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.

See also

• Iris dilator muscle

References

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
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	Keratocytes Ocular immune system Tapetum lucidum Eye care professional Eye disease Refractive error Accommodation Physiological Optics Visual perception

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