15.1.7—Rapid transmission in myelinated neurone

Two structural factors strongly affect how quickly an impulse travels along a neurone: whether the axon is insulated by myelin and the axon diameter.

• Myelin prevents ion diffusion and depolarisation across the insulated sections.
• Voltage-gated action potentials are regenerated at the exposed nodes of Ranvier.
• Local circuits between nodes depolarise the next node, so the impulse appears to jump from node to node: saltatory conduction.
• This gives a myelinated axon faster conduction than an otherwise comparable unmyelinated axon.

A thicker axon provides a larger membrane surface over which ions can diffuse. Faster ion movement supports faster changes in membrane potential and therefore faster regeneration of action potentials.

Myelin and diameter affect conduction speed; they do not change the basic all-or-nothing nature of an action potential. This card explains structural speed factors, while the basic adjacent-membrane mechanism and the refractory period are treated separately.