Visual

Explanationofterms

Referstothesensationproducedbytheimageofanobjectstimulatingtheretina.

HuShi's"AnswertoLanZhixian":"Actually,thepinyintextisbothsides,thepinyinis'auditory',andthespelledtextis'visual'."QuQiubai"Introduction""Thisshadow!Heisalwaysdanglinginfrontofmyeyes-itseemstocausemyvision."

Visionisreceivedbytheperipheralsensoryorgans(eyes)ofthevisualsystemThesubjectivesensationobtainedaftertheelectromagneticwavestimulationinacertainfrequencyrangeintheexternalenvironmentisprocessedandanalyzedbytherelevantpartofthecentralcenter.

Thehumaneyecanbedividedintotwoparts:theretinaofphotoreceptorcells(rodsandcones)andtherefractivesystem(cornea,aqueoushumor,lensandvitreous).Thesuitablestimulusiselectromagneticwaveswithafrequencyof300to750terahertz,thatis,thevisiblepartofthelight,about150colors.Thispartofthelightisimagedontheretinathroughtherefractionsystem,andthentransmittedtothebrain'svisualcenterviatheopticnerve,sothatthecolorandbrightnessoftheobjectcanbedistinguished.Therefore,youcanclearlyseetheoutline,shape,size,color,distanceandsurfacedetailsofluminousorreflectiveobjectsinthevisualrange.[1]

Itisworthnotingthattherelevantvisualdeceptiontesthintsthatwhatpeopleseeisrelatedtowhattheywanttosee.

Formationprocess

Light→cornea→pupil→lens(refractinglight)→vitreous(supportingandfixingtheeyeball)→retina(formingobjectimage)→opticnerve(conductingvisualinformation)→Thevisualcenterofthebrain(formingvision)

Photoreceptors

Evolution

Theformationofphotoreceptorsintheevolutionaryprocessisofgreatsignificancetothepreciseorientationofanimals.Thesimplestlight-sensitiveorganistheeyepointofthesingle-celledprotozoanEuglena,whichallowstheEuglenatomakeadirectionalphototaxismovement.Thestructureoftheeyepointsofthevortexflagellumismoreperfect,anditcanpreywiththelightperceptionofthiseyepoint.Thephotosensitiveorgansofmulticellularanimalsarebecomingmorecomplexanddiverse.Forexample,theretinaofajellyfishisjustaplate-likestructurecomposedofpigments,whichcanprovideanimalswithinformationabouttheintensityanddirectionoflight.Withtheevolutionofanimals,cup-shapedorsac-shapedphotoreceptorsappearedwithlensesthatcanfocuslight.Annelids,molluscs,andarthropodsoftenhavebutton-likeeyesorprotrudingretinas.Thistypeofphotoreceptorconsistsofmanystructurescalledeyesarrangedonthebodysurfacebulge,whichisstilllocatedinthesmallsac.Thephotoreceptivecellsintheommatidiumaresurroundedbypigment,andlightcanonlyentertheommatidiumfromonedirection,sothedirectionoflightcanbesensed.Intheevolutionaryprocessofthiskindofvisualorganpalace,differentkindsofanimalsshowedspecificpatterns,suchasthecompoundeyesofinsects.Thevisualsystemofvertebratesusuallyincludestheretina,relatednervepathwaysandnervecenters,aswellasvariousaccessorysystemsnecessaryforitsfunctions.Theseaccessorysystemsmainlyinclude:extraocularmuscles,whichcanmaketheeyeballmoveinalldirections;therefractivesystemoftheeye(cornea,lens,etc.),toensurethatexternalobjectsformaclearimageontheretina.

Classification

Photoreceptorscanbedividedintotwocategoriesaccordingtotheirshapes,namelyrodcellsandconecells.Thephotoreceptorsintheretinaofnocturnalanimals(suchasmice)aredominatedbyrodcells,whilethedaytimeanimals(suchaschickens,squirrels,etc.)aredominatedbyconecells.Butmostvertebrates(includinghumans)haveboth.Rodcellsareactivewhenthelightisdarker,andhavehigherphotosensitivity,buttheycannotmakefinespatialdiscriminationanddonotparticipateincolorvision.Conesdominateinbrighterenvironments,whichcanprovidecolorvisionandfinevision.Thisisthecoreofthevisualdualitytheory.Inthehumanretina,thereareabout6to8millioncones,andthetotalnumberofrodsreachesmorethan100million.Theyappeartobedistributedintheretinaintheformofmosaics;theirdistributionisuneven.Inthefoveaareaof​​themaculaoftheretina,therearealmostonlycones.Thisareahashighspatialresolution(visualacuity,alsocalledvision).Italsohasgoodcolorvision,whichisthemostimportantforvision.Intheareaoutsidethefovea,therearebothtypesofcells.Thefartherawayfromthefovea,themorerodcellsandthefewercones.Inthepartwheretheopticnerveleavestheretina(thenipple),thereisnophotoreceptor,whichformsablindspot.Fromthevisualphysiologicalcharacteristicsanddistributioncharacteristicsofthetwophotoreceptors,itisknownthattheobservationcolormainlyusesthecentralareaof​​theretinaoftheeye,thatis,thefieldofviewissmaller.Becausewhenthefieldofviewistoolarge,thesensationofredandgreendisappearsfirst,andonlyyellowandbluecanbeseen;whenlookingoutward,thesensationofyellowandbluewilldisappearandbecomeafullcolorblindzone.Atthistime,thecolorjudgmentwillbeAnerroroccurred.

