myelin

Myelin is a dielectric, electrically insulating material of protein and fat that forms a layer, the myelin sheath, usually around only the axon of a neuron. Its composition is well specified: about forty per cent of myelin is water, and its dry mass is roughly seventy to eighty-five per cent lipids and fifteen to thirty per cent proteins. Among its constituent proteins are myelin basic protein, myelin oligodendrocyte glycoprotein, and proteolipid protein, while its primary lipid is a glycolipid, galactocerebroside. The intertwining hydrocarbon chains of sphingomyelin serve to strengthen the sheath. That lipid-dominated composition is the basis of its insulating property, since the material surrounds the axon rather than the cell body or dendrites.