Editing Nervous system (section)

===Cells===
The nervous system contains two main categories or types of cells: [[neuron]]s and [[glial cell]]s.

====Neurons====
[[File:Neuron.svg|thumb|upright=1.4|Structure of a typical [[neuron]] with [[Schwann cell]]s in the [[peripheral nervous system]]]]

The nervous system is defined by the presence of a special type of cell—the [[neuron]] (sometimes called "neurone" or "nerve cell").<ref name=KandelCh2/> Neurons can be distinguished from other cells in a number of ways, but their most fundamental property is that they communicate with other cells via [[synapse]]s, which are membrane-to-membrane junctions containing molecular machinery that allows rapid transmission of signals, either electrical or chemical.<ref name=KandelCh2/> Many types of neuron possess an [[axon]], a protoplasmic protrusion that can extend to distant parts of the body and make thousands of synaptic contacts;<ref name=KandelCh4/> axons typically extend throughout the body in bundles called nerves.

Even in the nervous system of a single species such as humans, hundreds of different types of neurons exist, with a wide variety of morphologies and functions.<ref name=KandelCh4/> These include [[sensory neuron]]s that transmute physical stimuli such as light and sound into neural signals, and [[motor neuron]]s that transmute neural signals into activation of muscles or glands; however in many species the great majority of neurons participate in the formation of centralized structures (the brain and ganglia) and they receive all of their input from other neurons and send their output to other neurons.<ref name=KandelCh2/>

====Glial cells====
[[Glial cell]]s (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.<ref name=Allen2009/> 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.<ref name=Azevedo/> 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 [[pathogen]]s and remove dead neurons; and to provide guidance cues directing the axons of neurons to their targets.<ref name=Allen2009/> A very important type of glial cell ([[oligodendrocyte]]s in the central nervous system, and [[Schwann cell]]s 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. Recent findings indicate that glial cells, such as microglia and astrocytes, serve as important resident immune cells within the central nervous system.