Final answer:
The action potential does not reverse direction because voltage-gated Na+ channels are inactivated during the action potential's peak and cannot open again during the absolute refractory period. This unidirectionality is also supported by the action of voltage-gated K+ channels and the Na+/K+ pump.
Step-by-step explanation:
The action potential does not reverse direction on its own because the voltage-gated Na+ channels cannot transition directly from inactive to open states. At the peak of an action potential, the voltage-gated Na+ channels are inactivated, which means they cannot be opened again during the absolute refractory period. This ensures that depolarization spreading back toward previously opened channels has no effect, and the action potential must propagate toward the axon terminals.
The voltage-gated K+ channels also play a crucial role by opening slightly later than the Na+ channels, helping to repolarize the cell membrane by allowing K+ ions to exit the cell. The temporary hyperpolarization that occurs prevents the immediate reactivation of Na+ channels, thus maintaining the unidirectional flow of the action potential. Eventually, the membrane potential returns to its resting state due to the activity of non-gated channels and the Na+/K+ pump, which restores the resting potential and prepares the neuron for the possibility of a new action potential.