Electron Microscopy of the Centro-Peripheral Transition of Nerve Fibers in the Mouse Trigeminal Root

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The centro-peripheral transition of nerve fibers in the mouse trigeminal nerve root is located at about 0.5 to 0.6mm distal from the attachment of the nerve root to the pons, where "transition nodes" of the myelinated nerve fibers can be recognized. The transition of the peripheral part of the nerve fibers into the central part takes place abruptly in the transition node. Here, the fibers lose the sheath structure of the Schwann cell and the extracellular tissue space, and become ensheathed by glial components. The basement membrane on the distal surface of the glial limiting membrane in the centro-peripheral boundary zone is continuous with the basement membrane on the outer surface of the Schwann cell at the transition node. Near the transition node a direct contact between the process of the astrocyte and that of the Schwann cell without intervention of the basement membrane is present. This is referred to as the "glia-Schwann cell junction." A corresponding junction occurs also around the unmyelinated fibers near the transition node of the myelinated fibers. The nerve fiber changes its diameter at the centro-peripheral boundary: it is thicker on the peripheral side than on the central side. This seems to account for the abrupt fall in the conduction velocity of the peripheral nerve root as it enters the central nervous system.
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