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The nervous system derives its name from nerves, which are cylindrical bundles of fibers (the axons of neurons), that emanate from the brain and spinal cord, and branch repeatedly to innervate every part of the body.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.

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Recent findings indicate that glial cells, such as microglia and astrocytes, serve as important resident immune cells within the central nervous system.

The vertebrate nervous system can also be divided into areas called grey matter ("gray matter" in American spelling) and white matter.

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.

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.

In the insect nervous system, the brain is anatomically divided into the protocerebrum, deutocerebrum, and tritocerebrum.

Immediately behind the brain is the subesophageal ganglion, which is composed of three pairs of fused ganglia.

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.

Grey matter (which is only grey in preserved tissue, and is better described as pink or light brown in living tissue) contains a high proportion of cell bodies of neurons.

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