  Figure 1 A single oligodendrocyte may support myelin internodes for 60 or more neighboring axons. A major consequence of myelination is organization of the microenvironment of the larger-diameter axons. Sodium channels are clustered at segments of the axon between myelin segments (the nodes of Ranvier), and potassium channels are diffusely distributed under the myelin-invested segments of the axons. This arrangement allows myelinated axons to rapidly propagate action potentials from the axonal cell body distally to its synapse by saltatory conduction.93 Depolarization of the nodes of Ranvier in sequential fashion results in rapid propagation of the neural impulses in the largest-diameter nerve fibers at rates in excess of 100 m/sec. The upper part of the figure depicts a neuron with a myelinated axon. At the right corner, the neuron is magnified and cut in cross section, demonstrating the concentric lamellar structure of myelin. The middle of the figure shows a neuron with a myelinated axon forming a synapse with another neuron. Two oligodendrocytes are shown; each cell myelinates multiple segments of more than one axon. The bottom neuron has been partially demyelinated, and it is surrounded by T cells, which secrete inflammatory cytokines (interleukin-2 [IL-2], interferon gamma [IFN-y], and tumor necrosis factor-a [TNF-a]), and by macrophages, which are stripping myelin from the axon. The macrophages contain myelin debris in phagocytic vacuoles. Conduction in the demyelinated axon is blocked. A blood vessel in cross section shows T cells adhering to the lumen and crossing from the vessel into the brain.
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