Most axons in vertebrates haveadditional specialization: They are surrounded by a discontinuous myelinsheath consisting of many concentric layers of membrane.The myelin sheath is a reasonably effective electrical insulation for the segments of the axon that it envelops.Themyelin sheath of neurons in the CNS is formed byoligodendrocytes; in the peripheral nervous system(PNS), the myelin sheath is formed by Schwann cells (seeFigure 13-1), each of which wraps layer after layer of itsown plasma membrane around the axon in a tight spiral(Figure 13-13).For depolarization to spread from one siteto the rest of the neuron, the action potential must stillbe renewed periodically down the axon.