Iris sphincter muscle
| Iris sphincter muscle | |
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| Iris, front view. (Muscle visible but not labeled.) | |
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| 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 pupil of the iris, appropriate to its function as a constrictor of the pupil.
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[edit] Comparative Anatomy
It is found in vertebrates and some cephalopods.
[edit] 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.
[edit] Mode of Action
In humans, it functions to constrict the pupil in bright light (pupillary reflex) or during accommodation.
[edit] Innervation
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.
[edit] See also
[edit] References
- ^ a b Gest, Thomas R; Burkel, William E. "Anatomy Tables - Eye." Medical Gross Anatomy. 2000. University of Michigan Medical School. 5 Jan. 2010 <http://anatomy.med.umich.edu/nervous_system/eye_tables.html>.
- ^ jneurosci.org Muscarinic and Nicotinic Synaptic Activation of the Developing..
[edit] External links
- Overview of function at tedmontgomery.com
- Slide at mscd.edu
- sphincter+pupillae at eMedicine Dictionary
- Histology at BU 08010loa
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