Nerve fibres. Coloured scanning electron micrograph (SEM) of a bundle of nerve fibres (yellow) protruding from their sheath, or perineurium. A bundle of nerve fibres such as this is known as a fasciculus. The nerve fibres are made up of nerve cell axons, the output processes of nerve cells. Nerves in the body serve to collect, interpret and relay information. The fibres are used to pass the information on to other parts of the body, such as the muscles, the organs or the brain. Magnification unknown.