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.

Human nervous system The human nervous system represents the product of millions of years of evolution, particularly in the degree of encephalization or brain complexity. It can be divided into central and peripheral parts for descriptive purposes, although there is both anatomical and functional continuity between the two parts. The central nervous system consists of the brain and the spinal cord. The peripheral nervous system is not so clearly subdivided, but its anatomical parts are: (1) the spinal nerves; (2) the cranial nerves; and (3) the autonomic nervous system.

Positioning of the nervous system In the simplest nervous system, found in the coelenterates, certain sensory cells on the surface send signals to deeper muscle cells or glands either directly or via a diffuse network of interconnected nerve cells. A diffuse nervous system lying quite superficially is common to all coelenterates, and it is only in the flatworms that a head ganglion develops which receives information from specialized rather than non-specific sensory cells. In the higher invertebrates, the diffuse nerve net becomes condensed into longitudinal pathways which lead to the head ganglion. In addition the nervous system retreats from its superficial plane and comes to lie beneath the body musculature; this evolutionary migration of the nervous system is reflected in the development of the human nervous system which is derived from ectodermal (skin) layers of the embryo. In higher animals the sensory cells themselves come to lie close to the central nerve cord and maintain their connection with the skin by fibres with specialized nerve endings. In vertebrates, the cephalic or head end of the nervous system undergoes relative expansion to form the brain, which integrates the information received from the multitude of sensory cells via the dorsal nervous trunk or spinal cord.