Glial cells, commonly called neuroglia or simply glia, are non-neuronal cells that provide support and nutrition, maintaining homeostasis, forming 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 dogmata. 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.
Another misconception is that glia seem to retain the ability to undergo mitosis, while neurons lack this ability. The view is based on the general deficiency of the mature nervous system in replacing neurons after an insult or injury, such as a stroke or trauma, while very often there is a profound proliferation of glia, or gliosis near or at the site of damage. However, detailed studies found no evidence that 'mature' glia, such as astrocytes or oligodendrocytes, retain the ability of mitosis. Only the resident oligodendrocyte precursor cells seem to keep this ability after the nervous system matures. On the other hand, there are a few regions in the mature nervous system, such as the dentate gyrus of the hippocampus and the subventricular zone , that generation of new neurons can be observed.
Most glia are derived from ectodermal tissue of the developing embryo, particularly the neural tube and crest . The exception is microglia, which is believed to be derived from the macrophages from the blood.
Glia were discovered in 1891 by the early Spanish neuroanatomist Santiago Ramón y Cajal.
The brain contains about 9 times more glial cells than neurons. Following its discovery in the 20th century, this fact underwent significant media distortion, emerging as the famous myth claiming that "we are using only 10% of our brain". The role of glial cells as managers of communications in the synapse gap, thus modifying learning pace, has been discovered only very recently (2004).
Some glia function primarily as physical support for neurons. Others regulate the internal environment of the brain, especially the fluid surrounding neurons and their synapses, and provide nutrition to nerve cells. Some recent findings may add some functions to the known ones, for example with astrocytes ability to communicate.
Types of glia
Microglia are specialized macrophages capable of phagocytosis that protect neurons of the CNS. Though not technically glia because they are derived from monocytes rather than ectodermal tissue, they are commonly categorized as such because of their supportive role to neurons. Microglial cells are small relative to macroglial cells, with changing shapes and oblong nucleus. They are mobile within the brain. These cells, while normally only existing in small numbers, multiply in case of damage in the brain.
The most abundant type of glial cell, astrocytes have numerous projections that anchor neurons to their blood supply. They regulate the external chemical environment of neurons by removing excess ions, notably potassium, and recycling neurotransmitters released during synaptic transmission. The current theory suggests that astrocytes may be the predominant "building blocks" of the blood-brain barrier.
Astrocytes are also known to signal each other using calcium.
Oligodendrocytes are responsible for coating axons in the central nervous system (CNS) with a fatty substance called myelin, producing the so-called myelin sheath. The sheath provides insulation to the axon that allows electrical signals to propagate more efficiently.
Oligodendrocyte precursor cells
Oligodendrocyte precursor cells can be a misnomer. They serve as the precursors for oligodendrocytes during the development of nervous system, but quite a few of them remain in fully developed brain. There have been discussions of using different names, such as polydendrocytes or synantocytes, for these unique cells. They constitute about 5-8% of all cells in the nervous system, and have different properties from astrocytes or oligodendrocytes. They constitute the major group of cells undergoing mitosis in adult brain. Neurons make chemical synapses with these glial cells, a clear exception to the traditional view. The exact function of them is unknown.
Ependymal cells, also named ependymocytes, line the cavities of the CNS and beat their cilia to help circulate the cerebrospinal fluid. They make up the "walls" which segment different zones.
Similar in function to oligodendrocytes, Schwann cells provide myelination to axons in the peripheral nervous system (PNS). They also have phagocytotic activity and clear cellular debris that allows for regrowth of PNS neurons.
Satellite cells are small cells that line the exterior surface of PNS neurons and help regulate the external chemical environment.