Biology:Iris sphincter muscle

Short description: Muscle in the eye which constricts the pupil

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
Anatomical terms of muscle

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.

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

The pupil constricts when the iris sphincter muscle is stimulated and contracts

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 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.. 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

External links

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