Basicstructure

StructureTherodsandconesaredifferentiatedintoinnerandoutersegments,whichareconnectedbyslendercilia.Theinnersegment,includingmitochondriawithnumerousnucleiandotherorganelles,iscontinuouswiththeterminalendofthephotoreceptor;theoutersegmentformsasynapticconnectionwiththesecond-levelnervecellsoftheretina.Theoutersegmentcontainsagroupofstackeddiscs,whichareformedbyinnerpleatsofthecellmembrane.Mostofthesmalldisksofrodcellshavebeenseparatedfromthecellmembrane,whilethesmalldisksofconecellsarestillconnectedtothecellmembrane.Undernormalcircumstances,thesmalldisksatthetopoftheoutersegmentcontinuetofalloff,whilethesmalldisksatthebaseclosetotheinnersegmentcontinuetomigratetothetop.However,underpathologicalconditionssuchasretinitispigmentosa,therenewalofthissmalldiskwillbehindered.

VisualpigmentsOntheoutersmalldiskarearrangedlight-sensitivepigmentmolecules,whicharecommonlycalledvisualpigments.Aseriesofphotochemicalchangesthatoccurunderlightirradiationarethestartingpointoftheentirevisualprocess.

ThevisualpigmentofrodcellsThevisualpigmentofrodcellsiscalledrhodopsin.Ithascertainspectralabsorptioncharacteristicsandispinkinthedark.Eachrodcell’soutersegmentcontains109rhodopsins.Thequalitymolecule,rhodopsinisachromoprotein,composedoftwoparts.Oneisopsin,whichhas348aminoacidsandamolecularweightofabout38000;theotherpartisthechromophore—retinal,whichisanaldehydeofvitaminA.Becauseoftheexistenceofseveralcarbondoublebonds,ithasseveraldifferences.Spaceconfiguration.Itisadistorted11-typeisomerinthedark,butitturnsintoalinearall-transisomerafterexposuretolight.Thelattercannolongerbindtoopsin,andafteraseriesofunstableintermediateproducts,retinalseparatesfromopsin.Inthisprocess,thevisualpigmentmoleculeslosetheircolor(bleaching).Inthedark,undertheactionofenzymes,retinalbecomes11-cisagain,andrecombineswithopsin(resurrection)tocompletethevisualcycle.Afterstronglightirradiation,mostoftherhodopsinisbleached,anditsre-synthesistakesabout1hour.Withtherebirthofrhodopsin,thelightsensitivityoftheretinagraduallyrecovers,whichisthephotochemicalbasisofdarkadaptation.WhenanimalsaredeficientinvitaminA,thevisualcirculationisblocked,whichcanleadtonightblindness.

ThevisualpigmentofconecellsThestructureofvisualpigmentofconecellsissimilartorhodopsin.Thedifferenceisthetypeofopsin;itsdecompositionandrebirthprocessarealsosimilar.Inanimalswithcolorvision,therearethreetypesofconecells,whichcontainvisualpigmentswithspectralabsorptionpeaksintheyellow,green,andblueregionsofthespectrum.Thisdifferentspectralsensitivityisdeterminedbythespecificityoftheiropsin.

Excitement

Itisproducedbychangesinthepermeabilityofthecellmembranetoions.Thephotoreceptorisactivewhenitisnotstimulatedbylight,thatis,theionchannelofthecellmembraneisopeninthedark,andthecurrentofsodiumionscontinuouslyflowsintothecellfromtheoutsideofthecell,andthecellmembraneisdepolarized.Illuminationcausesionchannelstoclose,reducesmembraneconductance,hyperpolarizestheentirereceptors,andexcitescells.

