Why does shining a light in one eye constrict the opposite eye?
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Thereof, why do both pupils constrict in response to light in one eye?
A greater intensity of light causes the pupil to constrict (miosis/myosis; thereby allowing less light in), whereas a lower intensity of light causes the pupil to dilate (mydriasis, expansion; thereby allowing more light in). Thus, the pupillary light reflex regulates the intensity of light entering the eye.
Likewise, why does a doctor shine a light in your eyes? You've seen it on television: A doctor shines a bright light into an unconscious patient's eye to check for brain death. If the pupil constricts, the brain is OK, because in mammals, the brain controls the pupil. They then shined a bright light onto this muscle and measured any contraction.
Similarly, it is asked, what happens when you shine a light into one eye?
In bright light, it contracts. More light creates more impulses, causing the muscles to close the pupil. Part of the optic nerve from one eye crosses over and couples to the muscles that control the pupil size of the other eye. That's why the pupil of one eye can change when you shine the light into your other eye.
What happens if pupils are not constrict?
Also called Adie's tonic pupil or tonic pupil, this is a rare neurological disorder where one pupil is larger than normal and is slow to react to light or does not constrict at all. (When this occurs, the condition is called Adie's syndrome.)