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What is a coma?

A coma is a deep unconsciousness. The word comes from a Greek word meaning "state of sleep." But being in a coma is not the same as being asleep. A comatose person is alive and breathing, but so unconscious that he or she can't respond to pain, the sound of a voice or any other stimuli. Nor can he perform voluntary actions. The brain still functions, but at its most basic level.
To understand more about comas, we need to look at the brain and how it works.
The cerebrum is the largest part of the brain, controlling cognitive and sensory functions such as intelligence, reasoning, emotions and memory. The cerebellum in the back of the brain controls movement and balance. The brain stem links the two hemispheres of the cerebrum to the spinal cord, and it controls body functions like breathing, consciousness, sleep cycles and blood pressure. The thalamus, a large mass of neurons beneath the cerebrum, relays sensory impulses to the cerebral cortex.
Scientists believe that consciousness depends on the steady transmission of chemical signals from the brain stem and thalamus to the cerebrum. Any interruptions to the messages can lead to an altered state of consciousness, including comas.
One type of coma is a vegetative state, which represents an awake but unresponsive level of consciousness. Many of these patients were comatose previously and after days or weeks emerge to an unconscious state in which their eyelids are open, giving the impression they're awake. Family members sometimes falsely believe such patients are becoming awake and communicative, because he or she grunts, yawns or moves limbs or the head. However, these patients don't actually respond to any stimuli and evidence of extensive brain damage persists.
A coma, sometimes also called persistent vegetative state, is a profound or deep state of unconsciousness. Persistent vegetative state is not brain-death. An individual in a state of coma is alive, but unable to move or respond to his or her environment. Coma may occur as a complication of an underlying illness, or as a result of injuries, such as head trauma. Individuals in such a state lose their thinking abilities and the awareness of their surroundings, but retain their non-cognitive functions and normal sleep patterns. Patients in a persistent vegetative state may lose their higher brain functions, but other key functions, such as breathing and circulation remain relatively intact. Spontaneous movements may occur, and the eyes may open in response to external stimuli. Individuals may even occasionally grimace, cry, or laugh. Although individuals in a persistent vegetative state may appear somewhat normal, they do not speak and they are unable to respond to commands.
This answer is based on source information from National Institute of Neurological Disorders and Stroke.
Nicholas D Suite
Nicholas D Suite on behalf of MDLIVE
Neurology
Coma is a state of unarousable unresponsiveness. Patients who have suffered severe head injury, near-drowning, poisoning, or stroke may enter coma. Careful neurological assessment is crucial in this group of patients because even though this is a very serious state, it is important to realize that the cause of coma needs to be determined if there is any hope for treatment and recovery. There have been recent cases of individuals who have emerged from coma-like states after many years. Some of these patients report awareness of sound, recognition of voices, and even the perception of pain, even while they were thought to be unaware by caregivers.
A coma is a condition in which a person is unconscious and unresponsive, usually for a prolonged period of time. The arms and legs may respond reflexively, but there is no response to painful stimuli. Breathing may become irregular, but for the most part, the heart and lungs continue to function normally. A coma usually lasts a few days or weeks, but if someone is in a coma for much longer, full recovery is not likely.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.