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In medicine, coma (from the Greek ''koma'', meaning "deep sleep")〔(【引用サイトリンク】title=''Coma'' Origin )〕 is a state of unconsciousness in which a person: cannot be awakened; fails to respond normally to painful stimuli, light, or sound; lacks a normal wake-sleep cycle; and does not initiate voluntary actions. A person in a state of coma is described as being comatose. Typically, a distinction is made in the medical community between a coma and a medically induced coma, the former is generally understood to be a result of circumstances beyond the control of the medical community, while the latter is generally understood to be a means by which medical professionals may allow a patient's injuries to heal in a controlled environment. A comatose person exhibits a complete absence of wakefulness and is unable to consciously feel, speak, hear, or move. For a patient to maintain consciousness, two important neurological components must function. The first is the cerebral cortex—the gray matter that forms the outer layer of the brain. The other is a structure located in the brainstem, called reticular activating system (RAS).〔 See (diagram ).〕 Injury to either or both of these components is sufficient to cause a patient to experience a coma. The cerebral cortex is a group of tight, dense, "gray matter" composed of the nuclei of the neurons whose axons then form the "white matter", and is responsible for perception, relay of the sensory input (sensation) via the thalamic pathway, and many other neurological functions, including complex thinking. RAS, on the other hand, is a more primitive structure in the brainstem that is tightly in connection with reticular formation (RF). The RAS area of the brain has two tracts, the ascending and descending tract. Made up of a system of acetylcholine-producing neurons, the ascending track, or ascending reticular activating system (ARAS), works to arouse and wake up the brain, from the RF, through the thalamus, and then finally to the cerebral cortex. A failure in ARAS functioning may then lead to a coma. ==Signs and symptoms== Generally, a person who is unable to voluntarily open the eyes, does not have a sleep-wake cycle, is unresponsive in spite of strong tactile (painful), or verbal stimuli and who generally scores between 3 to 8〔(【引用サイトリンク】author=Russ Rowlett )〕 on the Glasgow Coma Scale is considered in a coma.〔 Coma may have developed in humans as a response to injury to allow the body to pause bodily actions and heal the most immediate injuries - if at all - before waking. It therefore could be a compensatory state in which the body's expenditure of energy is not superfluous. The severity and mode of onset of coma depends on the underlying cause. For instance, severe hypoglycemia (low blood sugar) or hypercapnia (increased carbon dioxide levels in the blood) initially cause mild agitation and confusion, but progress to obtundation, stupor and finally complete unconsciousness. In contrast, coma resulting from a severe traumatic brain injury or subarachnoid hemorrhage can be instantaneous. The mode of onset may therefore be indicative of the underlying cause. 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「coma」の詳細全文を読む スポンサード リンク
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