Glial cells, commonly called neuroglia or simply glia, are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system. In the human brain, glia are estimated to outnumber neurons by as much as 50 to 1.
Traditionally glia are thought to lack certain features of neurons. For example, glia are not believed to have chemical synapses, nor do they generate action potentials or release neurotransmitters. They were considered to be the passive bystanders of neural transmission. However, recent studies completely reversed these dogma. For example, astrocytes are crucial in clearance of neurotransmitter within the synaptic cleft, which temporally and spatially restricts neurotransmission and limit the toxicity of certain neurotransmitters such as glutamate. And at least in vitro astrocytes can release neurotransmitter glutamate in response to certain stimulation. Another unique type of glia, the oligodendrocyte precursor cells or OPCs, have very well defined and functional synapses from at least two major groups of neurons. The only notable differences between neurons and glia, by modern scrutiny, are the ability to generate action potentials and the polarity of neurons, namely the axons and dendrites which glia lack. It is inappropriate nowadays to consider glia as 'glue' in the nervous system as the name implies. They are also crucial in the development of nervous system and in processes such as synaptic plasticity and synaptogenesis.
