The Human Brain
The human brain is made up of three basic parts: the cerebrum, the cerebellum, and the brainstem. The cerebrum is the largest part of the brain and makes up about 85 percent of the brain's total weight. It's divided into two hemispheresthe cerebral hemispheresone on each side of the head. The cerebrum is where all of the body's most complicated mental and sensory functions are controlledintelligence, reasoning, memory, emotions, vision, the ability to feel, etc.
The cerebellum is a smaller part of the brain that lies behind the cerebrum. It plays an essential role in coordination, posture and balance. The brainstem is the stem-like part of the brain that connects the cerebral hemispheres to the spinal cord and is responsible for controlling many basic bodily functions, such as breathing, blood pressure, and being awake and alert.
The Biology of Unconsciousness
As with most mental processes that occur in the brain, the biology of consciousnessalso called arousalis very complicated and not well understood. There are many tissues located deep within the brain that play a role in how conscious and alert a person is.
Researchers believe that one of the important physiological processes that keeps a person conscious is the transfer, or neurotransmission, of chemical signals from the brainstem to the cerebral hemispheres of the brain. This continuous neurotransmission needs to be happening in order for a person to be aware of their environment. Abnormalities that interrupt it can lead to coma or other states of unconsciousness.
Abnormalities that can cause coma include injury or damage to the brain that leads to swelling (edema) in the brain, which results in an increased intracranial pressure (pressure within the skull). Increased pressure, whether it is localized in one particular spot or spread over the whole brain, decreases the flow of blood and can lead to unconsciousness.
Injury or damage to the brain can also cause some areas of the brain to shift within the skull and exert pressure on surrounding tissues and structures, including blood vessels. When a part of the brain shifts position like this, the event is called a brain herniation, and it can lead to coma and death if not treated immediately.