In order to understand the loss of function suffered by a comatose individual, consider the important characteristics of the conscious state. Consciousness is defined by two fundamental elements: awareness and arousal.
Awareness allows us to receive and process information communicated by the five senses and thereby relate to ourselves and the rest of the world. Awareness has psychological and physiological components. The psychological component is governed by an individual's mind and its mental processes. The physiological component refers to the functioning—the physical and chemical condition—of an individual's brain. Awareness is regulated by areas within the cerebral hemispheres, the outermost layer of the brain, which separates humans from other animals because it allows greater intellectual functioning.
Arousal is regulated solely by physiological functioning. Its primitive responsiveness to the world is demonstrated by predictable reflex (involuntary) responses to stimuli. Arousal is maintained by the reticular activating system (RAS). This is not an anatomical area of the brain but rather a network of structures (including the brainstem, the medulla, and the thalamus) and nerve pathways which function together to produce and maintain arousal.