Final answer:
Action potential depolarization ends due to the inactivation of Na+ channels after a brief period of opening. This inactivation couples with the opening of K+ channels, resulting in membrane repolarization and a refractory period where no new action potentials can be initiated.
Step-by-step explanation:
Action potential depolarization ends primarily when Na+ channels inactivate. The depolarization of a neuron's membrane potential is a result of the influx of Na+ ions through voltage-gated sodium channels. These channels have two gates: an activation gate that opens rapidly when the membrane potential crosses the threshold of approximately -55 mV, and an inactivation gate that closes after a short period, interrupting the influx of Na+ and leading to depolarization of the membrane potential.
As the membrane potential peaks, the Na+ channels' inactivation gates close, while voltage-gated K+ channels open, leading to an efflux of K+ ions and repolarizing the membrane. This sequence of events results in the neuron becoming temporarily resistant to the generation of another action potential, known as the refractory period. After a short period, the Na+ channels reset, returning to a state that will allow for a new action potential if a sufficient stimulus is presented.