Saltatory Conduction and Myelination Effects on Conduction Velocity

Myelination transforms how axons conduct. A myelin sheath, interrupted at regular gaps called nodes of Ranvier, insulates the internodal membrane and concentrates the regenerative current at the nodes, so the action potential effectively jumps from node to node. This saltatory conduction greatly increases speed and metabolic efficiency relative to continuous conduction in an unmyelinated fibre of the same diameter.

Definition

Saltatory conduction is the mode of action-potential propagation in myelinated axons in which depolarisation regenerates only at the nodes of Ranvier and spreads passively across the insulated internodes, so the impulse appears to jump from node to node, increasing conduction velocity.

Scope

This topic describes saltatory conduction, the role of the nodes of Ranvier, and how myelination and fibre geometry set conduction velocity. It contrasts continuous and saltatory propagation and outlines the structural determinants of speed, as reference physiology rather than clinical guidance.

Core questions

• How does the myelin sheath change the way current flows along an axon?
• Why does restricting regenerative current to the nodes of Ranvier increase conduction velocity?
• What structural factors determine the conduction velocity of a myelinated fibre?

Key concepts

• Myelin sheath
• Nodes of Ranvier
• Internode
• Saltatory vs. continuous conduction
• Conduction velocity
• Fibre diameter
• Membrane capacitance and insulation

Key theories

Saltatory conduction

The principle that in myelinated fibres the action potential is regenerated only at the nodes of Ranvier and skips across the insulated internodes, accounting for their much higher conduction velocity than unmyelinated fibres of similar size.

Fibre-size theory of conduction velocity

An analysis showing that for myelinated fibres conduction velocity scales approximately with fibre diameter, given the way internode length and membrane properties co-vary with size.

Mechanisms

The myelin sheath increases the resistance and decreases the capacitance of the internodal membrane, so little current is lost across it and the passive (electrotonic) spread of depolarisation along the axon is fast and far-reaching. Voltage-gated sodium channels are clustered at the nodes of Ranvier, where regenerative current entry occurs; depolarisation generated at one node spreads passively to the next, bringing it to threshold, so the impulse is regenerated only at the nodes and jumps between them. Huxley and Stampfli provided experimental evidence for this nodal, saltatory pattern. Because regeneration happens at discrete, widely spaced sites rather than continuously, conduction is faster and uses less ionic current; Rushton's analysis further showed how velocity scales with fibre diameter, and Waxman reviewed the geometric and membrane determinants of speed.

Clinical relevance

Saltatory conduction explains why loss of myelin slows or blocks nerve conduction, the physiological basis of demyelinating disorders and a key concept behind nerve conduction studies. This entry describes the normal mechanism and is not a basis for diagnosis or treatment of any individual.

Evidence & guidelines

The account rests on classic electrophysiological evidence for nodal conduction and on quantitative analyses of how fibre geometry determines velocity; these are mechanistic studies, not clinical guidelines.

History

Saltatory conduction was demonstrated experimentally in peripheral myelinated fibres in the late 1940s, showing that excitation is confined to the nodes of Ranvier. Rushton's 1951 cable analysis explained the dependence of velocity on fibre size, and later reviews integrated nodal channel distribution and internode geometry into a comprehensive picture of conduction velocity in myelinated nerve.

Key figures

• Andrew Huxley
• Robert Stampfli
• William Rushton
• Stephen Waxman

Seminal works

• huxley-stampfli-1949
• rushton-1951
• waxman-1980

Frequently asked questions

What are the nodes of Ranvier?

They are the regularly spaced gaps in the myelin sheath where the axon membrane is exposed and densely populated with voltage-gated sodium channels, and where the action potential is regenerated during saltatory conduction.

Why does myelination speed up conduction?

By insulating the internodes and lowering their capacitance, myelin lets depolarisation spread quickly and with little loss to the next node, so the impulse jumps from node to node instead of propagating slowly and continuously.