Glial Cells and Their Functions

Abstract

Despite the central role of neurons for information processing, there are about tenfold more glial cells in the CNS than there are neurons. Glial cells (or just glia) are close cousins to neurons; both arise from a common ancestral population of progenitor (or stem) cells at the earliest stages of CNS development. Stem cells give rise to neuroblasts and glioblasts that are committed by genetic programming to become, respectively, neurons and glia. The glia of the CNS arise from stem cells in the ventricular zone of the neural tube, and the glia of the PNS arise from progenitors in the neural crest (see Chapter 7). How do glia differ from neurons? Structurally, glia have smaller cell bodies (5 to 10 micrometers in diameter) than most neurons, and they extend only one kind of process rather than two. We can also draw three functional distinctions. First, glia retain the ability to proliferate under certain conditions, whereas neurons are postmitotic. The second and third distinctions are closely related: glia do not conduct action potentials and do not form chemical synapses with other cells. In other words, the glia do not directly participate in the conduction or transmission of information. However, the glia assist the signaling capacity of neurons in many essential ways.