Becausethevisualpigmentislocatedonthesmalldiskoftheoutersegment,thechangeinthepermeabilityoftheouterplasmamembranecausedbythechangeinthespatialconfigurationofthevisualpigmentmustbeachievedthroughthesecondmessenger.In1985,scientistsappliedpatchclamptechnologytoprovethatthissecondmessengeriscyclicguanosinemonophosphate(cGMP).Thebasicprocessofthephotoreceptivemechanismis:thevisualpigmentmoleculesarephotobleachedtoactivatetheadenosinetriphosphatebindingprotein,whichinturnactivatesthephosphodiesterase,whichhydrolyzescGMPtoguanylicacidandreducestheconcentrationofcGMP.Inthedark,itiscGMPthatkeepsthecellmembraneionchannelsopen,anditsdecreaseinconcentrationwillchangetheopeningofthesechannels,leadingtotheexcitementofphotoreceptors.

Thedevelopmentofultra-microelectrodetechnology(tiplessthan1micron)allowselectrodestopenetratevertebratephotoreceptorcells(afewmicronstomorethantenmicronsindiameter)torecordandanalyzethebioelectricactivityofasinglephotoreceptor.Inthedark,duetothecontinuousflowofsodiumionsfromoutsidethecellintothecell,therestingpotentialofthephotoreceptorcellmembraneislow,andtheintracellularrecordisabout-30millivolts.Whenilluminated,thesodiumchannelisclosedandthesodiumconductancedecreases,makingthemembranepotentialClosetotheequilibriumpotentialofpotassiumions,theintracellularpotentialofthephotoreceptorbecomesmorenegative,forminghyperpolarization.Thisisanimportantfeatureoftheelectricalresponseofphotoreceptors.Inaddition,itisagradedpotentialthatgraduallyincreaseswithincreasinglightintensity,anddoesnotproducethemostcommonformofbioelectricityinnervecells-actionpotentials.

Photoreceptorshavedifferentelectricalresponsestolightwiththesamephysicalintensitybutdifferentfrequencies.Thischaracteristicisusuallydescribedbyspectralsensitivity.Inanimalswithcolorvision(includinghumans),millionsofconecellscanbedividedinto3categoriesaccordingtotheirspectralsensitivity,whichhavethebestresponsetoredlight,greenlight,andbluelightrespectively,andthethreevisualpigmentsofconecellsTheabsorptionspectrumofisveryclose,andthecolorvisionhasthreevariables.Inprinciple,anycolorcanbematchedbythemixtureofthreeselectedprimarycolors(red,green,andblue).Theremaybethreetypesofphotoreceptorssensitivetored,green,andbluelightintheretina.Theirexcitatorysignalsareindependentlytransmittedtothebrain,andthenavarietyofcolorvisionsaresynthesized.Animportantcauseofcolorblindnessisthelackofoneortwoconecellpigmentsintheretina.

Becausethephotoreceptorsremaindepolarizedinthedark,theirendscontinuetoreleasetransmitterstothesecond-levelnervecellsinthedark.Thelightwillhyperpolarizethecellmembraneandreducethereleaseoftransmitters.Thetransmitterofthephotoreceptormaybeglutamicacidorasparticacid.

Thephotoreceptorofinvertebratesdepolarizesinresponsetolightandgeneratesnerveimpulses.Thereisnodifferenceinelectricalactivityfromotherreceptors(suchasstretchreceptors).

CentralInformation

Theinformationprocessedbytheretinalneuralnetworkistransmittedtothecentralnervoussystembytheaxonsofganglioncells-opticnervefibers.Atthesiteoftheopticchiasm,abouthalfofthe1millionopticnervefibersprojectedtothelateralgeniculatebodyofthethalamusonthesameside,theotherhalfcrossedtotheoppositeside,mostprojectedtothelateralgeniculatebody,andasmallpartprojectedtothesuperiorcolliculus.Inthesuperiorcolliculus,visualinformationisintegratedwithsomatosensoryinformationandauditoryinformationtocoordinatesensoryresponseswiththerelatedmovementsoftheears,eyes,andhead.Theprojectionsofthenervecellsofthelateralgeniculatebodyformthevisualradiationprojectedtotheprimaryvisualcortex(Broadman’s17area,orcorticalarea),andthentothehighervisualcenter(parastriatearea,orBrodmann).Deman’s18,19,etc.)projection.Therearefibersfromtheprimaryvisualcortexbacktothesuperiorcolliculusandlateralgeniculatebody.Thefunctionalsignificanceofthisfeedbackpathwayisunclear.

Duetotheintersectionoftheopticnerve,theleftlateralgeniculatebodyandcortexareconnectedtothetwolefthalvesoftheretina,sotheyarerelatedtotherighthalfofthevisualfield;therightlateralgeniculatebodyandrightcortexThesituationistheopposite.Thelateralgeniculatebodyandcortexononesidereceiveinformationinputfrombotheyes,andeachsideisrelatedtotheoppositehalfofthevisualworld.Whendifferentpartsofthevisualpathwayaredamaged,correspondingvisualfielddefectswillappear,whichisofgreatsignificanceinclinicaldiagnosis.

