Final answer:
The rapid depolarization phase of an action potential is due to the opening of voltage-gated Na+ channels, allowing a rapid influx of Na+ ions into the cell.
Step-by-step explanation:
The rapid depolarization phase of an action potential is due to the opening of voltage-gated Na+ channels. When a stimulus causes the membrane potential to reach the threshold, the voltage-gated Na+ channels open, allowing a rapid influx of Na+ ions into the cell. This influx of positively charged ions leads to depolarization and the generation of an action potential.