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Glia, also called glial cells ( gliocytes) or neuroglia, are non- cells in the central nervous system (the brain and the spinal cord) and in the peripheral nervous system that do not produce Action potential. The neuroglia make up more than one half the volume of neural tissue in the human body. They maintain homeostasis, form myelin, and provide support and protection for . In the central nervous system, glial cells include (that produce myelin), , and microglia, and in the peripheral nervous system they include (that produce myelin), and satellite cells.
Glial cells have far more cellular diversity and functions than neurons, and can respond to and manipulate neurotransmission in many ways. Additionally, they can affect both the preservation and consolidation of memories.
Glia were discovered in 1856, by the pathologist Rudolf Virchow in his search for a "connective tissue" in the brain. The term derives from Koine Greek γλία and γλοία "glue", . ( or ), and suggests the original impression that they were the glue of the nervous system.
| Astrocytes (also called astroglia) have numerous projections that link neurons to their blood supply while forming the blood–brain barrier. They regulate the external chemical environment of neurons by removing excess potassium and recycling released during synaptic transmission. Astrocytes may regulate vasoconstriction and vasodilation by producing substances such as arachidonic acid, whose are vasoactive.
Astrocytes signal each other using ATP. The (also known as electrical synapses) between astrocytes allow the messenger molecule IP3 to diffuse from one astrocyte to another. IP3 activates on cellular organelles, releasing calcium into the cytoplasm. This calcium may stimulate the production of more IP3 and cause release of ATP through channels in the membrane made of . 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. In general, there are 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. It has recently been shown that astrocyte activity is linked to blood flow in the brain, and that this is what is actually being measured in fMRI. They also have been involved in neuronal circuits playing an inhibitory role after sensing changes in extracellular calcium. Human astrocytes are larger and more abundant than any other animals'. |
| Oligodendrocytes are cells that coat axons in the CNS with their cell membrane, forming a specialized membrane differentiation called myelin, producing the myelin sheath. The myelin sheath provides insulation to the axon that allows action potential to propagate more efficiently. |
| Ependymal cells, also named ependymocytes, line the spinal cord and the ventricular system of the brain. These cells are involved in the creation and secretion of cerebrospinal fluid (CSF) and beat their cilia to help circulate the CSF and make up the blood-CSF barrier. They are also thought to act as neural stem cells. |
| Radial glia cells arise from neuroepithelial cells after the onset of neurogenesis. Their differentiation abilities are more restricted than those of neuroepithelial cells. In the developing nervous system, radial glia function both as neuronal progenitors and as a scaffold upon which newborn neurons migrate. 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 Müller cell is the glial cell that spans the thickness of the retina and, in addition to astroglial cells, participates in a bidirectional communication with neurons. |
| Similar in function to oligodendrocytes, Schwann cells provide myelination to axons in the peripheral nervous system (PNS). They also have phagocytosis activity and clear cellular debris that allows for regrowth of PNS neurons. |
| Satellite glial cells are small cells that surround neurons in sensory, sympathetic, and parasympathetic ganglia.Hanani, M. "Satellite glial cells in sensory ganglia: from form to function". Brain Res. Rev. 48:457–76, 2005 These cells help regulate the external chemical environment. Like astrocytes, they are interconnected by and respond to ATP by elevating the intracellular concentration of calcium ions. They are highly sensitive to injury and inflammation and appear to contribute to pathological states, such as chronic pain. |
| Are found in the intrinsic ganglia of the digestive system. Glia cells are thought to have many roles in the enteric system, some related to homeostasis and muscular digestive processes. |
These cells are found in all regions of the brain and spinal cord. Microglial cells are small relative to macroglial cells, with changing shapes and oblong nuclei. They are mobile within the brain and multiply when the brain is damaged. In the healthy central nervous system, microglia processes constantly sample all aspects of their environment (neurons, macroglia and blood vessels). In a healthy brain, microglia direct the immune response to brain damage and play an important role in the inflammation that accompanies the damage. Many diseases and disorders are associated with deficient microglia, such as Alzheimer's disease, Parkinson's disease and ALS.
The total number of glial cells in the human brain is distributed into the different types with being the most frequent (45–75%), followed by (19–40%) and microglia (about 10% or less).
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 glial cells in ganglia.
Glial cells are known to be capable of mitosis. By contrast, scientific understanding of whether neurons are permanently mitosis, or capable of mitosis, is still developing. In the past, glia had been considered to lack certain features of neurons. For example, glial cells were not believed to have synapse or to release neurotransmitter. They were considered to be the passive bystanders of neural transmission. However, recent studies have shown this to not be entirely true.
In addition to neurodegenerative diseases, a wide range of harmful exposure, such as hypoxia, or physical trauma, can lead to the result of physical damage to the CNS. Generally, when damage occurs to the CNS, glial cells cause apoptosis among the surrounding cellular bodies. Then, there is a large amount of activity, which results in inflammation, and, finally, there is a heavy release of growth inhibiting molecules.
Glia were first described in 1856 by the pathologist Rudolf Virchow in a comment to his 1846 publication on connective tissue. A more detailed description of glial cells was provided in the 1858 book 'Cellular Pathology' by the same author.
When markers for different types of cells were analyzed, Albert Einstein's brain was discovered to contain significantly more glia than normal brains in the left angular gyrus, an area thought to be responsible for mathematical processing and language.Diamond MC, Scheibel AB, Murphy GM Jr, Harvey T, "On the Brain of a Scientist: Albert Einstein" , Experimental Neurology 1985; 198–204", Retrieved February 18, 2017 However, out of the total of 28 statistical comparisons between Einstein's brain and the control brains, finding one statistically significant result is not surprising, and the claim that Einstein's brain is different is not scientific (cf. multiple comparisons problem).
Not only does the ratio of glia to neurons increase through evolution, but so does the size of the glia. Astroglial cells in human brains have a volume 27 times greater than in mouse brains.
These important scientific findings may begin to shift the neurocentric perspective into a more holistic view of the brain which encompasses the glial cells as well. For the majority of the twentieth century, scientists had disregarded glial cells as mere physical scaffolds for neurons. Recent publications have proposed that the number of glial cells in the brain is correlated with the intelligence of a species. Moreover, evidences are demonstrating the active role of glia, in particular astroglia, in cognitive processes like learning and memory. (2025). 9780195152227 ISBN 9780195152227
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