Neurons transmit action potentials in a unidirectional manner, much like one-way streets, primarily due to the properties of voltage-gated sodium (Na+) channels and the concept of the refractory period.
When a neuron is stimulated past its threshold, voltage-gated Na+ channels in the axon membrane open, allowing Na+ ions to rush into the cell. This influx of positive charge depolarizes the membrane and propagates the action potential down the axon. As the action potential travels, it activates the adjacent sections of the axon, opening more voltage-gated Na+ channels and propagating the signal in a forward direction.
However, once a segment of the axon has undergone depolarization and the action potential has passed, those voltage-gated Na+ channels become inactivated and enter a state where they cannot reopen immediately. This phenomenon is referred to as the refractory period. There are two phases of the refractory period: the absolute refractory period, during which no new action potential can be initiated, and the relative refractory period, during which a stronger-than-usual stimulus is needed to trigger another action potential.
The refractory period ensures that the action potential cannot propagate backward, as the region that has just undergone depolarization is temporarily unable to respond to further stimulation due to the inactivation of Na+ channels. As a result, the action potential progresses in one direction—away from the site of initiation and towards the axon terminal—while the previously activated region remains refractory and non-responsive.
In summary, the unidirectional flow of action potentials in neurons is primarily due to the inactivation of voltage-gated Na+ channels during the refractory period, which prevents any backward propagation of the signal.