Peripheral nervous system: The portion of the nervous system that is outside the brain and spinal cord. Abbreviated PNS. The nerves in the PNS connect the central nervous system (CNS) to sensory organs, such as the eye and ear, and to other organs of the body, muscles, blood vessels, and glands. The peripheral nerves include the 12 cranial nerves, the spinal nerves and roots, and the autonomic nerves. The autonomic nerves are concerned with automatic functions of the body, specifically with the regulation of the heart muscle, the tiny muscles that line the walls of blood vessels, and glands.