Misplaced Pages

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.)
Details
Origin	Encircles iris
Insertion	Encircles iris
Artery	Long posterior ciliary arteries
Nerve	Short ciliary nerves
Actions	Constricts pupil
Antagonist	Iris dilator muscle
Identifiers
Latin	musculus sphincter pupillae
TA98	A15.2.03.029
TA2	6762
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.

The ciliary muscle, pupillary sphincter muscle and pupillary dilator muscle sometimes are called intrinsic ocular muscles or intraocular muscles.

Comparative anatomy

This structure is found in vertebrates and in some cephalopods.

General structure

All the myocytes are of the smooth muscle type.

Its dimensions are about 0.75 mm wide by 0.15 mm thick.

Mode of action

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

Innervation

It is controlled by parasympathetic postganglionic fibers releasing acetylcholine acting primarily on the muscarinic acetylcholine receptor (M3) of iris sphincter muscle. Preganglionic fibers originate from the Edinger–Westphal nucleus, travel along the oculomotor nerve (CN III), and make nicotinic cholinergic synapses on neurons in the ciliary ganglion. Those neurons' postganglionic parasympathetic fibers then enter the eye through the short ciliary nerves. 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

• Iris dilator muscle
• Miosis

References

1. ^ Gest, Thomas R; Burkel, William E. (2000). "Anatomy Tables - Eye". Medical Gross Anatomy. University of Michigan Medical School. Archived from the original on 2010-05-26.{{cite web}}: CS1 maint: unfit URL (link).
2. Kels, Barry D.; Grzybowski, Andrzej; Grant-Kels, Jane M. (March 2015). "Human ocular anatomy". Clinics in Dermatology. 33 (2): 140–146. doi:10.1016/j.clindermatol.2014.10.006. PMID 25704934.
3. Ludwig, Parker E.; Aslam, Sanah; Czyz, Craig N. (2024). "Anatomy, Head and Neck: Eye Muscles". StatPearls. StatPearls Publishing. PMID 29262013.
4. 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. doi:10.1523/JNEUROSCI.07-12-03813.1987. PMC 6569112. PMID 2826718.
5. "Mouse eyes constrict to light without direct link to the brain". Phys.org. No. 19 June 2017. Retrieved 20 June 2017.
6. Ishizaka, N; Noda, M; Yokoyama, S; Kawasaki, K; Yamamoto, M; Higashida, H (March 1998). "Muscarinic acetylcholine receptor subtypes in the human iris". Brain Res. 787 (2): 344–7. doi:10.1016/s0006-8993(97)01554-0. PMID 9518684.
7. Berg, DK; Shoop, RD; Chang, KT; Cuevas, J (2000). "Nicotinic Acetylcholine Receptors in Ganglionic Transmission". In Clementi, F.; Fornasari, D; Gotti, C (eds.). Neuronal Nicotinic Receptors. Handbook of Experimental Pharmacology. Vol. 144. Springer. pp. 247–67. ISBN 978-3-642-63027-9.

External links

• Overview of function at tedmontgomery.com
• Slide at mscd.edu
• Histology image: 08010loa – Histology Learning System at Boston University

v t e Anatomy of the globe of the human eye
Fibrous tunic (outer)	Sclera Episcleral layer Schlemm's canal Trabecular meshwork Cornea Limbus layers Epithelium Bowman's Stroma Dua's layer Descemet's Endothelium
Uvea / vascular tunic (middle)	Choroid Capillary lamina of choroid Bruch's membrane Sattler's layer Ciliary body Ciliary processes Ciliary muscle Pars plicata Pars plana Iris Stroma Pupil Iris dilator muscle Iris sphincter muscle
Retina (inner)	Layers Inner limiting membrane Nerve fiber layer Ganglion cell layer Inner plexiform layer Inner nuclear layer Outer plexiform layer Outer nuclear layer External limiting membrane Layer of rods and cones Retinal pigment epithelium Cells Photoreceptor cells (Cone cell, Rod cell) → (Horizontal cell) → Bipolar cell → (Amacrine cell) → Retina ganglion cell (Midget cell, Parasol cell, Bistratified cell, Giant retina ganglion cells, Photosensitive ganglion cell) → Diencephalon: P cell, M cell, K cell, Muller glia Other Macula Perifoveal area Parafoveal area Fovea Foveal avascular zone Foveola Optic disc Optic cup Ora serrata
Anatomical regions of the eye	Anterior segment Adnexa (Eyebrow, Eyelid, Conjunctiva, Lacrimal system, Orbit) Fibrous tunic Anterior chamber Aqueous humour Iris Posterior chamber Ciliary body Lens Capsule of lens Zonule of Zinn Posterior segment Vitreous chamber Vitreous body Retina Choroid
Other	Keratocytes Ocular immune system Optical coherence tomography Eye care professional Eye disease Refractive error Accommodation Physiological Optics Visual perception

v t e Optical illusions (list)
Illusions	Afterimage Ambigram Ambiguous image Ames room Autostereogram Barberpole Bezold Café wall Checker shadow Chubb Cornsweet Delboeuf Ebbinghaus Ehrenstein Flash lag Fraser spiral Gravity hill Grid Hering Impossible trident Irradiation Jastrow Lilac chaser Mach bands McCollough Müller-Lyer Necker cube Oppel-Kundt Orbison Penrose stairs Penrose triangle Peripheral drift Poggendorff Ponzo Rubin vase Sander Schroeder stairs Shepard tables Spinning dancer Ternus Vertical–horizontal White's Wundt Zöllner
Popular culture	Op art Trompe-l'œil Spectropia (1864 book) Ascending and Descending (1960 drawing) Waterfall (1961 drawing) The dress (2015 photograph)
Related	Accidental viewpoint Auditory illusions Illusions Tactile illusions Temporal illusion

• Human iris
• Muscular system