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Iris sphincter muscle: Wikis


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Iris sphincter
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]
Nerve short ciliary nerves
Actions constricts pupil
Antagonist iris dilator muscle

The iris sphincter muscle (pupillary sphincter, 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.

It is found in vertebrates and some cephalopods.

In humans, it functions to constrict the pupil in bright light (pupillary reflex) or during accommodation. Its dimensions are about 0.75 mm wide by 0.15 mm thick.

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.

Initially, all the myocytes are of the smooth muscle type, but much later in life, so that ultimately most cells are of the striated muscle type.[2]

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..

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