Histology of Peripheral Nerve

The structures of the nerve can be identified microscopically. At different magnifications, the definition and various individual components of the nerves can be identified. The upper image (see Plate 6-2) shows a light micrograph transverse section of a peripheral nerve at a magnification of 200× (Masson trichrome stain). In this image, four individual nerve fascicles are seen. The epineurium (Ep) is the connective tissue that envelops and supports the individual fascicles. Each fascicle is surrounded by a dark-appearing band of connective tissue, the perineurium (Pe), which also provides tensile support of the axons. The individual axons within the perineurium and the connective tissue surrounding the axons (endoneurium) are difficult to identify at this magnification.

In the middle light micrograph image (see Plate 6-2), a transverse section of a single nerve fascicle is seen on the light micrography at medium magnification (280×, hematoxylin and eosin [H&E] stain). In this image, the darker-stained perineurium (Pe) can be seen forming a surrounding protective support for the axons within. Inside the perineurium are multiple nerve fibers that are sectioned transversely or obliquely. Many of the fibers are surrounded by myelin sheaths, although the myelin is difficult to see well as a result of the lipid content. Surrounding the individual axons are nuclei of fibroblasts, Schwann cells, and capillary endothelia cells.

The bottom electron micrograph image (see Plate 6-2), is a transverse section of a single individual axon as visualized on electron microscopy at very high magnification (30,000×). The axon is surrounded by perineurial cells (Pe) and collagen fibrils (CF) that constitute the supportive endoneurium. The Schwann cell surrounding the axon is composed of cytoplasm (SC) that wraps around the axon in lamellae forming the myelin sheath (MS); this appears as a thin “onion” covering wrapping around the axon. The thin external basal lamina of the myelin can also be identified. Within the myelin is the axon. Individual organelles, including mitochondria (Mi), neurofilaments, and microtubules can also be seen.

Peripheral nerves and their respective muscle fiber integrity and sensory pathway function are assessed through electrodiagnostic testing (electromyography [EMG]), including motor and sensory nerve conduction studies and needle electromyography. Injury or dysfunction at any motor unit site may lead to limited motor unit function. EMG abnormalities help to identify the site and physiologic basis of the motor unit dysfunction. The peripheral nerves are most commonly affected by genetically determined abnormalities, such as Charcot-Marie-Tooth disease (CMT1a), that are often autosomal dominant. Metabolic-derived conditions, including diabetes mellitus and chronic renal disorders and often occult malignancies producing a paraneoplastic process, as well as various toxins, including medications and certain environmental risks such as arsenic, can also affect peripheral nerve function.

Individual peripheral nerves may be affected by local factors, such as thickening of the transverse carpal ligament producing a carpal tunnel syndrome (CTS) or crossing one’s knees, thus entrapping the common peroneal nerve at the fibular head. With sufficient loss of motor units in a muscle, or with inability of an impulse to conduct along the motor unit, muscle strength diminishes. Primary or conjoint sensory dysfunction is studied with EMG, noting sensory nerve action potentials (SNAPs) absence secondary to an axonal process or prolonged conduction across the transverse carpal ligament typical of the carpal tunnel syndrome.

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