Glial cells

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.

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. Glia have important developmental roles, guiding migration of neurons in early development, and producing molecules that modify the growth of axons and dendrites. Recent findings in the hippocampus and cerebellum have indicated that glia are also active participants in synaptic transmission, regulating clearance of neurotransmitter from the synaptic cleft, releasing factors such as ATP which modulate presynaptic function, and even releasing neurotransmitters themselves.

Traditionally glia had been thought to lack certain features of neurons. For example, glia were not believed to have chemical synapses, nor were they believed to generate action potentials or release neurotransmitters. They were considered to be the passive bystanders of neural transmission. However, recent studies disproved this. 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.

Microglia
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 nuclei. They are mobile within the brain. These cells, while normally only existing in small numbers, multiply when the brain is damaged.

Astrocytes


The most abundant type of glial cell, astrocytes (also called astroglia) 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 may regulate vasoconstriction and vasodilation by producing substances such as arachidonic acid, whose metabolites are vasoactive.

Astrocytes signal each other using calcium. The gap junctions (also known as electrical synapses) between astrocytes allow the messenger molecule IP3 to diffuse from one astrocyte to another. IP3 activates calcium channels on cellular organelles, releasing calcium into the cytoplasm. This calcium may stimulate the production of more IP3. The net effect is a calcium wave that propagates from cell to cell. Extracellular release of ATP, and consequent activation of purinergic receptors on other astrocytes, may also mediate calcium waves in some cases.

There are generally two types of astrocytes, protoplasmic and fibrous, similar in function but distinct in morphology and distribution. Protoplasmic astrocytes have short, thick, highly branched processes and are typically found in gray matter. Fibrous astrocytes have long, thin, less branched processes and are more commonly found in white matter.

Oligodendrocytes
Oligodendrocytes are cells that coat axons in the central nervous system (CNS) with their cell membrane, called myelin, producing the so-called myelin sheath. The myelin sheath provides insulation to the axon that allows electrical signals to propagate more efficiently.

Ependymal cells
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.

Radial glia
In the developing nervous system, radial glia provide a scaffold for the outward migration of cortical cells. In the mature brain, the cerebellum and retina retain characteristic radial glial cells. In the cerebellum, these are Bergmann glia, which regulate synaptic plasticity. In the retina, the radial M�ller cell is the principal glial cell, and participates in a bidirectional communication with neurons.

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.

Capacity to divide
A misconception about glia is that they retain the ability to undergo mitosis in adulthood, while neurons cannot. 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.

Embryological development
Most glia are derived from ectodermal tissue of the developing embryo, particularly the neural tube and crest. The exception is microglia, which are derived from monocytes in the blood.

In the central nervous system, glia develop from the ventricular zone of the neural tube. These glia include the oligodendrocytes, ependymal cells, and astrocytes. In the peripheral nervous system, glia derive from the neural crest. These PNS glia include Schwann cells in nerves and satellite cells in ganglia.

History
Glia were discovered in 1891 by the early Spanish neuroanatomist Santiago Ram&oacute;n y Cajal.

The brain contains about nine 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).