<P> Glia were discovered in 1856, by the pathologist Rudolf Virchow in his search for a "connective tissue" in the brain . For over a century, it was believed that the neuroglia did not play any role in neurotransmission . However 21st century neuroscience has recognized that glial cells do have some effects on certain physiological processes like breathing, and in assisting the neurons to form synaptic connections between each other . </P> <P> Derived from ectodermal tissue . </P> <Table> <Tr> <Th> Location </Th> <Th> Name </Th> <Th> Description </Th> </Tr> <Tr> <Td> CNS </Td> <Td> Astrocytes </Td> <Td> <P> The most abundant type of macroglial cell in the CNS, 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 ions, and recycling neurotransmitters released during synaptic transmission . Astrocytes may regulate vasoconstriction and vasodilation by producing substances such as arachidonic acid, whose metabolites are vasoactive . </P> <P> Astrocytes signal each other using ATP . 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 and cause release of ATP through channels in the membrane made of pannexins . 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 . </P> <P> 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 . </P> <P> 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 . </P> </Td> </Tr> <Tr> <Td> CNS </Td> <Td> Oligodendrocytes </Td> <Td> <P> Oligodendrocytes are cells that coat axons in the central nervous system (CNS) with their cell membrane, forming a specialized membrane differentiation called myelin, producing the so - called myelin sheath . The myelin sheath provides insulation to the axon that allows electrical signals to propagate more efficiently . </P> </Td> </Tr> <Tr> <Td> CNS </Td> <Td> Ependymal cells </Td> <Td> <P> 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 . </P> </Td> </Tr> <Tr> <Td> CNS </Td> <Td> Radial glia </Td> <Td> <P> 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 . </P> </Td> </Tr> <Tr> <Td> PNS </Td> <Td> Schwann cells </Td> <Td> <P> 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 . </P> </Td> </Tr> <Tr> <Td> PNS </Td> <Td> Satellite cells </Td> <Td> <P> Satellite glial cells are small cells that surround neurons in sensory, sympathetic, and parasympathetic ganglia . These cells help regulate the external chemical environment . Like astrocytes, they are interconnected by gap junctions and respond to ATP by elevating intracellular concentration of calcium ions . They are highly sensitive to injury and inflammation, and appear to contribute to pathological states, such as chronic pain . </P> </Td> </Tr> <Tr> <Td> PNS </Td> <Td> Enteric glial cells </Td> <Td> <P> Are found in the intrinsic ganglia of the digestive system . They are thought to have many roles in the enteric system, some related to homeostasis and muscular digestive processes . </P> </Td> </Tr> </Table> <Tr> <Th> Location </Th> <Th> Name </Th> <Th> Description </Th> </Tr>

Where are glial cells located in the brain
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