Myelin's a dielectric (electrically insulating) material that forms a layer, the myelin sheath. Usually, myelin surrounds only the axon of a neuron. It's essential for proper functioning of the nervous system. Myelin's an outgrowth glial cell: Schwann cells supply the myelin for peripheral neurons, whereas oligodendrocytes supply it to those of the central nervous system. Myelin's considered a defining characteristic of the (gnathostome) vertebrates, but it's also arisen by parallel evolution… (More on Demyelination)