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| Nodes of Ranvier
Nodes of Ranvier occur regularly along the course of central myclinated fibres; the segment between two nodes is referred to as internodal myclin, and the region where the lamellae terminate is the paranode. The thickness of the myelin sheath is related to axonal diameter: larger axons are surrounded by thicker myelin sheaths. Evidence from experimental animals has convincingly shown that the formation of myelin is preceded by the proliferation of glial precursors which develop into young oligodendrocytes. The processes of these active oligodendrocytes are frequently wrapped around axons, but this direct connection is difficult to demonstrate in later life. Biologically, the formation of myclin is a two-step process: first, the oligodendrocyte relates to the axon in response to an axonal myelinogenic stimulus and, second, it produces myelin, the volume of which is determined by the internodal axonal surface area. Mvelination in man progresses slowly and generally proceeds centripetally. It commences peripherally and whist many axons, particularly in the spinal cord are myclinated at birth, the more central fibres in the frontal and parietal lobes remain unmyclinated. Completion of myelin formation is achieved largely during the first two years after birth. Immunohistochemistry for myelin basic protein confirms earlier findings of the chronological sequence of myelination: phylogenetically older regions acquire myelin first. The sequential nature of myelination reflects physiological demand: the time and rapidity of myelination are related to the relative significance of a fibre system at various periods of cerebral development.
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