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
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.

Comparative anatomy

This structure is found in vertebrates and in some cephalopods.^{[citation needed]}

General structure

All the myocytes are of the smooth 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]

Innervation

It is controlled by parasympathetic postganglionic fibers releasing acetylcholine acting primarily on the muscarinic acetylcholine receptor (M3) of iris sphincter muscle.^[4] 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.^[5] 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

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