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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.
The neurons that give rise to nerves do not lie entirely within the nerves themselves—their cell bodies reside within the brain, spinal cord, or peripheral ganglia Glial cells (named from the Greek for "glue") are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system.
In the human brain, it is estimated that the total number of glia roughly equals the number of neurons, although the proportions vary in different brain areas.
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.
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.
Immediately behind the brain is the subesophageal ganglion, which is composed of three pairs of fused ganglia.
Most nerves serve both functions and are called mixed nerves.
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.
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.
Nerves that transmit signals from the brain are called motor or efferent nerves, while those nerves that transmit information from the body to the CNS are called sensory or afferent.
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.