Iris sphincter muscle

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^[1]
Insertion	encircles iris^[1]
Artery	long posterior ciliary arteries
Nerve	short ciliary nerves
Actions	constricts pupil
Antagonist	iris dilator muscle
Identifiers
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.

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.

Mode of Action

In humans, it functions to constrict the pupil in bright light (pupillary light reflex) or during accommodation.

Innervation

It is controlled by parasympathetic fibers 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.. 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.

References

1 2 Gest, Thomas R; Burkel, William E. "Anatomy Tables - Eye." Medical Gross Anatomy. 2000. University of Michigan Medical School. 5 Jan. 2010 <http://anatomy.med.umich.edu/nervous_system/eye_tables.html>.
↑ 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.

External links

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

Anatomy of the globe of the 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

1:posterior segment 2:ora serrata 3:ciliary muscle 4:ciliary zonules 5:Schlemm's canal 6:pupil 7:anterior chamber 8:cornea 9:iris 10:lens cortex 11:lens nucleus 12:ciliary process 13:conjunctiva 14:inferior oblique muscule 15:inferior rectus muscule 16:medial rectus muscle 17:retinal arteries and veins 18:optic disc 19:dura mater 20:central retinal artery 21:central retinal vein 22:optic nerve 23:vorticose vein 24:bulbar sheath 25:macula 26:fovea 27:sclera 28:choroid 29:superior rectus muscle 30:retina

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 Foveola Fovea centralis Parafovea Perifovea Optic disc Optic cup Ora serrata

Anatomical regions of the eye

Anterior segment	Adnexa Fibrous tunic Anterior chamber Aqueous humor Iris Posterior chamber Ciliary body Lens Capsule of lens Zonule of Zinn

Posterior segment	Vitreous chamber Vitreous humor Retina Choroid

Other

Optical illusions

List of optical illusions

Afterimage Barberpole Bezold Blivet Café wall Cornsweet Delboeuf Ebbinghaus Ehrenstein Flash lag Fraser spiral Grid Hering Jastrow Mach McCollough Müller-Lyer Necker Orbison Penrose stairs triangle Peripheral drift Poggendorff Ponzo Rubin Sander Schroeder Ternus Vertical–horizontal White's Wundt Zöllner

Related	Auditory Tactile Temporal Op art Ascending and Descending Waterfall

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