Iris sphincter muscle

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Iris sphincter muscle
Iris, front view. (Muscle visible but not labeled.)
The upper half of a sagittal section through the front of the eyeball. ("Sphincter of pupil" labeled near bottom-center.)
Origin encircles iris[1]
Insertion encircles iris[1]
Artery long posterior ciliary arteries
Nerve short ciliary nerves
Actions constricts pupil
Antagonist iris dilator muscle
Latin Musculus sphincter pupillae
TA A15.2.03.029
FMA 49157
Anatomical terms of muscle
[edit on Wikidata]

The iris sphincter muscle (pupillary sphincter, pupillary constrictor, circular muscle of iris, circular fibers) is a muscle in the part of the eye called the iris. It encircles the pupil of the iris, appropriate to its function as a constrictor of the pupil.

Comparative Anatomy

It is found in vertebrates and some cephalopods.[citation needed]

General Structure

Initially, all the myocytes are of the smooth muscle type but, later in life, most cells are of the striated muscle type.[2]

Its dimensions are about 0.75 mm wide by 0.15 mm thick.[citation needed]

Mode of Action

In humans, it functions to constrict the pupil in bright light (pupillary light reflex) or during accommodation.[citation needed] In lower animals, the muscle cells themselves are photosensitive causing iris action without brain input.[3]


It is controlled by parasympathetic fibers of the muscarinic acetylcholine receptor (M3) that originate from the Edinger-Westphal nucleus, travel along the oculomotor nerve (CN III), synapse in the ciliary ganglion, and then enter the eye via the short ciliary nerves.[citation needed]. The short ciliary nerves then run forward and pierce the sclera at the back of the eye, traveling between the sclera and the choroid to innervate the iris sphincter muscle.

See also


  1. ^ a b Gest, Thomas R; Burkel, William E. "Anatomy Tables - Eye." Medical Gross Anatomy. 2000. University of Michigan Medical School. 5 Jan. 2010 <"Archived copy". Archived from the original on 2010-05-26. Retrieved 2010-01-05.CS1 maint: Archived copy as title (link)>.
  2. ^ Pilar, G; Nuñez, R; McLennan, I. S.; Meriney, S. D. (1987). "Muscarinic and nicotinic synaptic activation of the developing chicken iris". The Journal of Neuroscience. 7 (12): 3813–26. PMID 2826718.
  3. ^ "Mouse eyes constrict to light without direct link to the brain". (19 June 2017). Retrieved 20 June 2017.

External links

  • Overview of function at
  • Slide at
  • Histology image: 08010loa – Histology Learning System at Boston University
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