Basic organisation of nerve cells

The very title of this chapter would have been contentious in the nineteenth century, when detailed scientific study of the nervous system got underway. For it was then not generally agreed that the nervous system is composed of many individual nerve cells. This was mainly due to the fact that nerve cells are difficult to visualise with routine histological methods. Many cells are packed tightly together in nervous tissue (there are 100000 nerve cells in 1mm3 of human brain) and they give off fine, branched processes, so that it is almost impossible to determine the limits of a single cell. Many scientists therefore believed that nerve cells were fused together in a continuous network of branched processes, rather like the capillary beds that link small arteries and veins.

The technique that was most powerful in challenging this view was silver staining, first discovered by Camillo Golgi in 1873 and developed by others, particularly Santiago Ramon y Cajal from 1888 onwards. Ramon y Cajal examined many parts of the nervous system in a wide range of animal species. He realised that the special feature of silver staining is that it only stains a small percentage of cells in a piece of tissue but it stains them in their entirety, so that the structure of an individual nerve cell can be described (Fig. 2.1). Nowadays, single neurons are often stained by the intracellular injection of dye through a microelectrode, or by the use of methods that recognise chemicals characteristically found in particular neurons.