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

, running radially in the

iris Iris most often refers to: *Iris (anatomy), part of the eye *Iris (mythology), a Greek goddess * ''Iris'' (plant), a genus of flowering plants * Iris (color), an ambiguous color term Iris or IRIS may also refer to: Arts and media Fictional ent ...

and therefore fit as a dilator. The pupillary dilator consists of a spokelike arrangement of modified contractile cells called

myoepithelial cell Myoepithelial cells (sometimes referred to as myoepithelium) are cells usually found in glandular epithelium as a thin layer above the basement membrane but generally beneath the luminal cells. These may be positive for alpha smooth muscle actin ...

s. 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 Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic chemical in the catecholamine family that functions in the brain and body as both a hormone and neurotransmitter. The name "noradrenaline" (from Latin '' ad'' ...

, which acts on α1-receptors. 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 des ...

, this innervation contracts the muscle and dilates the

pupil The pupil is a black hole located in the center of the iris of the eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company. It appears black ...

, 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 In the autonomic nervous system, fibers from the ganglion to the effector organ are called postganglionic fibers. Neurotransmitters The neurotransmitters of postganglionic fibers differ: * In the parasympathetic division, neurons are ''chol ...

sympathetic nerves arising from the

superior cervical ganglion The superior cervical ganglion (SCG) is part of the autonomic nervous system (ANS); more specifically, it is part of the sympathetic nervous system, a division of the ANS most commonly associated with the fight or flight response. The ANS is com ...

as the sympathetic root of ciliary ganglion. From there, they travel via the

internal carotid artery The internal carotid artery (Latin: arteria carotis interna) is an artery in the neck which supplies the anterior circulation of the brain. In human anatomy, the internal and external carotids arise from the common carotid arteries, where these ...

through the

carotid canal The carotid canal is a passageway in the temporal bone of the skull through which the internal carotid artery enters the middle cranial fossa from the neck. Structure The carotid canal is located within the middle cranial fossa, at the petrous ...

foramen lacerum The foramen lacerum ( la, lacerated piercing) is a triangular hole in the base of skull. It is located between the sphenoid bone, the apex of the petrous part of the temporal bone, and the basilar part of the occipital bone. Structure The fora ...

. They then enter the

middle cranial fossa The middle cranial fossa, deeper than the anterior cranial fossa, is narrow medially and widens laterally to the sides of the skull. It is separated from the posterior fossa by the clivus and the petrous crest. It is bounded in front by the po ...

above foramen lacerum, travel through the cavernous sinus in the middle cranial fossa and then travel with the

ophthalmic artery The ophthalmic artery (OA) is an artery of the head. It is the first branch of the internal carotid artery distal to the cavernous sinus. Branches of the ophthalmic artery supply all the structures in the orbit around the eye, as well as some str ...

in the

optic canal The ''optic foramen'' is the opening to the optic canal. The canal is located in the sphenoid bone; it is bounded medially by the body of the sphenoid and laterally by the lesser wing of the sphenoid. The superior surface of the sphenoid bone is b ...

or on the

ophthalmic nerve The ophthalmic nerve (V1) is a sensory nerve of the face. It is one of three divisions of the trigeminal nerve (CN V). It has three branches that provide sensory innervation to the eye, the skin of the upper face, and the skin of the anterior scal ...

through the

superior orbital fissure The superior orbital fissure is a foramen or cleft of the skull between the lesser and greater wings of the sphenoid bone. It gives passage to multiple structures, including the oculomotor nerve, trochlear nerve, ophthalmic nerve, abducens nerve ...

. From there, they travel with the nasociliary nerve and then the long ciliary nerve. They then pierce the

sclera The sclera, also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective, outer layer of the human eye containing mainly collagen and some crucial elastic fiber. In humans, and some ...

, travel between sclera and

choroid The choroid, also known as the choroidea or choroid coat, is a part of the uvea, the vascular layer of the eye, and contains connective tissues, and lies between the retina and the sclera. The human choroid is thickest at the far extreme rear of ...

to reach the iris dilator muscle. They will also pass through ciliary ganglion and travel in

short ciliary nerves The short ciliary nerves are nerves of the orbit around the eye. They are branches of the ciliary ganglion. They supply parasympathetic and sympathetic nerve fibers to the ciliary muscle, iris, and cornea. Damage to the short ciliary nerve may ...

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 ''Terminologia Anatomica'' is the international standard for human anatomical terminology. It is developed by the Federative International Programme on Anatomical Terminology, a program of the International Federation of Associations of Anatomis ...

'', 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. 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 per ...

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 Aulus Cornelius Celsus ( 25 BC 50 AD) was a Roman encyclopaedist, known for his extant medical work, ''De Medicina'', which is believed to be the only surviving section of a much larger encyclopedia. The ''De Medicina'' is a primary source on d ...

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 A philosopher is a person who practices or investigates philosophy. The term ''philosopher'' comes from the grc, φιλόσοφος, , translit=philosophos, meaning 'lover of wisdom'. The coining of the term has been attributed to the Greek th ...

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

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 ''Nomina Anatomica'' (''NA'') was the international standard on human anatomic terminology from 1895 until it was replaced by ''Terminologia Anatomica'' in 1998. In the late nineteenth century some 30,000 terms for various body parts were in use ...

'') 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 , french: link=no, Bâlois(e), it, Basilese , neighboring_municipalities= Allschwil (BL), Hégenheim (FR-68), Binningen (BL), Birsfelden (BL), Bottmingen (BL), Huningue (FR-68), Münchenstein (BL), Muttenz (BL), Reinach (BL), Riehen ( ...

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 popu ...

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