Nerve fibres. Coloured scanning electron micrograph (SEM) of a section through bundles of nerve fibres (green). Each is within a sheath, or perineurium (purple). The nerve fibre bundles (fasciculi) are held together by connective tissue (orange). Each fibre consists of a nerve cell axon, the output process of a nerve cell, surrounded by a fatty insulating layer known as a myelin sheath. Myelin sheaths increase the transmission speed of the electrical nerve signals. Nerves in the body serve to collect, interpret and relay information. The fibres pass the information on to other parts of the body, such as the muscles and organs.