The peripheral nervous system includes the nerves leading from the brain and spinal cord to the rest of the body. It is made up of 31 pairs of nerves coming out of the spinal column openings (spinal nerves) and 12 pairs of nerves coming out of the opening in the skull (cranial nerves).

A peripheral nerve may be made up of cells carrying information from the brain to the body (motor messages) or carrying information from the body to the brain (sensory messages). The spinal nerves carry both sensory and motor messages and are called "mixed nerves." Among the cranial nerves are five pairs of motor nerves, three pairs of sensory nerves, and four pairs of mixed nerves.