{{short description|Smooth muscle of the eye}} {{Infobox muscle | Name = Iris dilator muscle | Latin = musculus dilatator pupillae | Image = Gray878.png | Caption = [[Iris (anatomy)|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. (Iris dilator muscle is not labeled and not to be confused with "Radiating fibers" labeled near center, which are part of the [[ciliary muscle]].) | Origin = Outer margins of iris<ref name="Gest">{{cite web |last1=Gest |first1=Thomas R |last2=Burkel |first2=William E. |title=Anatomy Tables – Eye |work=Medical Gross Anatomy |year=2000 |publisher=University of Michigan Medical School |url=http://anatomy.med.umich.edu/nervous_system/eye_tables.html |url-status=unfit |archive-url=https://web.archive.org/web/20100526064035/http://anatomy.med.umich.edu/nervous_system/eye_tables.html |archive-date=2010-05-26}}</ref> | Insertion = Inner margins of iris<ref name=Gest/> | Blood = | Nerve = [[Long ciliary nerves]] ([[sympathetic nervous system|sympathetics]]) | Action = [[Pupillary dilation|Dilates]] [[pupil]] | Antagonist = [[Iris sphincter muscle]] }} The '''iris dilator muscle''' ('''pupil dilator muscle''', '''pupillary dilator''', '''radial muscle of iris''', '''radiating fibers'''), is a [[smooth muscle]]<ref>{{cite journal |doi=10.1523/JNEUROSCI.07-12-03813.1987 |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=3813–3826 |last2=Nuñez |first2=R |last3=McLennan |first3=I. S. |last4=Meriney |first4=S. D.|pmc=6569112 |doi-access=free }}</ref> of the [[eye]], running radially in the [[iris (anatomy)|iris]] and therefore fit as a dilator. The pupillary dilator consists of a spokelike arrangement of modified contractile cells called [[myoepithelial cell]]s. These cells are stimulated by the sympathetic nervous system.<ref name="saladin">{{cite book|last=Saladin|first=Kenneth|title=Anatomy and Physiology|year=2012|publisher=McGraw-Hill|pages=616–617}}</ref> When stimulated, the cells contract, widening the pupil and allowing more light to enter the eye.
The [[ciliary muscle]], [[pupillary sphincter muscle]] and [[pupillary dilator muscle]] sometimes are called [[intrinsic ocular muscle]]s<ref>{{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 }}</ref> or [[intraocular muscle]]s.<ref>{{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 }}</ref>
==Structure== ===Innervation=== It is innervated by the sympathetic system, which acts by releasing [[noradrenaline]], which acts on [[α1-receptors]].<ref name=Rang163>{{cite book |author=Rang, H. P. |title=Pharmacology |publisher=Churchill Livingstone |location=Edinburgh |year=2003 |isbn=0-443-07145-4|page =163}}</ref> Thus, when presented with a threatening stimulus that activates the [[fight-or-flight response]], this innervation contracts the muscle and dilates the [[pupil]], thus temporarily letting more light reach the [[retina]].
The dilator muscle is innervated more specifically by [[postganglionic]] sympathetic nerves arising from the [[superior cervical ganglion]] as the [[sympathetic root of ciliary ganglion]]. From there, they travel via the [[internal carotid artery]] through the [[carotid canal]] to [[foramen lacerum]]. They then enter the [[middle cranial fossa]] above foramen lacerum, travel through the [[cavernous sinus]] in the middle cranial fossa and then travel with the [[ophthalmic artery]] in the [[optic canal]] or on the [[ophthalmic nerve]] through the [[superior orbital fissure]]. From there, they travel with the [[nasociliary nerve]] and then the [[long ciliary nerve]]. They then pierce the [[sclera]], travel between sclera and [[choroid]] to reach the iris dilator muscle. They will also pass through ciliary ganglion and travel in [[short ciliary nerves]] to reach the iris dilator muscle.
