The brain and spinal cord together form the central nervous sytem (CNS). Twelve cranial nerves leave each side of the brain (see NERVES, below) and 31 spinal nerves from each side of the cord: together these nerves form the peripheral nervous system. Complex chains of nerves lying within the chest and abdomen, and acting largely independently of the peripheral system, though linked with it, comprise the AUTONOMIC NERVOUS SYSTEM and govern the activities of the VISCERA.
The control centre of the whole nervous system is the brain, which is located in the skull or cranium. As well as controlling the nervous system it is the organ of thought, speech and emotion. The central nervous system controls the body’s essential functions such as breathing, body temperature (see HOMEOSTASIS) and the heartbeat. The body’s various sensations, including sight, hearing, touch, pain, positioning and taste, are communicated to the CNS by nerves distributed throughout the relevant tissues. The information is then sorted and interpreted by specialised areas in the brain. In response these initiate and coordinate the motor output, triggering such ‘voluntary’ activities as movement, speech, eating and swallowing. Other activities – for example, breathing, digestion, heart contractions, maintenance of BLOOD PRESSURE, and ﬁltration of waste products from blood passing through the kidneys – are subject to involuntary control via the autonomic system. There is, however, some overlap between voluntary and involuntary controls.