Light micrograph of a section through the spinal cord, showing grey matter (yellow) and white matter (brown). The spinal cord is a downwards extension of the brain, and is responsible for relaying sensory and motor signals between the brain and the body. The grey matter consists of neuron (nerve cell) bodies, unmyelinated nerve fibres, support cells and capillaries. White matter consists solely of myelinated nerve fibres. Myelin is an insulating fatty layer that surrounds some nerve fibres. Its presence increases the speed at which nerve impulses travel.