Nervous System

nervous system, organized group of cells specialized for the conduction of electrochemical stimuli from sensory receptors through a network to the site at which a response occurs.

Follow the electrical and chemical changes undergone to transmit an impulse through the human nervous systemSee all videos for this article

All living organisms are able to detect changes within themselves and in their environments. Changes in the external environment include those of light, temperature, sound, motion, and odour, while changes in the internal environment include those in the position of the head and limbs as well as in the internal organs. Once detected, these internal and external changes must be analyzed and acted upon in order to survive. As life on Earth evolved and the environment became more complex, the survival of organisms depended upon how well they could respond to changes in their surroundings. One factor necessary for survival was a speedy reaction or response. Since communication from one cell to another by chemical means was too slow to be adequate for survival, a system evolved that allowed for faster reaction. That system was the nervous system, which is based upon the almost instantaneous transmission of electrical impulses from one region of the body to another along specialized nerve cells called neurons.

invertebrate: nervous system

cnidarian nervous system

brain structure of the cat

Nervous systems are of two general types, diffuse and centralized. In the diffuse type of system, found in lower invertebrates, there is no brain, and neurons are distributed throughout the organism in a netlike pattern. In the centralized systems of higher invertebrates and vertebrates, a portion of the nervous system has a dominant role in coordinating information and directing responses. This centralization reaches its culmination in vertebrates, which have a well-developed brain and spinal cord. Impulses are carried to and from the brain and spinal cord by nerve fibres that make up the peripheral nervous system.

This article begins with a discussion of the general features of nervous systems—that is, their function of responding to stimuli and the rather uniform electrochemical processes by which they generate a response. Following that is a discussion of the various types of nervous systems, from the simplest to the most complex.