Myelin is a critical element of the central and peripheral nervous systems of all higher vertebrates. As nervous systems evolved to be more complex, high densities of conducting axons required a design shift in
favor of much smaller energy-efficient fibers, with higher conduction velocities. The solution was myelination, which is a tight wrapping of axons by multiple layers of lipid-rich membrane synthesized by Schwann cells in the PNS and oligodendrocytes in the CNS. The myelin wraps are interrupted at regular intervals by nodes of Ranvier, where
the unmyelinated axolemma, rich in Na channels, is exposed to the extracellular space; it is here that action currents are regenerated allowing rapid electrotonic current flow to the next node, repeating the excitation/ conduction cycle, and effecting saltatory action potential propagation
(Debanne et al., 2011; Nave and Trapp, 2008).