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 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 through 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[]

• 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