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Horizontal cell



Thecellbodyofthehorizontalcellislocatedoutsidetheinnercorelayerandsendsoutmanyhorizontalbranchesextendingintotheoutermeshlayertoformsynapseswithrodcells,bipolarcellsandinterreticularcells.Therearegapjunctionsbetweenadjacenthorizontalcells.

Twofunctionsofhorizontalcells:

Oneistoadjustthebrightnessofthevisualsignaloutputbythephotoreceptorcellstoachievevisualbrightnessadaptation

Thesecondistopassthecenter-Peripheralantagonisticreaction,enhancethecontrastoftheedgeoftheimage,highlightthecontourlinesofthescene

Thedensityofhorizontalcellsinthecentralareaof​​theretinaislarge.Incateyes,thedensityofA-typehorizontalcellsinthecentralareaof​​theretinais225cells/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.

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