D is the efferent or motor nerve fibre, taking an instruction from the central nervous system (in this case the spinal cord) to a muscle (the biceps) that will contract in response to the instruction. The body of the motor nerve cell is located inside the spinal cord. Motor nerve fibres are subtly different from sensory nerve fibres and are subject to a dreadful disease, called motor neurone disease, where motor functions are gradually lost until death occurs.
