Final answer:
An action potential moves from the axon hillock to the axon terminals, with backward movement prevented by the refractory period of inactivated voltage-gated sodium channels. Myelinated axons enhance the speed of this process through saltatory conduction at the nodes of Ranvier, where action potentials are regenerated.
Step-by-step explanation:
An action potential moves through an axon from the axon hillock, near the cell body, toward the axon terminals. This sequential excitation of the axon membrane transmits neural signals over long distances. To prevent the action potential from moving backwards, the voltage-gated sodium channels that open during depolarization become inactivated at the peak of this process and enter a state known as the absolute refractory period. During this period, depolarization has no effect on the already opened channels, ensuring the action potential only propagates toward the axon terminals.
In a myelinated axon, the action potential occurs at the nodes of Ranvier, where there are gaps in the myelin sheath. These gaps are rich in voltage-gated Na+ and K+ channels, allowing for the regeneration and propagation of the action potential. The myelin sheath itself acts as an insulator, preventing current leak and enabling faster signal transmission – known as saltatory conduction. Conversely, in demyelinating diseases such as multiple sclerosis, the loss of insulation slows down the propagation of action potentials.