Miosis is constriction of the pupil of the eye. This is a normal response to an increase in light but can also be associated with certain pathological conditions, microwave radiation exposure and certain drugs.

The opposite, 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. These give information about ambient light levels, and react sluggishly compared to the rods and cones. Signals from these intrinsically photosensitive 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 pertaining to 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 constrictor muscle of the iris, producing miosis. This occurs because sympathetic activity from the ciliary ganglion is lost thus parasympathetics are not inhibited.



The likelihood of suffering miosis increases with age.




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.

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