nervous system

System of interconnected nerve cells of most invertebrates and all vertebrates. It is composed of the central and autonomic nervous systems. It may be as simple as the nerve net of coelenterates (for example, jellyfishes) or as complex as the mammalian nervous system, with a central nervous system comprising brain and spinal cord and a peripheral nervous system connecting up with sensory organs, muscles, and glands.

In a nervous system, specialized cells called nerve cells or neurones carry messages as nerve impulses quickly from one part of the body to another. These impulses may be carrying information about the outside world (stimuli) which allows the body to respond quickly to them. However, much of the information being carried around is to do with organizing processes inside the body (nervous coordination).

In mammals, some examples of processes regulated by the nervous system are changes in heart rate, changes in ventilation rate (breathing rate), the movement of food along the alimentary canal, and changes in the size of the iris which alter the amount of light entering the eye.

The nervous system includes the brain, the area in which collected information is used to make decisions and from where responses are initiated. Information is collected from all over the body, including from specialized sense organs such as the eye. The rest of the nervous system includes the spinal cord and nerve cells carrying information to and from the brain. For example, responses to stimuli involve nerve impulses which are carried along nerve cells from the brain, through part of the spinal cord, and then to muscles or other cells of the body.