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Хоризонтална клетка



Thecellbodyofthehorizontalcellislocatedoutsidetheinnercorelayerandsendsoutmanyhorizontalbranchesextendingintotheoutermeshlayertoformsynapseswithrodcells,bipolarcellsandinterreticularcells.Therearegapjunctionsbetweenadjacenthorizontalcells.

Две функции на хоризонтални клетки:

Oneistoadjustthebrightnessofthevisualsignaloutputbythephotoreceptorcellstoachievevisualbrightnessadaptation

Thesecondistopassthecenter-Peripheralantagonisticreaction,enhancethecontrastoftheedgeoftheimage,highlightthecontourlinesofthescene

Плътността на хоризонталните клетки в централната област​​на нея е голяма. Incateyes, плътността на хоризонталните клетки от тип А в централната област​​на нея е 225 клетки/mm2, а плътността в периферните области е 120 клетки/mm2.

Horizontalcellscanbedividedintorod-levelcellsthatareonlyincontactwithrodsandcone-levelcellsthatareonlyincontactwithcones.

Конусните хоризонтални клетки могат да бъдат разделени на светимост (LHC) и цветност (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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