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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.
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.
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.
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.
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).
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.
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.