The Microscopic structure of the nervous system: Its function

Abstract

The essential component of the system is the nerve cell with its dendrites and its prolongation, the axon (Figs. 2.1-2.3). Young (1945) characterized the axon as a very long cylinder of a semi fluid nature. It is a column of neuronal cytoplasm, the axoplasm enclosed by a cell membrane, the axolemma. Thomas et al. (1993a) described the axoplasm as a fluid cytosol in which are suspended formed elements. The most conspicuous of the latter is the cytoskeleton consisting of neurotubules, neurofilaments and matrix. In addition, there are mitochondria, axoplasmic reticulum, lamellar and multivesicular bodies, and membranous cisterns, tubes and vesicles. It is the cytoskeleton that provides the apparatus for axoplasmic transport. Berthold et al. (2005) describe the axolemma as a three-layered unit membrane about 8 nm thick and consider that it: conveys signals between the neurone and its Schwann cells that control the proliferative and myelin producing functions of the Schwann cells and partly regulate axon size. The glial cells of the peripheral nervous system are essential for the development, maturation, survival and regeneration of the neurone. The relationship between the axon and the Schwann cell is lifelong. The myelinating and non myelinating Schwann cells are the main peripheral glial cells. There are others, which include the satellite cells surrounding cell bodies in the dorsal root and autonomic ganglia, the glia of the enteric system; the teloglia (terminal Schwann cells) at the terminals of somatic motor axons and the glia associated with sensory terminals such as the Pacinian corpuscle. Mirsky and Jessen (2005) observe: evidence to date suggests that the molecular and morphological differences between these various cells depend on the specific location and cellular environment in which they are found and that the glial cells of the PNS retain unusual plasticity throughout life. King (Berthold et al. 2005) estimated that about 10% of nuclei within the endoneurium of a normal peripheral or spinal nerve root are fibroblasts, and that endogenous macrophages account for between 2% and 9%. Most of the remainder are Schwann cells. Whilst mast cells are also seen their function is not well understood. © 2011 Springer-Verlag London Limited.