How Comas Work

Parts of the Brain

The brain is made up of three major parts: the cerebrum, the cerebellum and the brain stem. The cerebrum is the largest part of the brain. It makes up most of the total brain. The cerebrum controls cognitive and sensory functions such as intelligence, memory, reasoning and emotions. The cerebellumin the back of the brain controls balance and movement. The brain stem connects the two hemispheres of the cerebrum to the spinal cord. It controls breathing, blood pressure, sleep cycles, consciousness and other body functions. In addition, there is a large mass of neurons beneath the cerebrum, called the thalamus. This small, but critical area relays sensory impulses to the cerebral cortex. For a more detailed explanation of the brain's function, see How Your Brain Works.

Scientists believe that consciousness depends on the constant transmission of chemical signals from the brainstem and thalamus to the cerebrum. These areas are connected by neural pathways called the Reticular Activating System (RAS). Any interruptions to these messages can put someone into an altered state of consciousness.

A vegetative state is a type of coma that represents an awake but unresponsive state of consciousness. Many of these patients were comatose previously and after a few days or weeks emerge to an unconscious state in which their eyelids are open, giving the impression that they are awake. Patients in this state of consciousness may exhibit behaviors that lead family members to incorrectly believe they are becoming awake and communicative. These behaviors can include grunting, yawning and moving the head and limbs. However, these patients do not actually respond to any internal or external stimuli and evidence of extensive brain damage still persists. The outcome of patients in whom a vegetative state lasts for a month or more is generally poor and doctors use the term persistent vegetative state.

Next, we'll find out how someone can become comatose.

Other States of Consciousness

Catatonia - People in this state do not move or speak, and tend not to make eye contact with others. It can be a symptom of psychiatric disorders such as schizophrenia.
Stupor - The patient can be awakened only by vigorous stimuli, accompanied by motor behavior that leads to the avoidance of uncomfortable or aggravating stimuli.
Drowsiness - This simulates light sleep characterized by easy arousal and periods of alertness.
Locked-in syndrome - People with this rare neurological condition are fully able to think and reason, but they are completely paralyzed with the exception of opening and closing their eyes (which they sometimes use to communicate). Strokes or other conditions that damage the brain stem, but not the cerebrum, can cause this syndrome.
Brain death - People with this condition show no signs of brain function. Although their heart still beats, they cannot think, move, breathe or perform any bodily function. A person who is pronounced "brain dead" can't respond at all to pain, breathe unassisted or digest food. Legally, the patient is declared dead and the wishes of the patient or the family regarding organ donation can be considered. For more on brain death, check out How Brain Death Works.