The Nerve Fiber
The axon (AFG1) is surrounded by a sheath: in unmyelinated nerve fibers by the cyto-plasm of the sheath cells, and in myelinatednerve fibers by themyelin sheath(ABG2). Axon and sheath together are called the nerve fiber. The myelin sheath begins be-hind the initial segment of the axon and ends just before the terminal ramification. It consists of myelin, a lipoprotein produced by the sheath cells. The sheath cells in the CNS are oligodendrocytes; in the peripheralnerves they are Schwann cells, which origi-nate from the neural crest. The myelin sheath of fresh, unfixed nerve fibers appears highly refractile and without structure. Its lipid content makes it birefringent in polarized light. The lipids are removed upon fixation, and the denatured protein scaffold remains as a gridlike structure (neurokeratin) (D3).
At regular intervals (1 – 3 mm), the myelin sheath is interrupted by deep constrictions, the nodes of Ranvier (AB4 F). The segment be-tween two nodes of Ranvier in peripheral nerves, the internode or interannular segment (F), corresponds to the expansion of one sheath cell. The cell nucleus (ADF5) and perinuclear cytoplasm form a slight bulge on the myelin sheath in the middle of the in-ternode. Cytoplasm is also contained in ob-lique indentations, the Schmidt–Lantermanincisures (C, F6) (see also p. 40, A4). The mar-gins of the sheath cells define the node of Ranvier at which axon collaterals (E) may branch off or synapses may occur.