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Glial cells, sometimes called neuroglia or simply glia (Greek γλοία "glue"; pronounced in English as either or ), are non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for neurons in the central nervous system and peripheral nervous system. As the Greek name implies, glia are commonly known as the glue of the nervous system; however, this is not fully accurate. Glia were discovered in 1856, by the pathologist Rudolf Virchow in his search for a "connective tissue" in the brain. Neuroscience currently identifies four main functions of glial cells: # To surround neurons and hold them in place # To supply nutrients and oxygen to neurons # To insulate one neuron from another # To destroy pathogens and remove dead neurons. 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. ==Functions== Some glial cells function primarily as the physical support for neurons. Others regulate the internal environment of the brain, especially the fluid surrounding neurons and their synapses, and nutrify neurons. During early embryogenesis glial cells direct the migration of neurons and produce molecules that modify the growth of axons and dendrites. Recent research indicates that glial cells of the hippocampus and cerebellum participate in synaptic transmission, regulate the clearance of neurotransmitters from the synaptic cleft, and release gliotransmitters such as ATP, which modulate synaptic function. Glial cells are known to be capable of mitosis. By contrast, scientific understanding of whether neurons are permanently post-mitotic, 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 chemical synapses or to release transmitters. They were considered to be the passive bystanders of neural transmission. However, recent studies have shown this to be untrue.〔The Other Brain, by R. Douglas Fields, Ph. D. Simon & Schuster, 2009〕 For example, astrocytes are crucial in clearance of neurotransmitters from within the synaptic cleft, which provides distinction between arrival of action potentials and prevents toxic build-up of certain neurotransmitters such as glutamate (excitotoxicity). It is also thought that glia play a role in many neurological diseases, including Alzheimer's disease. Furthermore, at least in vitro, astrocytes can release gliotransmitter glutamate in response to certain stimulation. Another unique type of glial cell, the oligodendrocyte precursor cells or OPCs, have very well-defined and functional synapses from at least two major groups of neurons.〔Feezel, Charlie. World Cocoa Foundation: Knowledge Creation in Rulal West Africa. US Aid Education Workshop.〕 The only notable differences between neurons and glial cells are neurons' possession of axons and dendrites, and capacity to generate action potentials. Glia ought not to be regarded as "glue" in the nervous system as the name implies; rather, they are more of a partner to neurons.〔The Root of Thought: Unlocking Glia, by Andrew Koob, FT Science Press, 2009〕 They are also crucial in the development of the nervous system and in processes such as synaptic plasticity and synaptogenesis. Glia have a role in the regulation of repair of neurons after injury. In the central nervous system (CNS), glia suppress repair. Glial cells known as astrocytes enlarge and proliferate to form a scar and produce inhibitory molecules that inhibit regrowth of a damaged or severed axon. In the peripheral nervous system (PNS), glial cells known as Schwann cells promote repair. After axonal injury, Schwann cells regress to an earlier developmental state to encourage regrowth of the axon. This difference between the CNS and the PNS, raises hopes for the regeneration of nervous tissue in the CNS. For example, a spinal cord may be able to be repaired following injury or severance. Schwann cells are also known as neuri-lemmocytes. These cells envelop nerve fibers of the PNS by winding repeatedly around a nerve fiber with the nucleus inside of it. This process creates a myelin sheath, which not only aids in conductivity but also assists in the regeneration of damaged fibers. Oligodendrocytes are another type of glial cell of the CNS. These dendrocytes resemble an octopus bulbous body and contain up to fifteen arm-like processes. Each “arm” reaches out to a nerve fiber and spirals around it, creating a myelin sheath. This myelin sheath insulates the nerve fiber from the extracellular fluid as well as speeds up the signal conduction in the nerve fiber.〔Saladin, Kenneth. Anatomy and Physiology, 6th Edition. McGraw Hill 2012. Page 446-448.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Neuroglia」の詳細全文を読む スポンサード リンク
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