization occurs via Ca2+ influx; repolarization via Na+ influx.
• D. Resting potential; depolarization via K+ efflux; repolarization via Na+ influx.

Answer: (A)

An action potential is driven by a precise sequence of ion movements through voltage-gated channels. It starts from the resting membrane potential; a depolarizing stimulus brings the membrane to threshold, at which point voltage-gated Na+ channels open and Na+ rushes into the cell. This rapid influx of positive charge causes the membrane potential to spike upward (depolarization) toward a positive value. As the peak is reached, the Na+ channels inactivate, and voltage-gated K+ channels open, allowing K+ to exit the cell. The outward K+ current restores the membrane potential back toward the negative resting level (repolarization). The K+ channels stay open briefly, producing a brief overshoot below resting potential (hyperpolarization) before the voltage-gated channels reset and the Na+/K+ ATPase helps restore the original ion gradients so the neuron is ready for another signal. The other options don’t fit because they describe ion movements that would not produce the rapid depolarization and repolarization seen in a typical action potential: Cl- influx would oppose depolarization; Ca2+ influx is not the main driver in most neurons; and depolarization from K+ efflux contradicts the known direction of the depolarizing current.