Thecellbodyofthehorizontalcellislocatedoutsidetheinnercorelayerandsendsoutmanyhorizontalbranchesextendingintotheoutermeshlayertoformsynapseswithrodcells,bipolarcellsandinterreticularcells.Therearegapjunctionsbetweenadjacenthorizontalcells.
Twofunctionsofhorizontalcells:
Oneistoadjustthebrightnessofthevisualsignaloutputbythephotoreceptorcellstoachievevisualbrightnessadaptation
Thesecondistopassthecenter-Peripheralantagonisticreaction,enhancethecontrastoftheedgeoftheimage,highlightthecontourlinesofthescene
Thedensityofhorizontalcellsinthecentralareaoftheretinaislarge.Incateyes,thedensityofA-typehorizontalcellsinthecentralareaoftheretinais225cells/mm2,andthedensityintheperipheralareais120cells/mm2.
Horizontalcellscanbedividedintorod-levelcellsthatareonlyincontactwithrodsandcone-levelcellsthatareonlyincontactwithcones.
TheconehorizontalcellscanbedividedintoLuminosity(LHC)andChromaticity(CHC).
CHChastwocommonreactiontypes:biphasicreaction,R/Gtypehyperpolarizationforgreenlightanddepolarizationforredlight,depolarizationforgreenlightandhyperpolarityforbluelightModifiedG/Btype;three-phasereaction,hyperpolarizationofredandbluelight,anddepolarizationofgreenlight.
LHCreceivesglutamatergicexcitatorysignalsfromredandgreensensitivecones,andfeedbackinhibitorysignals.AlthoughLHCcontainsGABA,theinteractionofredandgreenconesignalsisnotdeterminedbyGABA.
Intheabsenceoflight,photoreceptorcellsreleaseglutamate,whichdepolarizeshorizontalcells.Depolarizedhorizontalcellswillhyperpolarizeadjacentphotoreceptorcells.Conversely,inalightenvironment,thephotoreceptorcellsreducethereleaseofglutamate,thehorizontalcellsarehyperpolarized,andtheneighboringphotoreceptorcellsaredepolarized.Inthisway,thehorizontalcellsprovidenegativefeedbacktothephotoreceptorcells.
Horizontalcellssummarizethereceivedsignalintensityofphotoreceptorcells,measuretheaveragebrightnessofthelightontheretina,feedbackinhibitionsignaladjuststheoutputsignalofphotoreceptorcellstoanappropriatelevel,sothatthesignalreceivedbybipolarcellsisbothItwillnotbetoosmalltodrowninthenoiseoftheneuralpathway,anditwillnotbetoolargetooversaturatetheneuralpathway.
Theexactmechanismbywhichdepolarizedhorizontalcellshyperpolarizephotoreceptorcellsisparticularlyunknown.AlthoughhorizontalcellscontainGABA,themechanismbywhichhorizontalcellsinhibitconesmaynotinvolvethereleaseofGABAfromhorizontalcellstophotoreceptorcells.
Horizontalcellsandotherretinalinterneuroncellsformexclusiveareasthatareseparatedfromeachother.Thismosaicarrangementprovidesamechanismforevenlydistributingvariousfunctionsub-unitsintheretina,ensuringthatallpartsofthefieldofviewcanreceiveafullsetofvisualprocessing.
Confocalmicrographofmouseretina.Thebluedotsaretheendfeetofconecellsandtheredarehorizontalcells.Thedendritesofthehorizontalcellsarewellconnectedtoeachcone.(Credit:ConfocalmicrographbyPatrickKeeley)
Thecenter-peripheralantagonisticresponseofbipolarcellsisthoughttobeinheritedfromhorizontalcells.Thehorizontalcellsestablishinvaginatedsynapsestowardtheendofthephotoreceptorcells,andthecenter-peripheralantagonisticresponseobviouslyoccursattheendofthecone.
Thehorizontalcellsofthebonyfishformseveralsublayersatthedistaledgeoftheinnernuclearlayeroftheretina.Theouterhorizontalcells(shortaxons)locatedatthemostdistalendformsynapsesonlywiththecones,whilethemiddlehorizontalcellsformsynapses.Cells(withoutaxons)onlyformsynapseswithrods,sotheyareoftencalledconehorizontalcellsandrodhorizontalcells,respectively.
Thestudyofcarpretinafoundthatafteralongperiodofdarkadaptation(2h),thehorizontalcellsoftheconesarestronglysuppressedinresponsetolight,andtheprotrudingendsaresmoothandblunt.Aftertheapplicationofweakbackgroundlight(15min),theresponseofthesecellsincreasedsignificantly,andalargenumberofthorn-shapedstructuresdeepintheinnerconesoftheconesappearedattheirends.Thisthorn-shapedstructureincreasedtheprotrusionsbetweentheconesandthehorizontalcells.Tactiletransmissionefficiencymaybethemorphologicalbasisfortheincreasedlightresponsivenessofcone-levelcells.
ThestudyofcruciancarpretinafoundthatundertheactionofGABA(0.5～5mmol/L),thedarkmembranepotentialofL-conehorizontalcellshyperpolarizedandtheirresponsetolightdecreased.TheeffectofGABAismainlymediatedbytheGABAAreceptorontheendofthecone.However,GABAhasnosignificanteffectonthemembranepotentialandlightresponseoftherod-levelcells.