Saltatory Conduction

Mechanism

When a neuron receives a suprathreshold stimulus, the voltage-gated Na⁺ channels at the axon hillock open, therefore allowing the influx of sodium ions for the initiation of an action potential. The initial action potential then causes depolarization in the adjacent region of the axonal membrane.

Myelinated axons exhibit a highly structured distribution of voltage-gated ion channels, with Na⁺ channels profoundly concentrated at the myelin sheath gaps (nodes of Ranvier). The myelin sheath effectively prevents the leakage of ions across the membrane, and the depolarization spreads passively along the axon underneath the myelin sheath until reaching a myelin sheath gap. There, it is regenerated by the voltage-gated Na⁺ channels. By this process of regenerative depolarization, the action potential 'leaps' from gap to gap, until it reaches the axon terminal. This whole process is called saltatory conduction and it plays an important role in the rapid propagation of action potentials in the nervous system.

Advantages

As described, saltatory action potential propagation in myelinated neurons is faster than the continuous propagation in nonmyelinated neurons of the same diameter, as it bypasses the myelinated areas. Additionally, saltatory conduction is metabolically efficient; depolarization passively spreads underneath the myelin sheath and actively regenerates at the nodes of Ranvier, thus requiring less energy compared to continuous conduction, where every segment of the axon undergoes active depolarization. Preservation of the signal strength is achieved via the insulating properties of the myelin sheath, preventing ion 'leakage' across the neuron and ensuring the reliable long-distance transmission of impulses.