Glial cell
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.
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.
Function
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.
Types of glia
Astrocytes
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
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.
Schwann cells
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
Satellite cells are small cells that line the exterior surface of PNS neurons and help regulate the external chemical environment.
Ependymal cells
Ependymal cells line the cavities of the CNS and beat their cilia to help circulate the cerebrospinal fluid.
Microglia
Microglia are specialized macrophages 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.
External links
- Role of glia in synapse development (http://pfrieger.gmxhome.de/work/publications/pfrieger_2002.pdf)
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