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The nervous system is the part of an animal's body that coordinates its actions and transmits signals to and from different parts of its body.Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago.Typically, each body segment has one ganglion on each side, though some ganglia are fused to form the brain and other large ganglia.The head segment contains the brain, also known as the supraesophageal ganglion.White matter includes all of the nerves, and much of the interior of the brain and spinal cord.Grey matter is found in clusters of neurons in the brain and spinal cord, and in cortical layers that line their surfaces.
The PNS is divided into a) somatic and b) autonomic nervous system, and c) the enteric nervous system. The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems.
The sensory information from these organs is processed by the brain.
In insects, many neurons have cell bodies that are positioned at the edge of the brain and are electrically passive—the cell bodies serve only to provide metabolic support and do not participate in signalling.
Both autonomic and enteric nervous systems function involuntarily.
Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves.