The myelin sheath of peripheral nerve fibers is surrounded by the cytoplasm of the Schwann cell. The outer cell membrane borders on a basal lamina, which envelops the entire peripheral nerve fiber. In the three-dimensional reconstruction, they appear as spirals in which the cytoplasm communicates between the inside and outside. At the node of Ranvier, the Schwann cell processes slide over the paranodal region and over the axon. They interdigitate and thus form a dense envelope around the node of Ranvier. In the peripheral nerve fiber, there is a regular relationship between the circumference of the axon, the thickness of its myelin sheath, the distance between the nodes of Ranvier, and the conduction velocity of a nerve fiber. The larger the circumference of an axon, the thicker the enclosingmyelin sheath and the longer the internodes. When myelinated nerve fibers are still growing (e.g., in the nerves of the limbs), the internodes are growing in length. The longer the internodes, the faster the conduction velocity of the fiber. We distinguish between myelinated, poorly myelinated, and unmyelinated nerve fibers; also referred to as A, B, and C fibers.
The myelinated A fibers have an axonal diameter of 3–20 µm and a conduction velocity of up to 120 m/s; the poorly myelinated B fibers are up to 3 µm in diameter and have a conduction velocity of up to 15 m/s. Conduction velocity is the slowest in the unmyelinated C fibers (up to 2 m/s); it is a continuous spread of excitation. By contrast, conduction in myelinated nerves is saltatory, that is, it takes place in jumps. The morphological basis of saltatory conduction is the alternation of myelinated internodes and unmyelinated nodes of Ranvier; the current inside the axon jumps from one node to the next, and the current circuit is closed each time at the nodes through changes in the permeability of the axolemma (triggered by voltage-gated ion channels). This mode of conduction is much faster and requires less energy than the continuous spread of excitation.
The peripheral nerve fiber is surrounded by longitudinal collagenous connective-tissue fibrils; together with the basal membrane, they form the endoneural sheath. The nerve fibers are embedded in a loose connective tissue, the endoneurium. A variable number of nerve fibers is collected into bundles or fascicles by the perineurium which consists mainly of circular fibers. The innermost layer of the perineurium is formed by endothelial cells that enclose the endoneural space in several thin layers. The perineural endothelial cells possess a basal membrane at their perineural and endoneural surfaces and are joined together by zonulae occludentes (tight junctions). They represent a barrier between nerve and surrounding tissue, similarly to the endothelial cells of cerebral capillaries. The mechanical strength of the peripheral nerve is based on its content of circular elastic fibers. In the nerves of the limbs, the perineurium is reinforced in the joint regions.