The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the colored part of the eye ...NCBISkiptomaincontentSkiptonavigationResourcesHowToAboutNCBIAccesskeysMyNCBISignintoNCBISignOutNCBIBookshelf.AserviceoftheNationalLibraryofMedicine,NationalInstitutesofHealth.StatPearls[Internet].TreasureIsland(FL):StatPearlsPublishing;2021Jan-.StatPearls[Internet].ShowdetailsTreasureIsland(FL):StatPearlsPublishing;2021Jan-.SearchtermAnatomy,HeadandNeck,EyeIrisSphincterMuscleJeffreyBloom;MahsawMotlagh;CraigN.Czyz.AuthorInformationAuthorsJeffreyBloom;MahsawMotlagh1;CraigN.Czyz2.Affiliations1UniversityofArizona2OhioUniversityLastUpdate:July25,2021.IntroductionTheirissphinctermuscle,alsoknownasthepupillarysphincterorsphincterpupillae,isamusclelocated inthecoloredpartoftheeyecalledtheiris.Thesphinctermusclefibersarelocatednearthepupillarymarginandareslightlyanteriortothepigmentedepitheliumoftheiris.Itencirclesthepupiloftheirisandfunctionstoconstrictthepupilinbrightlightviathepupillarylightreflexorduringaccommodation.Bycontrollingthediameterofthepupil,theiriscontrolstheamountoflightthatreachesthebackoftheeyeattheretina.StructureandFunctionTheirisisacircular,pigmenteddiaphragmthatliesanteriortothelensinacoronalplane.Itisattachedperipherallytotheciliarybody,withitsunboundcentralborderforminganapertureknownasthepupil.Theirisdividesthespacebetweenthecorneaandthelensintotwochambers:anteriorandposterior.Theanteriorchamberisboundbythecorneaandiris.Theanteriorchambercommunicatesthroughthepupilwiththeposteriorchamber,whichisboundbytheiris,ciliaryprocesses,zonules,andlens.Afluidsubstanceknownasaqueoushumorfillsbothchambers.Theiris,viathesphincterpupillae,isapartoftheaccommodativereflex.Theaccommodativereflexoftheeyeisaninvoluntaryresponsewhenthevisualfocusswitchesfromanobjectatadistancetoonethatiscloser.Thereflexis controlledbytheparasympatheticnervous systemandinvolvesthreeactions-alterationofpupilaperture,lensshape,andconvergence.Thesphinc