Many axons are wrapped in myelin, a multilayered membrane sheath produced by glial cells, non-neuronal cells that support nervous-system function. Myelin electrically insulates the axon and changes how local current spreads.

The sheath is interrupted at short gaps called nodes of Ranvier. Voltage-gated Na$^+$ channels are concentrated strongly at these nodes.

Why insulation speeds conduction

Without myelin, current can leak across membrane along much of the axon, so each nearby patch must be depolarized and regenerate the action potential.

Myelin reduces ion leakage across the covered membrane and allows local current to spread farther along the axon before substantially dissipating.

The next node can therefore reach threshold quickly:

node fires
   ↓
current spreads rapidly under myelin
   ↓
next node reaches threshold
   ↓
new action potential

This node-to-node regeneration is called saltatory conduction.

The action potential does not literally disappear at one node and teleport to the next. Electrical current spreads continuously through the internodal region, but full regenerative spikes occur mainly at the nodes.

Myelin changes efficiency as well as speed

Because much less membrane must undergo large ion flux during each propagated spike, myelination can reduce the amount of Na$^+$ and K$^+$ movement that later has to be corrected by active transport.

Loss of myelin impairs signaling

If myelin is damaged, current leaks more strongly through the internodal membrane. The next node may depolarize more slowly or fail to reach threshold, slowing or blocking conduction.

Myelin therefore increases conduction performance by combining electrical insulation with spatially concentrated regenerative ion channels.