The human brain contains more than a billion neurons which interconnect
to form networks that process, store, and recall sensory information.
These neuronal activities are supported by a group of accessory brain
cells coll- tively known as neuroglia. Surprisingly, glial cells are ten
times more - merous than neurons, and occupy more than half the brain
volume (Hydén, 1961). Although long considered a passive, albeit
necessary, component of the nervous system, many interesting and unusual
functional properties of glial cells are only now being brought to
light. As a result, the status of these cellular elements is approaching
parity with nerve cells as a subject for experimental study. The term
glia (or glue) seems today to be a misnomer in view of the diverse
functions attributed to glial cells. Experimental studies in the last
three decades have clearly established that the behavior of glial cells
is far from passive, and that they are at least as complex as neurons
with regard to their membrane properties. In addition, glial cells are
of importance in signal processing, cellular metabolism, nervous system
development, and the pathophysiology of neurological diseases. The
Müller cell of the ver- brate retina provides a splendid example of an
accessory cell that exhibits features illustrating every aspect of the
complex behavior now associated with glial cells.
