A nerve signal doesn't crawl down the fiber. It leaps.
Myelin wraps the axon in fatty insulation, broken only by tiny gaps called the nodes of Ranvier. The impulse rebuilds itself at each gap instead of crawling along the whole membrane.
So it skips from node to node — which is why myelinated nerves fire far faster than bare ones.
Strip that insulation away, as multiple sclerosis does, and messages between brain and body slow, scramble, or never arrive.