The Structural Heterogeneity of Central Nervous Tissue

Abstract

Of all the tissues in the body, the nervous system possesses cells with the most extended form and the greatest range of size. In an animal like man, nerve cells can be from 20 or 30 μm to 2 m long. Their shapes can be just as various. In nearly all parts of the nervous system, neurons and parts of neurons of various dimensions and shapes are intertwined together in a tissue of extraordinary complexity and heterogeneity. This heterogeneity expresses two of the organizational principles of the nervous system. The first principle is the segregation of functions, which is evidenced, at least morphologically, by the appearance of white matter and grey matter, the clustering of the peri-karya of neurons having similar functional relations into nuclei, and the bundling of nerve fibres into tracts, commissures, and fasciculi; in short, by the whole apparatus of traditional neuroanatomy. This anatomical complexity is actually a consequence of a more fundamental principle inherent in the architecture of neurons, the segregation of functions within the cell. The second principle is expressed at a cellular level, by the enormous extent of the cell surface, the spatial separation of receptive, conducting, and transmitting portions of the cell, the almost complete restriction of protein synthesis to the perikaryon, and the differential distribution of intracellular organelles in the cytoplasm. The nerve cell itself is thus divided up into compartments, some of which have clear morphological boundaries set by quantitative differences in their contents or by membranous interfaces.