Under myelin, there is increased resistance to movement of charges across the membrane, resulting in rapid propagation of the voltage signal within the axonal cytoplasm to the next node. In non-myelinated axons, the action potential must propagate continuously along the plasma membrane. The purpose of the myelin sheath is to increase the speed of transmission by acting as an electrical insulator. The long, myelin-covered section of axon between nodes is the internode. For efficient propagation of the action potential, it is crucial that the voltage-gated sodium and potassium channels remain in distinct regions. On the side of this junction opposite the node, termed juxtaparanode, there is a high concentration of voltage-gated potassium ion channels that facilitate the return of the membrane voltage to baseline following an action potential. This junction provides scaffolding within the axon to compartmentalize molecules within the axon and restrict the movement of ion channels within the axonal membrane. In the regions directly adjacent to the nodes, termed the paranodes, loops of myelin form a tight, septate-like junction (SpJ) with the axonal membrane. The nodes contain high concentrations of voltage-gated sodium ion channels, responsible for raising membrane voltage during the creation of an all-or-nothing action potential. Although it is bare of myelin at the node, the axon is in direct contact with the microvilli of the Schwann cells in the PNS, or with processes of astrocytes in the CNS (Figure 1). The nodes of Ranvier are characterized by short (1um), specialized regions in the axonal membrane that are not insulated by myelin.
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