Mydriasis iris sphincter, composed of circularly arranged muscle fibers, and the iris dilator, composed of radially arranged muscle fibers. The sphincter is innervated by (signaled by nerves of) the parasympathetic nervous system, and the dilator by the sympathetic nervous system. Sympathetic stimulation of the adrenergic receptors causes the contraction of the radial muscle and subsequent dilation of the pupil. Conversely, parasympathetic stimulation causes contraction of the circular muscle and constriction of the pupil. The mechanism of mydriasis depends on the agent being used. It usually involves either a disruption of the parasympathetic nerve supply to the eye (which normally constricts the
Mydriasis instance, the adrenoceptor agonist phenylephrine (e.g. Cyclomydril) is used if strong mydriasis is needed for a surgical intervention. Alternatively, anticholinergics (in particular, antimuscarinics) such as atropine, hyoscyamine, and scopolamine antagonize the muscarinic acetylcholine receptors in the eye. Blocking acetylcholine receptors, reduces the pupilary muscles' ability to constrict and causes dilation (which is critical in eye surgery procedures such as cataract surgery which require uninterrupted access to the inner eye via the pupilary aperture, thus requiring that the eye be both paralyzed and anesthetized before the procedure can go ahead. The antimuscarinic, tropicamide, may be used as a mydriastic agent during