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There is an anatomical convention that a cluster of neurons in the brain or spinal cord is called a nucleus, whereas a cluster of neurons in the periphery is called a ganglion.There are, however, a few exceptions to this rule, notably including the part of the forebrain called the basal ganglia Arthropods, such as insects and crustaceans, have a nervous system made up of a series of ganglia, connected by a ventral nerve cord made up of two parallel connectives running along the length of the belly.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.

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 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.The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state.

The enteric nervous system functions to control the gastrointestinal system.Among the most important functions of glial cells are to support neurons and hold them in place; to supply nutrients to neurons; to insulate neurons electrically; to destroy pathogens and remove dead neurons; and to provide guidance cues directing the axons of neurons to their targets.A very important type of glial cell (oligodendrocytes in the central nervous system, and Schwann cells in the peripheral nervous system) generates layers of a fatty substance called myelin that wraps around axons and provides electrical insulation which allows them to transmit action potentials much more rapidly and efficiently.It was in the decade of 1960 that we became aware of how basic neuronal networks code stimuli and thus basic concepts are possible (David H. The molecular revolution swept across US universities in the decade of 1980.It was in the decade of 1990 that molecular mechanisms of behavioral phenomena became widely known (Eric Richard Kandel)." A microscopic examination shows that nerves consist primarily of axons, along with different membranes that wrap around them and segregate them into fascicles.It controls the mouthparts, the salivary glands and certain muscles.