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.)
Latin m. sphincter pupillae
Gray's subject #225 1013
Origin encircles iris[1]
Insertion    encircles iris[1]
Artery long posterior ciliary arteries
Nerve short ciliary nerves
Actions constricts pupil
Antagonist iris dilator 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 eye, appropriate to its function as a constrictor.


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 reflex) or during accommodation.


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.

See also


  1. ^ a b Gest, Thomas R; Burkel, William E. "Anatomy Tables - Eye." Medical Gross Anatomy. 2000. University of Michigan Medical School. 5 Jan. 2010 <>.
  2. ^ Muscarinic and Nicotinic Synaptic Activation of the Developing..

External links

Wikimedia Foundation. 2010.

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