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In medicine, a coma is a state of unconsciousness lasting more than six hours, in which a person: cannot be awakened; fails to respond normally to painful stimuli, light, or sound; lacks a normal sleep-wake cycle; and, does not initiate voluntary actions. A person in a state of coma is described as being comatose. Although a coma patient may appear to be awake, they are unable to consciously feel, speak, hear, or move. For a patient to maintain consciousness, two important neurological components must function impeccably. The first is the cerebral cortex which is the gray matter covering the outer layer of the brain. The other is a structure located in the brainstem, called reticular activating system. 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 nucleus of the neurons whose axons then form the "white matter", and is responsible for perception, relay of the sensory input via the thalamic pathway, and most importantly directly or indirectly in charge of all the neurological functions, from simple reflexes to complex thinking. (via Freebase)