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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.

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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.

"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).

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.

White matter is composed mainly of myelinated axons, and takes its color from the myelin.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.Typically, each body segment has one ganglion on each side, though some ganglia are fused to form the brain and other large ganglia.

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