Grey and White Matter
Grey matter (or gray matter) is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil (dendrites and unmyelinated axons), glial cells (astroglia and oligodendrocytes), and capillaries (Figure 1). Grey matter is distributed at the surface of the cerebral hemisphere's cerebellum, as well as in the depths of the cerebrum, cerebellar, brainstem, and spinal grey matter. In living tissue, grey matter actually has a grey-brown color, which comes from capillary blood vessels and neuronal cell bodies. Grey matter includes regions of the brain involved in muscle control, sensory perception, such as seeing and hearing; memory, emotions, and speech.
A second major component of the central nervous system is white matter and it is composed of bundles of myelinated axons that connect various grey matter regions of the nervous system to each other and carry nerve impulses between neurons (Figure 2). White matter only contains the axons of the nerve cells, and not the cell bodies, which are found in grey matter. Myelin is a lipid that forms a thin layer, known as the myelin sheath, around the axons of white matter neurons It acts as an electrical insulator, increasing the transmission speed of nerve signals by allowing the signal to jump down the axon. Myelin also gives white matter its characteristic color. At the age of 20, the total length of myelinated fibers in the body, if places end to end, is 176,000 km (109.4 miles) in males and is 149,000 km (92.6 miles) in females.
Using a computer network as an analogy, the gray matter can be thought of as the actual computers themselves, whereas the white matter represents the network cables connecting the computers together.