Physiological Basis of Conduction in Myelinated Nerve Fibers

Speculating in his Lecons sur l’Histologie du Systeme Nerveux on the physiological function of myelin and nodes in conduction in myelinated nerve, Ranvier (1878) concluded that the myelin sheath, because of its lipid makeup, would protect the axon cylinder against compression. While this conclusion does not play a prominent part in current thinking, his second conclusion does—namely, that the myelin might serve as a nonconducting envelope to surround the conducting axon and insulate it electrically from the external conducting medium. Referring to the analogy between conduction in a myelinated nerve and conduction in a marine telegraph cable, Ranvier wrote: But what is the role of the myelin sheath itself? It clearly has a protective role; it preserves the axis-cylinder from compression. As it is liquid, or almost liquid, pressures exerted on it are transmitted in every direction and are thus distributed over many points, so that their constrictive action on the axis-cylinder is much reduced. The myelin has also perhaps another role; it is probably an insulating sheath. One knows that electric wires that are immersed in a conducting medium must be insulated from this medium by a nonconducting sheath; it is on this principle that the construction of submarine cables rests. It would be possible—certain facts lead one to believe—that the transmission of sensory or motor impulses has some analogy with the transmission of electricity, and maybe it is convenient that each nerve tube is insulated so that this transmission is more effective. I do not say, I wish you to note, that this insulating myelin sheath is necessary for the transmission of impulses, since we will see to the contrary, in the next lesson, that this transmission is achieved equally well by nerve fibers that lack myelin; nevertheless, I think that this insulation serves to make it more perfect.