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The enteric nervous system functions to control the gastrointestinal system.
Both autonomic and enteric nervous systems function involuntarily.
Immediately behind the brain is the subesophageal ganglion, which is composed of three pairs of fused ganglia.
Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago.
In vertebrate species it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The PNS consists mainly of nerves, which are enclosed bundles of the long fibers or axons, that connect the CNS to every other part of the body.
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.
A protoplasmic fiber runs from the cell body and branches profusely, with some parts transmitting signals and other parts receiving signals.
Nerves are large enough to have been recognized by the ancient Egyptians, Greeks, and Romans, but their internal structure was not understood until it became possible to examine them using a microscope.