The nervous system is a network of cells specialized for the rapid transfer and integration of information. The fundamental unit of the nervous system is the neurone which with its processes transmits electrical signals. The cell bodies of neurones tend to be segregated into compact groups (nuclei, ganglia) or into sheets (laminae) that lie within the grey matter of the CNS or are located in specialized ganglia in the PNS. Groups of nerve fibres running in a common direction usually form a compact bundle (nerve, tract, peduncle, brachium, pathway). Many of these nerve fibres are surrounded by sheaths of lipid material called myelin which gives rise to the characteristic appearance of the white matter. In addition to neurones there are glial cells which play a supporting role. There are about five times more glial cells than neurones and they occupy approximately half the volume of the brain.