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. 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 covers 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 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 many other neurological functions, including complex thinking.