There are two types of muscle that control the size of the iris: the iris sphincter, composed of circularly arranged muscle fibers, and the iris dilator, composed of radially arranged muscle fibers. The sphincter is controlled 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<unk>  depends on the agent being used. It usually involves either a disruption of the parasympathetic nerve supply to the eye (which normally constricts the pupil) or overactivity of the sympathetic nervous system (SNS). Pupil diameter also increases in reaction to cognitive tasks requiring memory and attention, and this phenomenon is used as an indicator of mental activation (‘arousal’) in psychophysiological experiments. Drugs A mydriatic is an agent that induces dilation of the pupil. Drugs such as tropicamide 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 (see cycloplegia). One effect of administration of a mydriatic is intolerance to bright light (photophobia). Purposefully-induced<unk>  via mydriatics is also used as a diagnostic test for Horner's syndrome.<unk>  can be induced via modulation of adrenergic or cholinergic signalling. Drugs that can cause<unk>  include: * Stimulants (typically monoaminergics) such as amphetamines, cocaine, MDMA, and mephedrone. * Anticholinergics such as diphenhydramine, atropine, hyoscyamine, and scopolamine antagonize the muscarinic acetylcholine receptors in the eye. Blocking acetylcholine receptors reduces the pupillary 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 pupillary 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 surgery. * Serotonergics such as LSD, psilocybin mushrooms, mescaline and 2C-B. These drugs are typically hallucinogens. Similarly, selective serotonin reuptake inhibitors can cause<unk> . * Dissociatives such as dextromethorphan (an SSRI and sigma-1 agonist). * Certain GABAergic drugs, such as phenibut and GHB. * Adrenergic agonists, such as phenylephrine and cyclomydril. Adrenergic agonists may be used if strong<unk>  is needed in surgery. Norepinephrine is a hormone and neurotransmitter that regulates the involuntary muscles of the autonomic nervous system, including dilation of the pupil aperture via the muscles of the iris. Hence adrenergic agonists mimic the activity of norepinephrine, which is how they induce<unk> . Natural release of the hormone oxytocin can cause mild to moderate<unk> . Long term effects of drugs can also cause<unk> , for example opioid withdrawal. Autonomic neuropathy Parasympathetic fibers travel with cranial nerve III, the oculomotor nerve, to innervate the circular layer of muscle of the eye (sphincter pupillae). Damage to this nerve typically manifests itself as<unk> , because the sympathetic supply to the pupil, which causes<unk> , remains unaffected, and therefore unopposed. Multiple central nervous system disorders e.g. epilepsy, stroke, and impending brain herniation are known to lead to temporal<unk>  as well. A brain catastrophe, or a rapidly increasing brain mass, can cause compression of the oculomotor nerve. Trauma In cases of head injury or orbit trauma (eye injury), the iris sphincter (the muscle responsible for closing the pupil) or the nerves controlling it can be damaged, reducing or eliminating the normal pupillary light reflex.
Mydriasis
Periocular injection
Conjunctival suffusion
Ocular tremor
Iontophoresis
Subconjunctival injection
Nephroptosis
Pityriasis