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Myosis due to opiate use.jpg
Miosis due to opiate use
  • /maɪˈoʊ sɪs/
SpecialtyOphthalmology Edit this on Wikidata

Miosis is excessive constriction of the pupil.[1][2][3][4] The term is from Ancient Greek μύειν, mūein, "to close the eyes".

The opposite condition, mydriasis, is the dilation of the pupil. Anisocoria is the condition of one pupil being more dilated than the other.



  • senile miosis (a reduction in the size of a person's pupil in old age)



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

See also[edit]


  1. ^ Farlex medical dictionary citing:
  2. ^ Seidel, Henry M.; Jane W. Ball; Joyce E. Dains; G. William Benedict (2006-03-29). Mosby's Guide to Physical Examination. Mosby. ISBN 978-0-323-03573-6.
  3. ^ Farlex medical dictionary citing: Millodot: Dictionary of Optometry and Visual Science, 7th edition.
  4. ^ Farlex medical dictionary citing: Mosby's Medical Dictionary, 8th edition.

External links[edit]