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Dilation and constriction of the pupil. Pupillary response is a physiological response that varies the size of the pupil, via the optic and oculomotor cranial nerve.. A constriction response (), [1] is the narrowing of the pupil, which may be caused by scleral buckles or drugs such as opiates/opioids or anti-hypertension medications.
Nerves involved in the resizing of the pupil connect to the pretectal nucleus of the high midbrain, bypassing the lateral geniculate nucleus and the primary visual cortex. From the pretectal nucleus neurons send axons to neurons of the Edinger-Westphal nucleus whose visceromotor axons run along both the left and right oculomotor nerves .
Mydriasis is frequently induced by drugs for certain ophthalmic examinations and procedures, particularly those requiring visual access to the retina. Fixed, unilateral mydriasis could be a symptom of raised intracranial pressure. The opposite, constriction of the pupil, is referred to as miosis. Both mydriasis and miosis can be physiological.
Pilocarpine is a drug that acts as a muscarinic receptor agonist. It acts on a subtype of muscarinic receptor (M 3) found on the iris sphincter muscle, causing the muscle to contract - resulting in pupil constriction . Pilocarpine also acts on the ciliary muscle and causes it to contract.
Tropicamide, sold under the brand name Mydriacyl among others, is a medication used to dilate the pupil and help with examination of the eye. [3] Specifically it is used to help examine the back of the eye. [4] It is applied as eye drops. [3] Effects occur within 40 minutes and last for up to a day. [3]
Demecarium causes constriction of the pupil , which improves the drainage of the fluid in the eye (aqueous humour). [2] As demecarium reversibly inhibits cholinesterase, it can be administered less frequently than other parasympathomimetic drugs, such as carbachol. [3]
If the drug pilocarpine is administered, the pupils will constrict and accommodation is increased due to the parasympathetic action on the circular muscle fibers, conversely, atropine will cause paralysis of accommodation (cycloplegia) and dilation of the pupil. Certain drugs cause constriction of the pupils, such as opioids. [7]
All cycloplegics are also mydriatic (pupil dilating) agents and are used as such during eye examination to better visualize the retina. When cycloplegic drugs are used as a mydriatic to dilate the pupil, the pupil in the normal eye regains its function when the drugs are metabolized or carried away.