Why does wrapping an axon in myelin make the signal so much faster? · Grey Matter

In a bare axon every patch of membrane must be charged to threshold in turn, and current leaks out through the wall as it spreads, so the spike crawls (about 0.5 to 2 m/s in thin unmyelinated fibres).

Why does wrapping an axon in myelin make the signal so much faster?

In a bare axon every patch of membrane must be charged to threshold in turn, and current leaks out through the wall as it spreads, so the spike crawls (about 0.5 to 2 m/s in thin unmyelinated fibres). Myelin wraps the axon in many layers of fatty membrane, which cuts the leak and the charge needed to move the voltage, so current from one node of Ranvier flows quickly along the insulated stretch to the next node. The spike is regenerated only at the nodes and seems to jump between them, reaching tens of metres per second in a fibre a few micrometres wide and up to about 120 m/s in the thickest ones. Matching that speed without myelin would need an axon hundreds of times thicker, which is how the squid gets its fast giant axon.