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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.

Both autonomic and enteric nervous systems function involuntarily.

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"It is difficult to believe that until approximately year 1900 it was not known that neurons are the basic units of the brain (Santiago Ram? Equally surprising is the fact that the concept of chemical transmission in the brain was not known until around 1930 (Henry Hallett Dale) and (Otto Loewi).

We began to understand the basic electrical phenomenon that neurons use in order to communicate among themselves, the action potential, in the decade of 1950 (Alan Lloyd Hodgkin, Andrew Huxley and John Eccles).

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.

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.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.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.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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