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nerve fiber[′nərv ‚fī·bər]
the structural unit along which nerve impulses travel, consisting of the nerve cell outgrowth, or axon, and its surrounding sheath. The axon is also called the axis cylinder.
Nerve fibers are usually 0.5–30 μ thick; some nerve fibers in lower vertebrates and in invertebrates are much thicker (as in the annelid Myxicola, with a nerve fiber thickness of about 1,700 μ). The length of a nerve fiber varies in direct proportion to the size of the animal and can be greater than 1 m. The vertebrate nervous system has medullated, or myelinated, nerve fibers and unmedullated, or unmyelinated, nerve fibers. In both types of fiber, the sheath consists of Schwann’s cells. In unmedullated fibers the Schwann’s cells form the neurilemma, or Schwann’s sheath, which encloses one or more axons. Schwann’s cells in medullated fibers also form the myelin sheath. This consists of a white protein-lipid complex—myelin—which is formed by the repeated convolutions of the Schwann’s cell around the axon. The cytoplasm of the Schwann’s cell is displaced to the periphery of the cell, while the cell membranes “swathe” the axon. The portion of the axon covered in this manner can measure 200 μ to several millimeters in length.
The intervals between adjacent Schwann’s cells are called the nodes of Ranvier (see Figure 1); these are approximately 1 μ long and are free of the myelin sheath. Since the myelin sheath is an insulator, it prevents the current created by a stimulus from jumping to adjacent portions of the axon’s membrane. As a result, the nerve impulse does not spread continuously as it would in an unmyelinated fiber; rather, the impulse in a myelinated fiber passes in stepwise fashion from one node of Ranvier to the next. This method of propagating an impulse is called saltatory conduction. It is a more rapid method than that in unmyelinated fibers.
The rate at which nerve impulses travel along nerve fibers increases in proportion to the thickness of the axons. Several species of animals develop giant nerve fibers to provide for rapid nerve reactions in response to emergency situations. The giant nerve fiber of a squid is approximately 800 μ in diameter; studies using this nerve fiber have revealed several common patterns in the physiology of excitable cell membranes.
D. A. SAKHAROV