Lakhasly

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.

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). Because each Schwann cell surrounds

short segment (about 1 mm) of a single axon, manySchwann cells are requiredencase a PNS axon with adiscontinuous sheath of myelin. Myelination decreases theability of the neuronal membraneretain electric charge(i.e., myelination decreases its capacitance), permitting adepolarization event to spread farther and faster thanwould along a nonmyelinated axon.

Myelination doeseliminate the need for propagation, however. For depolarization to spread from one siteto the rest of the neuron, the action potential must stillbe renewed periodically down the axon. This happens atthe nodes of Ranvier, interruptions in the myelin layerthat are spaced just close enough together (1-2 mm) toensure that the depolarization spreading out from an action potential at one node is still strong enough to
bring an adjacent node above its threshold potential
(Figure 13-14).