A bundle of conducting fibres called axons (see AXON) that come from neurones (see NEURON(E)) – the basic units of the NERVOUS SYSTEM. Nerves make up the central nervous system (BRAIN and SPINAL CORD) and connect that system to all parts of the body, transmitting information from sensory organs via the peripheral nerves to the centre and returning instructions for action to the relevant muscles and glands.

Nerves vary in size from the large pencil-sized sciatic nerve in the back of the thigh muscles to the single, hair-sized fibres distributed to the skin. A nerve, such as the sciatic, possesses a strong, outer fibrous sheath, called the epineurium, within which lie bundles of nerve-fibres, divided from one another by partitions of fibrous tissue, in which run blood vessels that nourish the nerve. Each of these bundles is surrounded by its own sheath, known as the perineurium, and within the bundle fine partitions of fibrous tissue, known as endoneurium, divide up the bundle into groups of fibres. The finest subdivisions of the nerves are the fibres, and these are of two kinds: medullated and non-medullated fibres. (See NEURON(E) and NERVOUS SYSTEM for more details on structure and functions of neurons and nerves.)