Miosis

Miosis (or myosis, from Ancient Greek μύειν, mūein, "to close the eyes") is the constriction of the pupil of the eye to two millimeters or less. The pupillary response (enlargement or reduction of the pupil) is a normal response to changes in light levels, but changes in pupil size can also be associated with certain pathological conditions, microwave radiation exposure, and certain drugs, especially opioids. Parasympathetic stimulation of the iris sphincter muscle (pupillary sphincter muscle) decreases the pupillary aperture (size of the pupil). This constriction is referred to as miosis.

The opposite condition, mydriasis, is the dilation of the pupil.

Physiology of the photomotor reflex
Light entering the eye strikes three different photoreceptors in the retina: the familiar rods and cones used in image forming and the more newly discovered photosensitive ganglion cells. The ganglion cells give information about ambient light levels, and react sluggishly compared to the rods and cones. Signals from ganglion cells have three functions: acute suppression of the hormone melatonin, entrainment of the body's circadian rhythms and regulation of the size of the pupil.

The retinal photoceptors convert light stimuli into electric impulses. 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. Visceromotor nerve axons (which constitute a portion of cranial nerve III, along with the somatomotor portion derived from the Edinger-Westphal nucleus) synapse on ciliary ganglion neurons, whose parasympathetic axons innervate the iris sphincter muscle, producing miosis. This occurs because sympathetic activity from the ciliary ganglion is lost thus parasympathetics are not inhibited. Image

Diseases

 * Horner syndrome (a set of abnormalities in the nervous supply of the face due to damage to the sympathetic nervous system).
 * Hemorrhage into pons (intracranial hemorrhage)
 * Cluster Headaches with ptosis
 * Iridocyclitis
 * Fatal familial insomnia

Drugs

 * Opioids such as fentanyl, morphine, heroin and methadone (the notable exception being demerol)
 * Antipsychotics, including haloperidol, thorazine, olanzapine, quetiapine and others
 * Cholinergic agents such as acetylcholine
 * Some cancer chemotherapy drugs, including camptothecin derivatives
 * Mirtazapine, a noradrenergic and specific serotonergic antidepressant (NaSSA)
 * Trazodone
 * Some MAO Inhibitors.
 * In some rare cases, when exposed to mustard gas.
 * Organophosphates

Miotics
A miotic substance causes the constriction of the pupil of the eye (or miosis). It is the opposite of a mydriatic substance, which causes dilation of the pupil.