In medicine, a coma (from the Greek κῶμα koma, meaning deep sleep) 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 (RAS or ARAS).

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 (sensation) via the thalamic pathway, and most importantly directly or indirectly in charge of all the neurological functions, from simple reflexes to 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. It is therefore necessary to investigate the integrity of the bilateral cerebral cortices, as well as that of the reticular activating system (RAS) in a comatose patient.

Read more about Coma:  Causes, Signs and Symptoms, Diagnosis and Findings, Prognosis, Society and Culture

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