The brain contains two major classes of cells: neurons and glia. While both are neural cells, conventionally, the fundamental difference between the two classes has been understood to be that neurons are electrically excitable, whereas glia are non-excitable. Various types of glial cells make up about 90% of all cells in the human brain. In the central nervous system, they are referred to as macroglial or neuroglial cells, and can be categorized into three types: astrocytes, oligodendrocytes, and ependymal cells (Verkhratsky & Butt, 2007).
Glial cells have traditionally been regarded as passive supporting cells. The name glia is from Greek, literally meaning “glue”. Rudolf Ludwig Karl Virchow (1821–1902), who coined the term for neuroscience, thought of glia as a sort of “nerve putty”—a connective tissue void of any cellular elements (Verkhratsky & Butt, 2007). However, today we have a very different understanding of the function of glia in the nervous system. Glia play a vital role in the growth and development of neurons as well as their maintenance and death. Atroglial cells provide the stem elements to birth neurons; they compartmentalize neurons, synapses, and capillaries into functional units and help modulate chemical signals between neurons. Oligodendroglia even myelinate the axons of neurons in the central nervous system (as Schwann cells do in the peripheral nervous system), creating faster communication pathways.
In 1984, researchers discovered that astrocytes and oligodendrocytes possessed GABA receptors. GABA (Gamma-aminobutyric acid) is an inhibitory neurotransmitter that plays a counterpart role to the glutamates, if glutamate is the accelerator of the brain, GABA is the brake. A few years later, others found that astroglial cells can communicate over long distances by propagating calcium waves. Since then it has been demonstrated that glial cells can express practically every type of neurotransmitter and can detect the activity of neighbouring neurons. In fact, far from being mere passive, non-excitable glue for neurons, glia form a whole other communication circuit in tandem with the neuronal circuit, the two systems communicating with each other via chemical and electrical signalling.
Verkhratsky, A., & Butt, A. (2007). Glial neurobiology: A textbook. Chichester, United Kingdom: John Wiley & Sons.
