Glial cells, commonly called neuroglia or simply glia, are non-neuronal cells that provide support and nutrition to cells of the nervous system. In the human brain, glia are estimated to outnumber neurons by as much as 50 to 1.
Unlike neurons, glia do not have chemical synapses, nor do they communicate via action potentials or neurotransmitters. They do, however, provide a kind of glue between nerve cells, and appear to contribute to synaptic development. They are fundamentally different from neurons in that they can undergo mitosis. Any damaged or destroyed neurons are gone forever, while glial cells are not.
Glia are derived from ectodermal tissue of the developing embryo, particularly the neural tube and crest .
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.
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 cell s are small cells that line the exterior surface of PNS neurons and help regulate the external chemical environment.
- Role of glia in synapse development http://pfrieger.gmxhome.de/work/publications/pfrieger_2002.pdf
Last updated: 02-08-2005 08:18:23
Last updated: 05-03-2005 17:50:55