peripheral nerves

The principal structural component of peripheral nerve is the nerve fiber (an axon with its Schwann cells and myelin sheath).

A nerve consists of numerous fibers that are grouped into fascicles by connective tissue sheaths. Myelinated and unmyelinated nerve fibers are intermingled within the fascicle.

In the sural nerve, the nerve that is most commonly examined by biopsy and a relatively pure sensory nerve, myelinated fibers range between 2 and 16 μm in diameter and have a bimodal distribution; the smaller axons, which average 4 μm, are about twice as numerous as the larger axons, which average 11 μm.

Structure

Peripheral nerves consist of fascicles that contain myelinated and unmyelinated axons. Endoneurium is the small amount of matrix that is present between individual axons. The perineurium is a sheath of special, fiber-like cells that ties the axons of each fascicle together. Epineurium is the connective tissue that surrounds the entire nerve trunk and gives off vascular connective tissue septa that traverse the nerve and separate fascicles from one another.

Myelin sheath

Peripheral nervous system (PNS) axons are myelinated in segments (internodes) separated by nodes of Ranvier. A single Schwann cell supplies the myelin sheath for each internode.

The thickness of the myelin sheath is directly proportional to the diameter of the axon, and the larger the axon diameter, the longer is the internodal distance.

Myelin in the PNS is similar in overall lipid and protein composition to central nervous system (CNS) myelin; however, PNS myelin contains a higher proportion of sphingomyelins and glycoproteins. Some myelin proteins are specific to PNS myelin; others are shared with CNS myelin.

Myelin protein zero (MPZ) is the major protein, making up over 50% of PNS myelin protein. MPZ is a transmembrane protein, which functions in the compaction of apposing lipid bilayers of myelin.

Myelin basic protein is the second most abundant protein; it is located topographically on the internal surface of bilayers at the major dense line of myelin.

Peripheral myelin protein 22 (PMP22) is a 22-kDa transmembrane protein located in compacted myelin. The periaxin gene encodes two proteins that are involved in the maintenance of myelin.

Early growth response 2 is a transcription factor that regulates the expression of these myelin proteins and is essential for the maintenance of myelin.

Unmyelinated axons, which are far more numerous than myelinated axons, range in size from 0.2 to 3 μm.

Schwann cells

The cytoplasm of one Schwann cell envelops, and isolates from each other, a variable number of unmyelinated fibers (5 to 20 axons in humans). The Schwann cells associated with either myelinated or unmyelinated fibers have pale oval nuclei with an even chromatin distribution and an elongated bipolar cell body. By electron microscopy, Schwann cells have a basement membrane, unlike neighboring endoneurial fibroblasts and histiocytes.

Axon structure

Peripheral axons contain organelles and cytoskeletal structures, including microfilaments, neurofilaments, microtubules, mitochondria, vesicles, smooth endoplasmic reticulum, and lysosomes. Dense-core granules and coated vesicles are located in the nerve terminals. Protein synthesis does not occur in the axon. Instead, axoplasmic flow delivers proteins and other substances synthesized in the perikaryon down the axon.7,8 A retrograde transport system serves as a feedback system for the cell body.

Connective tissue components

There are three major connective tissue components of peripheral nerve:

 the epineurium, which encloses the entire nerve;
 the perineurium, a multilayered concentric connective tissue sheath that encloses each fascicle;
 the endoneurium, which surrounds individual nerve fibers. The nerve microenvironment is regulated by the perineurial barrier (formed by the tight junctions between perineurial cells), the blood-nerve barrier,9 and the nerve-cerebrospinal fluid (CSF) barrier.

Endoneurial capillaries derive from the vasa nervorum, and their endothelial cells form tight junctions to establish the blood-nerve barrier.

This barrier has been found to be relatively less competent within nerve roots, dorsal root ganglia, and autonomic ganglia than along the rest of the nerve.

The nerve-CSF barrier is formed by the tight junctions between the cells that form the outer layer of the arachnoid membrane. These cells fuse with the perineurium of the roots and cranial nerves as they leave the subarachnoid space.

The motor and sensory fibers, which are separated within anterior and posterior roots, intermingle within the mixed sensorimotor nerves that exit the spinal canal.

Pathology

 peripheral nervous lesions
 peripheral nervous lesional syndromes
 peripheral nervous diseases
 peripheral nervous tumors

Videos

 Histology of the peripheral nerve (by Washington Deceit)

@<@object width="425" height="350">@<@param name="movie" value="http://www.youtube.com/v/Z0Ci42y-UJY">@<@/param>@<@param name="wmode" value="transparent">@<@/param>@<@embed src="http://www.youtube.com/v/Z0Ci42y-UJY" type="application/x-shockwave-flash" wmode="transparent" width="425" height="350">@<@/embed>@<@/object>

See also

 Nervous system

• peripheral nervous system