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
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
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
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.
- Opioids such as tramadol, codeine, fentanyl, 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 pilocarpine, carbachol (Miostat) and neostigmine
- Some cancer chemotherapy drugs, including camptothecin derivatives
- Mirtazapine, a noradrenergic and specific serotonergic antidepressant (NaSSA)
- Some MAO Inhibitors.
- In some rare cases, when exposed to mustard gas.
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.