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Iris sphincter muscle

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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.)
Details
OriginEncircles iris[1]
InsertionEncircles iris[1]
ArteryLong posterior ciliary arteries
NerveShort ciliary nerves
ActionsConstricts pupil
AntagonistIris dilator muscle
Identifiers
Latinmusculus sphincter pupillae
TA98A15.2.03.029
TA26762
FMA49157
Anatomical terms of 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[2] or intraocular muscles.[3]

Comparative anatomy

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This structure is found in vertebrates and in some cephalopods.[citation needed]

General structure

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All the myocytes are of the smooth muscle type.[4]

Its dimensions are about 0.75 mm wide by 0.15 mm thick.[citation needed]

Mode of action

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The pupil constricts when the iris sphincter muscle is stimulated and contracts

In humans, it functions to constrict the pupil in bright light (pupillary light reflex) or during accommodation.[citation needed] In lower animals, the muscle cells themselves are photosensitive causing iris action without brain input.[5]

Innervation

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It is controlled by parasympathetic postganglionic fibers releasing acetylcholine acting primarily on the muscarinic acetylcholine receptor (M3) of iris sphincter muscle.[6] 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.[7] 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

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References

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  1. ^ a b Gest, Thomas R; Burkel, William E. (2000). "Anatomy Tables - Eye". Medical Gross Anatomy. University of Michigan Medical School. Archived from the original on 2010-05-26.{{cite web}}: CS1 maint: unfit URL (link).
  2. ^ Kels, Barry D.; Grzybowski, Andrzej; Grant-Kels, Jane M. (March 2015). "Human ocular anatomy". Clinics in Dermatology. 33 (2): 140–146. doi:10.1016/j.clindermatol.2014.10.006. PMID 25704934.
  3. ^ Ludwig, Parker E.; Aslam, Sanah; Czyz, Craig N. (2024). "Anatomy, Head and Neck: Eye Muscles". StatPearls. StatPearls Publishing. PMID 29262013.
  4. ^ Pilar, G; Nuñez, R; McLennan, I. S.; Meriney, S. D. (1987). "Muscarinic and nicotinic synaptic activation of the developing chicken iris". The Journal of Neuroscience. 7 (12): 3813–26. doi:10.1523/JNEUROSCI.07-12-03813.1987. PMC 6569112. PMID 2826718.
  5. ^ "Mouse eyes constrict to light without direct link to the brain". Phys.org. No. 19 June 2017. Retrieved 20 June 2017.
  6. ^ Ishizaka, N; Noda, M; Yokoyama, S; Kawasaki, K; Yamamoto, M; Higashida, H (March 1998). "Muscarinic acetylcholine receptor subtypes in the human iris". Brain Res. 787 (2): 344–7. doi:10.1016/s0006-8993(97)01554-0. PMID 9518684.
  7. ^ Berg, DK; Shoop, RD; Chang, KT; Cuevas, J (2000). "Nicotinic Acetylcholine Receptors in Ganglionic Transmission". In Clementi, F.; Fornasari, D; Gotti, C (eds.). Neuronal Nicotinic Receptors. Handbook of Experimental Pharmacology. Vol. 144. Springer. pp. 247–67. ISBN 978-3-642-63027-9.
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