Mydriasis
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Mydriasis is an excessive dilation of the pupil due to disease, drugs, or the like. Although the pupil will dilate in the dark normally, it is usually quite constricted (small) in the light. With mydriasis, a pupil will remain excessively large, even in a bright environment.
Abnormal constriction of the pupil is called miosis.
Mechanism
The mechanism that causes 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 over-activity of the sympathetic nervous system (SNS).
Atropine blocks muscarinic acetylcholine receptors. Acetylcholine (ACh) is the neurotransmitter of the parasympathetic nervous system and blocking its action means the pupil cannot constrict.
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.
Mydriatics
A mydriatic is an agent which induces dilation of the pupil. An example is tropicamide. Such drugs are used in medicine to permit examination of the retina and other deep structures of the eye, and also to reduce painful ciliary muscle spasm.