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Mydriasis is an excessive dilation of the pupil due to disease or drugs. Although the pupil will normally dilate in the dark, it is usually quite constricted in the light. A mydriatic pupil will remain excessively large, even in a bright environment.
Constriction of the pupil is called miosis.
There are two types of muscle which control the size of the iris: circular and radial. The former is innervated by the parasympathetic nervous system, the latter by the sympathetic nervous system. Sympathetic stimulation of α1 adrenergic receptors causes the contraction of the radial muscle, and subsequent dilation of the iris. Conversely, parasympathetic stimulation causes contraction of the circular muscle and constriction of the iris.
The mechanism of mydriasis depends on the agent being used. It will usually involve either a disruption of the parasympathetic nerve supply to the eye (which causes contraction of the pupil), or overactivity of the sympathetic nervous system (SNS).
Cocaine inhibits the reuptake of noradrenaline within a nerve synapse. When a solution of cocaine is dropped into the eye, noradrenaline is no longer reabsorbed by neurons, and its levels increase. Noradrenaline, the neurotransmitter for the SNS, causes dilation of the pupil. Another drug known to cause mydriasis is MDMA (Ecstasy).
A mydriatic is an agent which induces dilation of the pupil. Drugs such as tropicamide and atropine can be used in medicine to permit examination of the retina and other deep structures of the eye, and also to reduce painful ciliary muscle spasm (see cycloplegia).
Ophthalmologicals: mydriatics and cycloplegics (S01F)
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