The iris dilator muscle (pupil dilator muscle, pupillary dilator, radial muscle of iris, radiating fibers), is a

smooth muscle Smooth muscle is an involuntary non- striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit ...

of the

eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...

, running radially in the iris and therefore fit as a dilator. The pupillary dilator consists of a spokelike arrangement of modified contractile cells called myoepithelial cells. These cells are stimulated by the sympathetic nervous system. When stimulated, the cells contract, widening the pupil and allowing more light to enter the eye.

Structure

Innervation

It is innervated by the sympathetic system, which acts by releasing noradrenaline, which acts on

α1-receptors alpha-1 (α1) adrenergic receptors are G protein-coupled receptors (GPCRs) associated with the Gq heterotrimeric G protein. α1-adrenergic receptors are subdivided into three highly homologous subtypes, i.e., α1A-, α1B-, and α1D-adrenerg ...

. page 163 Thus, when presented with a threatening stimulus that activates the

fight-or-flight response The fight-or-flight or the fight-flight-or-freeze response (also called hyperarousal or the acute stress response) is a physiological reaction that occurs in response to a perceived harmful event, attack, or threat to survival. It was first desc ...

, this innervation contracts the muscle and dilates the pupil, thus temporarily letting more light reach the

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...

. The dilator muscle is innervated more specifically by postganglionic sympathetic nerves arising from the superior cervical ganglion as the

sympathetic root of ciliary ganglion The ciliary ganglion is a parasympathetic ganglion located just behind the eye in the posterior orbit. Three types of axons enter the ciliary ganglion but only the preganglionic parasympathetic axons synapse there. The entering axons are arranged ...

. 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

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. 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".

History

Etymology

The English name ''dilator pupillae muscle''Federative Committee on Anatomical Terminology (FCAT) (1998). ''Terminologia Anatomica''. Stuttgart: Thieme as currently used in the list of English equivalents of the '' Terminologia Anatomica'', the reference-work of the official anatomic nomenclature, can be considered as a corruption of the full Latin expression ''musculus dilatator pupillae''.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. The full Latin expression exhibits three words that each can be traced back to

Roman antiquity In modern historiography, ancient Rome refers to Roman civilisation from the founding of the city of Rome in the 8th century BC to the collapse of the Western Roman Empire in the 5th century AD. It encompasses the Roman Kingdom (753–50 ...

. The

Classical Latin Classical Latin is the form of Literary Latin recognized as a literary standard by writers of the late Roman Republic and early Roman Empire. It was used from 75 BC to the 3rd century AD, when it developed into Late Latin. In some later pe ...

name ''musculus'' is actually a diminutive of the Classical Latin name ''mus'',Lewis, C.T. & Short, C. (1879). ''A Latin dictionary founded on Andrews' edition of Freund's Latin dictionary''. Oxford: Clarendon Press. and can be translated as ''little mouse''. 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. Latin ''musculus'' can be explained by the fact that a muscle looks like a little mouse that moves under the skin.Kraus, L.A. (1844). ''Kritisch-etymologisches medicinisches Lexikon'' (Dritte Auflage). Göttingen: Verlag der Deuerlich- und Dieterichschen Buchhandlung. In the writings of Greek philosopher

Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical Greece, Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatet ...

the Ancient Greek word for ''mouse'', i.e. μῦςLiddell, 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. is also used to refer to a muscle. ''Dilatator'' in the Latin expression ''musculus dilatator pupillae'' is derived from the classical Latin verb ''dilatare'',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. to dilate, to spread out. Two possible explanations exist concerning the etymological derivation of this verb. The first explanation considers ''dilatare'' as frequentative of ''differere''. The Latin verb ''differe'' can mean, ''to carry different ways'', ''to spread abroad'', ''to scatter'', but also ''to delay''. The other explanation considers ''dilatare'' as a compound from ''di-'' and ''latus'', with the latter word meaning, ''broad'' or ''wide'', hence the German name ''Erweiterer'' for Latin ''dilatator''. 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), 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'',Donáth, T. & Crawford, G.C.N. (1969). ''Anatomical dictionary with nomenclature and explanatory notes.'' Oxford/London/Edinburgh/New York/Toronto/Syney/Paris/Braunschweig: Pergamon Press.International Anatomical Nomenclature Committee (1966). ''Nomina Anatomica''. Amsterdam: Excerpta Medica Foundation.International Anatomical Nomenclature Committee (1977). ''Nomina Anatomica, together with Nomina Histologica and Nomina Embryologica''. Amsterdam-Oxford: Excerpta Medica.International Anatomical Nomenclature Committee (1983). ''Nomina Anatomica, together with Nomina Histologica and Nomina Embryologica''. Baltimore/London: Williams & WilkinsInternational Anatomical Nomenclature Committee (1989). ''Nomina Anatomica, together with Nomina Histologica and Nomina Embryologica''. Edinburgh: Churchill Livingstone. The ''Nomina Anatomica'' as authorized in 1895 in Basle and in 1935 in

Jena Jena () is a German city and the second largest city in Thuringia. Together with the nearby cities of Erfurt and Weimar, it forms the central metropolitan area of Thuringia with approximately 500,000 inhabitants, while the city itself has a po ...

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.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 edition). Jena: Verlag Gustav Fischer. used the full Latin expression.

Additional images

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) arasympathetic pathway in blueFile:Iris_Dilator_Muscle_012909.jpg, The iris dilator muscle fibers course radially through the iris.

References

External links

Description of function at tedmontgomery.com

at mscd.edu * {{DEFAULTSORT:Iris Dilator Muscle Muscular system Human iris