==Function== [[File:Human eye in dim light.jpg|thumb|260x260px|The pupil widens when the iris dilator muscle is stimulated and contracts. An extreme, though natural, case is shown]] The pupillary dilator acts to increase the size of the pupil to allow more light to enter the eye. It works in opposition to the pupillary constrictor.<ref name="clev1">{{cite web |title=Reflexes of the Eye |url=https://my.clevelandclinic.org/health/articles/24674-reflexes-of-the-eye |publisher=Cleveland Clinic |access-date=3 August 2023}}</ref> Pupil dilation occurs when there is insufficient light for the normal function of the eye, and during heightened sympathetic activity, for example in the "fight-or-flight reflex".<ref name="clev1" /><ref>{{cite web |title=What Happens to Your Body During the Fight-or-Flight Response? |url=https://health.clevelandclinic.org/what-happens-to-your-body-during-the-fight-or-flight-response/ |publisher=Cleveland Clinic |access-date=3 August 2023 |date=9 December 2019}}</ref>
==History== ===Etymology=== The English name ''dilator pupillae muscle''<ref name="FCAT">Federative Committee on Anatomical Terminology (FCAT) (1998). ''Terminologia Anatomica''. Stuttgart: Thieme</ref> as currently used in the list of English equivalents of the ''[[Terminologia Anatomica]]'', the reference-work of the official anatomic nomenclature,<ref name="Kachlik2008">{{cite journal |doi=10.1007/s00276-008-0357-y |pmid=18488135 |title=Anatomical terminology and nomenclature: Past, present and highlights |journal=Surgical and Radiologic Anatomy |volume=30 |issue=6 |pages=459–466 |year=2008 |last1=Kachlik |first1=David |last2=Baca |first2=Vaclav |last3=Bozdechova |first3=Ivana |last4=Cech |first4=Pavel |last5=Musil |first5=Vladimir|s2cid=2273591 }}</ref> can be considered as a corruption<ref name="Mar2001">{{cite journal |doi=10.1016/S0940-9602(01)80215-6 |pmid=11396787 |title=On the new anatomical nomenclature |journal=Annals of Anatomy - Anatomischer Anzeiger |volume=183 |issue=3 |pages=201–207 |year=2001 |last1=Marečková |first1=Elena |last2=Šimon |first2=František |last3=Červený |first3=Ladislav |pmc=2413596 }}</ref> of the full Latin expression ''musculus dilatator pupillae''.<ref name="His">His (1895). ''Die anatomische Nomenclatur. Nomina Anatomica. Der von der Anatomischen Gesellschaft auf ihrer IX. Versammlung in Basel angenommenen Namen''. Leipzig: Verlag von Veit & Comp.{{page needed|date=May 2015}}</ref> The full Latin expression exhibits three words that each can be traced back to [[ancient Rome|Roman antiquity]]. The [[Classical Latin]] name ''musculus'' is actually a diminutive of the Classical Latin name ''mus'',<ref name="Lewis & Short">Lewis, C.T. & Short, C. (1879). ''A Latin dictionary founded on Andrews' edition of Freund's Latin dictionary''. Oxford: Clarendon Press.{{page needed|date=May 2015}}</ref> and can be translated as ''little mouse''.<ref name="Lewis & Short"/> In the medical writings of [[Aulus Cornelius Celsus]] we can also find this specific name to refer to a muscle instead of its literal meaning.<ref name="Lewis & Short"/> Latin ''musculus'' can be explained by the fact that a muscle looks like a little mouse that moves under the skin.<ref name="Kraus1844">Kraus, L.A. (1844). ''Kritisch-etymologisches medicinisches Lexikon'' (Dritte Auflage). Göttingen: Verlag der Deuerlich- und Dieterichschen Buchhandlung.{{page needed|date=May 2015}}</ref> In the writings of Greek [[philosophy|philosopher]] [[Aristotle]] the Ancient Greek word for ''mouse'', i.e. μῦς<ref name="Liddell & Scott">Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with the assistance of. Roderick McKenzie.'' Oxford: Clarendon Press.{{page needed|date=May 2015}}</ref> is also used to refer to a muscle.<ref name="Liddell & Scott"/>
