grey matter

grey matter comprises those regions in the brain and spinal cord that consist predominantly of the ‘cell bodies’ of nerve cells (neurons).

The nervous system is composed of many types of body tissue, but the most important is nervous tissue itself. The component cells are nerve cells or neurons. Just like cells elsewhere in the body, each neuron has a cell body, with a nucleus (containing the chromosomes with the genetic programs) and cytoplasm for metabolism and the production of proteins. But neurons also have several rather special features, in particular two types of processes extending out from the cell body. Typically, there is a nerve fibre or axon, which can be a metre or more in length, along which impulses travel to convey information to other neurons, or to muscles or glands. In addition, there are several dendrites (from the Greek for ‘wood’ or ‘tree’), up to 2 mm long, on which the axons of other neurons terminate, forming synapses.

In the central nervous system (CNS) — made up of the brain and spinal cord — nerve cell bodies are usually found closely packed together in characteristic regions. Such a concentration of cell bodies, together with their dendrites, but with a relatively small proportion of axons, appears grey in a fresh cut through the brain or cord and is called ‘grey matter’. It forms a relatively thin surface layer, called ‘cortex’ over the cerebral hemispheres and the cerebellum. The spinal cord has a central core of grey matter, which has the shape of a butterfly when cut across. Elsewhere, there are clumps of grey matter called ‘nuclei’, making up such structures as the hypothalamus, the thalamus, and the basal ganglia.

Where mainly axons are concentrated, in most of the rest of the brain and around the outside of the grey matter of the spinal cord, the predominant colour when cut is whitish, because many of the axons are surrounded by a pale, fatty sheath, consisting of myelin. This forms the white matter.

The brain, and particularly the cerebral cortex, has for long been associated with ‘intellect’. The use of the expression ‘grey matter’ has, therefore, appeared in common parlance to mean intelligence, but there is little evidence to suggest that this quality is directly connected to the size of an individual brain.

Laurence Garey

See nervous system.See also central nervous system; nerves.