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| Iris sphincter | |
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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. ("Circular fibers" labeled near center.) | |
| Latin | m. sphincter pupillae |
| Gray's | subject #225 1013 |
| Origin | |
| Insertion | |
| Artery | |
| Nerve | short ciliary nerves |
| Action | 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 hatch later in life, so that ultimately most cells are of the striated muscle type.[1]
References
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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Wikipedia content modification information:
- This page was last modified on 3 October 2008, at 03:31.
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