Pathophysiology of Peripheral Nerve Injury

Abstract

Peripheral nervous system (PNS) is a complex construction, which serves dual purpose. Firstly, it disseminates information from the central nervous systems and ensures that this information is interpreted to the target end organs. Secondly, it collects information from the periphery, translates it to nerve signals, processes it and feeds it back to the central nervous system. The PNS consists of a complex arborisation of peripheral nerves. In order to set a stage for the information that will be presented further on, I will shortly review the relevant anatomy first. The peripheral nerves are long extension of neuronal cells, which cells bodies are located in the spinal chord and dorsal root ganglia (spinal nerves) or in the brain (cranial nerves). The peripheral nerve consists of nerve fibres and supportive connective tissue. The connective tissue is organised longitudinally surrounding the nerve fibres and serves a double function. Firstly, it provides mechanical support for the nerve fibres to withstand stretching and compression during the body movements. Secondly, it contains blood vessels – vasa nervorum, which ensure trophic support for the fibres (Gray 1995). The connective tissue is organised in three “layers”. The outermost layer – epineurium – is a thick layer of connective tissue which ensheaths the nerve and isolates it from the external environment (Fig.1). The vasa nervorum are continued within this layer and these vessels communicate abundantly with the network of arterioles and venules found in the connective tissues in the depth of the nerve. The amount of epineurium differs depending on the individual, thickness of the nerve and location. There is an evidence that epineurium is thicker around joints (Sunderland 1978). Deep to epineurium, the axonal fibres are organised in one (unifascicular) or more (multifascicular) fascicles. The fascicles are enclosed within the second layer of connective tissue – perineurium (Fig.1). The perineurium is a thick and mechanically strong layer, which is composed of epithelium-like cells and collagen fibres. The cells are typically organised in several layers separated by collagen with ample vascular structures running longitudinally (Thomas and Jones 1967). This stratification gives perineurium a great endurance and ability to withstand a pressure in excess of 200 mmHg (Selander and Sjostrand 1978). Deep to perineurium the endoneurium is found (Fig. 1). It consists of loose collagenous matrix enveloping the nerve fibres and providing further protection from mechanical forces. The endoneurium also contains several important cell types. The most abundant one are Schwann cells, followed by fibroblasts, endothelial-like cells, macrophages and mastocytes (Causey and Barton 1959). It is important to note that endoneurium contains ample extracellural matrix and fluid, which is contained at a slightly higher pressure that that surrounding perineurium (Myers et al. 1978). The reason for that is unknown, although we can speculate that it protects