Introduction

Virchow initially described glial cells in 1846 as cellular elements that filled in the spaces between neurons but were distinct from neurons. Dieters further defined the description of glial cells in 1865 as cells that did not possess an axon. The concept that glial cells were a heterogeneous population of cells was realized by Golgi, who developed a technique of staining cells that allowed details of cellular morphology to be closely scrutinized. Golgi's studies indicated that glial cells had variation in their morphologies and led to subsequent subclassification of glial cells. Glial cells in fact outnumber neurons in a ratio of 10:1 and constitute 50% of the cellular volume of the central nervous system (CNS; not more than 50% because of their relatively smaller size compared to neurons).

The implication that glial cells act as a form of inert Styrofoam packing between neurons became an entrenched idea from these initial studies, and it is only within the last two decades that the true range and diversity of glial cell function have been fully realized. The impression of glial cells as inert entities with a purely supportive role in part may have arisen during the pioneering electrophysiological work in the 1930s to 1950s, when all interest was focused on cells that were electrically excitable and could fire action potentials. Because glia are incapable of firing action potentials, there was relatively little interest in them. It was not until the 1960s that the studies of Steven Kuffler suggested that glial cells, although incapable of firing action potentials, did indeed have specific roles in the nervous system. It is now realized that neurons cannot function as they should without normally functioning glial cells in close proximity. Under normal conditions there is a steady flow of information between neurons and glia. Glial cells are equally important under pathological conditions in the CNS.

Encyclopedia of the Human Brain Volume 3

Copyright 2002, Elsevier Science (USA).

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They can contribute to the pathology, for example, by adding the excitotoxin glutamate to brain extracellular space (ECS) or potentially limit injury by minimizing ionic changes in ECS or providing energy substrate to neurons, which do not possess any internal energy stores. Thus, to understand pathological processes it is necessary to understand the glial reactions to the pathology. Current research on glial cells indicates that new properties and functions are still being assigned to them.

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