''Dilatator'' in the Latin expression ''musculus dilatator pupillae'' is derived from the classical Latin verb ''dilatare'',<ref name="Foster1891">Foster, F.D. (1891–1893). ''An illustrated medical dictionary. Being a dictionary of the technical terms used by writers on medicine and the collateral sciences, in the Latin, English, French, and German languages.'' New York: D. Appleton and Company.{{page needed|date=May 2015}}</ref> to dilate, to spread out.<ref name="Lewis & Short"/> Two possible explanations exist concerning the etymological derivation of this verb. The first explanation considers ''dilatare'' as frequentative of ''differere''.<ref name="Lewis & Short"/> The Latin verb ''differe'' can mean, ''to carry different ways'', ''to spread abroad'', ''to scatter'',<ref name="Lewis & Short"/> but also ''to delay''.<ref name="Lewis & Short"/> The other explanation<ref name="Kraus1844"/> considers ''dilatare'' as a compound from ''di-'' and ''latus'', with the latter word meaning, ''broad'' or ''wide'',<ref name="Lewis & Short"/> hence the German name ''Erweiterer'' for Latin ''dilatator''.<ref name="Foster1891"/>
The expression ''dilator pupillae muscle'', as used in the list of English equivalents of the ''Terminologia Anatomica'', is actually partly Latin, i.e. ''dilator pupillae'', with ''pupillae'' (=of the pupil<ref name="Lewis & Short"/>), a noun in the genitive case modifying ''dilator'', a noun in the nominative case, and partly English, i.e. ''muscle''. In previous editions (''[[Nomina Anatomica]]'') this muscle was officially called the ''musculus dilator pupillae'',<ref name="Donáth1969">Donáth, T. & Crawford, G.C.N. (1969). ''Anatomical dictionary with nomenclature and explanatory notes.'' Oxford/London/Edinburgh/New York/Toronto/Sydney/Paris/Braunschweig: Pergamon Press.{{page needed|date=May 2015}}</ref><ref name="NA3">International Anatomical Nomenclature Committee (1966). ''Nomina Anatomica''. Amsterdam: Excerpta Medica Foundation.{{page needed|date=May 2015}}</ref><ref name="NA4">International Anatomical Nomenclature Committee (1977). ''Nomina Anatomica, together with Nomina Histologica and Nomina Embryologica''. Amsterdam-Oxford: Excerpta Medica.{{page needed|date=May 2015}}</ref><ref name="NA5">International Anatomical Nomenclature Committee (1983). ''Nomina Anatomica, together with Nomina Histologica and Nomina Embryologica''. Baltimore/London: Williams & Wilkins{{page needed|date=May 2015}}</ref><ref name="NA6">International Anatomical Nomenclature Committee (1989). ''Nomina Anatomica, together with Nomina Histologica and Nomina Embryologica''. Edinburgh: Churchill Livingstone.{{page needed|date=May 2015}}</ref> The ''Nomina Anatomica'' as authorized in 1895 in [[Basel]]<ref name="His"/> and in 1935 in [[Jena]]<ref name="Kopsch">Kopsch, F. (1941). ''Die Nomina anatomica des Jahres 1895 (B.N.A.) nach der Buchstabenreihe geordnet und gegenübergestellt den Nomina anatomica des Jahres 1935 (I.N.A.)'' (3. Auflage). Leipzig: Georg Thieme Verlag.{{page needed|date=May 2015}}</ref><ref name="Stieve1949">Stieve, H. (1949). ''Nomina Anatomica. Zusammengestellt von der im Jahre 1923 gewählten Nomenklatur-Kommission, unter Berücksichtigung der Vorschläge der Mitglieder der Anatomischen Gesellschaft, der Anatomical Society of Great Britain and Ireland, sowie der American Association of Anatomists, überprüft und durch Beschluß der Anatomischen Gesellschaft auf der Tagung in Jena 1935 endgültig angenommen.'' (4th ed.). Jena: Verlag Gustav Fischer.{{page needed|date=May 2015}}</ref> used the full Latin expression.
==Additional images== <gallery> File:Horner's Syndrome and Autonomic innervation of the eye.svg|Scheme showing sympathetic and parasympathetic innervation of the pupil and sites of lesion in a Horner's syndrome. File:Gray840.png|Sympathetic connections of the ciliary and superior cervical ganglia (red) (parasympathetic pathway in blue) File:Iris_Dilator_Muscle_012909.jpg|The iris dilator muscle fibers course radially through the iris. </gallery>
==See also== * [[Iris sphincter muscle]] * [[Mydriasis]] * [[Pupillary response]]
==References== {{Reflist}}
==External links== * [http://www.tedmontgomery.com/the_eye/iris.html Description 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}}
{{DEFAULTSORT:Iris Dilator Muscle}} [[Category:Muscular system]] [[Category:Human iris]]