The action potential, or as is often said "the nerve impulse" (in English literature often "the spike") is a short-term discharge of the electrical voltage difference (membrane voltage/membrane potential) that exists between the inside and outside of the nerve cell/axon (nerve fiber). The discharge, once it has occurred, has the ability to spread out along the nerve fibre (axon). The spreading or conduction velocity depends on the thickness of the axon and whether the axon is bare, i.e. unmyelinated, or enclosed by an insulating fat sheath, i.e. myelinized.
The thickness/diameter of an axon varies between different neurons, from 0.1 up to 20 micrometers. The thicker the axon, the faster the nerve impulse is conducted. Axons that are around 1 micrometer or more in thickness are surrounded by a myelin sheath. One therefore speaks of "myelinated axons", i.e. those that have myelin sheaths, and of "unmyelinated axons", i.e. those that lack myelin sheaths. The line speed is less than 0.1 m/sec. in the thinnest unmyelinated axons, but just over 120 m/sec. (= approx. 450 km/h) in the thickest myelinated axons.
Thus, the presence of a myelin sheath causes a very significant increase in conduction speed. Even the thinnest myelinated axons conduct the nerve impulse by 2-3 m/sec.