Organisation of the Human Nervous System

The human nervous system as a whole is organised into two principal divisions that work together. The central nervous system (CNS) comprises the brain and the spinal cord, enclosed and protected within the skull and vertebral column respectively; it is the main integrating and command centre of the body, receiving information, processing and interpreting it, and issuing the appropriate response. The peripheral nervous system (PNS) comprises all the nerve fibres and ganglia that lie outside the CNS — chiefly the cranial nerves arising from the brain and the spinal nerves arising from the spinal cord — and it functions as the communication network that carries information from receptors all over the body inward to the CNS, and carries the CNS's commands outward to muscles and glands.

Functionally, the PNS is further divided into two systems. The somatic nervous system conveys motor commands to skeletal muscle, producing voluntary, conscious movements, and also carries sensory information from the skin, joints and muscles. The autonomic nervous system carries motor commands, largely outside conscious control, to smooth muscle, cardiac muscle and glands, thereby regulating the internal environment of the body; because the organs it controls are the visceral (internal) organs, this division is also referred to as the visceral nervous system. The autonomic/visceral nervous system itself has two further, functionally antagonistic sub-divisions — the sympathetic and the parasympathetic — which are described in a later section, along with local networks of neurons embedded within the walls of organs such as the gut (sometimes called the enteric nervous system) that can operate with a degree of independence while still being influenced by the wider autonomic system. …