Coma

In medicine, coma (from the Greek κῶμα koma, meaning "deep sleep") 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). 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.