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- Miosis should not be confused with meiosis, the cellular division process involved in sexual reproduction.
Miosis is a medical term for constriction of the pupil. It is the opposite of mydriasis. It is seen in a variety of medical conditions, and can also be caused by certain drugs and chemicals. Eye drops used to intentionally cause miosis are known as "miotics". Extreme miosis is commonly called "pinpoint pupils".
Physiology of the photomotor reflex
Visual stimuli enters the eye, and through the retinal photoceptors is converted into an electric impulse, carried through the optic nerve (cranial nerve II) to the brain, where it connects to the pretectal nucleus of the high midbrain. It bypasses 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. Visceromotor nerve axons (which constitute a portion of the III cranial nerve, along with the somatomotor portion derived from the Edinger-Westphal nucleus) synapse on ciliary ganglion neurons, whose parasympathetic axons innervate the constrictor muscle of the iris, producing miosis. 
- Horner syndrome (a set of abnormalities in the nervous supply of the face due to damage to the sympathetic nervous system).
- Pancoast tumor (a tumor of the apical lung), due to damage to the ascending sympathetic tract that would normally cause the pupil to dilate.
- Hemorrhage into pons (intracranial hemorrhage)
- Opioids such as codeine, morphine, heroin and methadone
- Antipsychotics, including haloperidol, thorazine, olanzapine, quetiapine and others
- Cholinergic agents such as those used to treat Alzheimer's disease and nerve gases; cholinergic drugs such as carbachol (Miostat) and neostigmine
- Some cancer chemotherapy drugs, including camptothecin derivatives
- Mirtazapine, a noradrenergic and specific serotonergic antidepressant (NaSSA)
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