iris sphincter muscle
{{short description|Muscle in the eye which constricts the pupil}}
{{Infobox muscle
| Name = Iris sphincter muscle
| Latin = musculus sphincter pupillae
| Image = Gray878.png
| Caption = Iris, front view. (Muscle visible but not labeled.)
| Image2 = Gray883.png
| Caption2 = The upper half of a sagittal section through the front of the eyeball. ("Sphincter of pupil" labeled near bottom-center.)
| Action = Constricts pupil
| Blood = Long posterior ciliary arteries
| Nerve = Short ciliary nerves
| 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.
The ciliary muscle, pupillary sphincter muscle and pupillary dilator muscle sometimes are called intrinsic ocular muscles{{cite journal|last1=Kels|first1=Barry D.|last2=Grzybowski|first2=Andrzej|last3=Grant-Kels|first3=Jane M.|title=Human ocular anatomy|journal=Clinics in Dermatology|date=March 2015|volume=33|issue=2|pages=140–146|doi=10.1016/j.clindermatol.2014.10.006|pmid=25704934}} or intraocular muscles.{{cite book|last1=Ludwig|first1=Parker E.|last2=Aslam|first2=Sanah|last3=Czyz|first3=Craig N.|title=StatPearls|date=2024|publisher=StatPearls Publishing|chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK470534/|chapter=Anatomy, Head and Neck: Eye Muscles|pmid=29262013}}
Comparative anatomy
This structure is found in vertebrates and in some cephalopods.{{citation needed|date=May 2015}}
General structure
All the myocytes are of the smooth muscle type.{{cite journal|pmid=2826718|year=1987|last1=Pilar|first1=G|title=Muscarinic and nicotinic synaptic activation of the developing chicken iris|journal=The Journal of Neuroscience|volume=7|issue=12|pages=3813–26|last2=Nuñez|first2=R|last3=McLennan|first3=I. S.|last4=Meriney|first4=S. D.|doi=10.1523/JNEUROSCI.07-12-03813.1987|pmc=6569112|doi-access=free}}
Its dimensions are about 0.75 mm wide by 0.15 mm thick.{{citation needed|date=May 2015}}
Mode of action
File:Human eye in bright light.jpg
In humans, it functions to constrict the pupil in bright light (pupillary light reflex) or during accommodation.{{citation needed|date=May 2015}} In {{Clarify|reason=What is a lower animal? Needs further definition on distinction.|date=March 2025|text=lower animals}}, the muscle cells themselves are photosensitive causing iris action without brain input.{{cite news|title=Mouse eyes constrict to light without direct link to the brain|url=https://phys.org/news/2017-06-mouse-eyes-constrict-link-brain.html|access-date=20 June 2017|work=Phys.org|issue=19 June 2017}}
Innervation
It is controlled by parasympathetic postganglionic fibers releasing acetylcholine acting primarily on the muscarinic acetylcholine receptor (M3) of iris sphincter muscle.{{cite journal|last1=Ishizaka|first1=N|last2=Noda|first2=M|last3=Yokoyama|first3=S|last4=Kawasaki|first4=K|last5=Yamamoto|first5=M|last6=Higashida|first6=H|title=Muscarinic acetylcholine receptor subtypes in the human iris.|journal=Brain Res.|date=March 1998|volume=787|issue=2|pages=344–7|doi=10.1016/s0006-8993(97)01554-0|pmid=9518684}} Preganglionic fibers originate from the Edinger–Westphal nucleus, travel along the oculomotor nerve (CN III), and make nicotinic cholinergic synapses on neurons in the ciliary ganglion.{{cite encyclopedia|last1=Berg|first1=DK|last2=Shoop|first2=RD|last3=Chang|first3=KT|last4=Cuevas|first4=J|editor1-last=Clementi|editor1-first=F.|editor2-last=Fornasari|editor2-first=D|editor3-last=Gotti|editor3-first=C|encyclopedia=Neuronal Nicotinic Receptors|title=Nicotinic Acetylcholine Receptors in Ganglionic Transmission|date=2000|publisher=Springer|series=Handbook of Experimental Pharmacology|volume=144|isbn=978-3-642-63027-9|pages=247–67}} Those neurons' postganglionic parasympathetic fibers 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
References
{{Reflist}}
External links
- [http://www.tedmontgomery.com/the_eye/iris.html Overview of function at tedmontgomery.com]
- [https://web.archive.org/web/20051221054237/http://clem.mscd.edu/~raoa/eyeweb/sld026.htm Slide] at mscd.edu
- {{BUHistology|08010loa}}
{{Eye anatomy}}
{{Optical illusions}}
{{Authority control}}