Miosis (or myosis, from Ancient Greek μύειν, mūein, "to close the eyes") is a term with various definitions, which generally include constriction of the pupil.
The opposite condition,
mydriasis, is the dilation of the pupil. Anisocoria is the condition of one pupil being more dilated than the other.
Definitions [ edit ]
Definitions of miosis include:
Constriction of the pupil that is excessive,
relative to the amount of light the pupil receives [1 ] Constriction of the pupil to a diameter of less than two millimeters
[2 ] [3 ] Constriction of the pupil with causes including both abnormal and physiological ones.
[4 ] Pupillary constriction by abnormal causes.
Physiology of the photomotor reflex [ edit ]
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 photosensitive ganglion cells have multiple functions including 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
senile miosis (a reduction in the size of a person's pupil in old age)
Diseases [ edit ]
Opioids such as fentanyl, morphine, heroin and methadone (the notable exception being demerol/pethidine)
Imidazolines such as clonidine, naphazoline, Oxymetazoline and Tetrahydrozoline
Antipsychotics, including risperdal, 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)
Pilocarpine Eye drops and all other parasympathomimetics In some rare cases, when exposed to
See also [ edit ]
References [ edit ]
External links [ edit ]