Abstract
Nerve trunks are composed of bundles of axons organized by connective tissues into fascicles. Nonneural glial (Schwann) cells, fibroblasts, and endothelial cells are also present. Nerves can contain motor, sensory, or mixed fibers, depending on their target and related function. Motor fibers conduct electrical impulses at a more rapid rate than sensory fibers. The presence of myelin profoundly enhances the velocity of impulse transmission by allowing saltatory conduction. A myelinated fiber has a compact myelin sheath, a lipid and protein bilayer formed by a Schwann cell wrapped spirally around the axon. Individual Schwann cells meet along fibers at the nodes of Ranvier; here, small gaps in the myelin exist and the axon is surrounded only by the Schwann cell basal lamina. Small bundles of nonmyelinated axons can be encircled by a single Schwann cell, but no myelin is present.
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