Visualinformationundergoesfurtherprocessingatalllevelsofthevisualcentralpathway.Thelateralgeniculatebodyisonlyarelaystationforvisualinformationtransmission,anditscellreceptivefieldmaintainsaconcentricsymmetricalcenter-peripheralanteriorconfiguration.However,intheprimaryvisualcortex,exceptforasmallnumberofcellsthatstillmaintainacircularreceptivefield,mostofthecellsshowaspecialresponsepattern.Theynolongerrespondwelltothelightspots,butrequireaspecialeffectivestimulus.

Theprimaryvisualcortexcanbedividedintosimplecellsandcomplexcellsaccordingtoitsspecificrequirementsforstimulation.Simplecellsrespondtolinesegments,lightbandsorcertainlinearedgesinacertainpartofthefieldofvision.Inparticular,theyrequirethatthelinesegmentshaveaspecificorientation,andthestimulationwiththisorientation(thebestorientationofthecell)makesthecellshowthebestresponse(thepulsefrequencyisthehighest).Theoptimalorientationvarieswiththecell,andisusuallyverystrict,sothatchangingthestimulusorientationclockwiseorcounterclockwiseby10°or20°cansignificantlyreduceorevendisappearthecellresponse.Therefore,whatasimplecellreflectsisnolongerasingleisolatedone.Thelightpointsareaspecialarrangementofpointgroups,whichisobviouslyanimportantfeatureinformationextraction.Complexcellshavethebasicresponsecharacteristicsofsimplecells,buttheirmainfeatureisthattheydonothavestrictrequirementsfortheexactpositionofthelinesegmentinthefieldofview.Aslongasthelinesegmentisinthereceptivefieldofthesecells,ithasaspecificorientationandposition.Evenwithaslightdisplacement,thechangeinresponseisnotobvious.Anothercharacteristicofcomplexcellsisthattheinformationfromtheeyesstartstoconverge.Unlikethecellsofthelateralgeniculatebodyandsimplecells,theyonlyrespondtothestimulationofoneeye,buttothestimulationofbotheyes,buttheamountofresponseisusuallydifferent,andoneeyeisalwaysdominant.,Thatis,thestimulationoftheeyecancausethecellstoemithigherfrequencypulses.Thisshowsthatthecomplexcellshavebeguntoprocesstheinformationoftheeyes.

Cellswiththesameoptimalorientationorsimilareyedominanceareclusteredintheprimaryvisualcortex,andtheyformasmallcolumnarstructureextendingfromthesurfaceofthecortextothedeeppart.Betweenadjacentcolumns,theoptimalorientationofthecellsmovesregularly,andtheeyedominancealsochanges,oftenchangingfromleft-eyedominancetoright-eyedominance,orviceversa.Thissmallpieceof1mmsquareand2mmdeepisthebasiccomponentoftheprimaryvisualcortex,andtheentire17areaismainlycomposedofthistypeofbasicunit.Therefore,theunderstandingofthefunctionofarea17canbeattributedtothestudyoftheinternalfunctionofeachsmallcolumntoaconsiderableextent.Thecharacteristicsofthisfineperiodicdivisionareuniversalinthecerebralcortex,andthesomatosensorycenterandauditorycenterhavesimilarconditions.

Theprimaryvisualcortexhasbeenregardedastheendpointofthevisualpathwayforalongtime.Judgingbythedegreeofabstractionoftheinformationprocessed,itmayonlybeanearlystage.OthermoreadvancedThevisualcortexperformsfurtherfineprocessingofvisualinformation.Forexample,inarea18,therearesuper-complexcells,whichhavemorespecificrequirementsforstimulation.Onlylinesorcornerswithendpointscancausethebestresponseofthecells.Super-complexcellscanthenbedividedintoseveralsubcategories.

Basedontheseresults,someoneproposedahierarchicalhypothesisofvisualinformationprocessing.Theybelievethatfromtheconcentricreceptivefieldsoftheganglioncellsandthelateralgeniculatebodytothespecialrequirementsofsimple,complex,andsuper-complexcellsforstimulationreflectthedifferentlevelsofvisualinformationprocessing.Ateachlevel,whatthecell"sees"Itwillgetmorethanthelowerlevel,andthehigherthelevel,thehighertheinformationextractionability.Thisgradehypothesisissupportedbymanyexperiments.Itisgenerallybelievedthatinadditiontothishierarchicalinformationprocessing,thereisalsoaparallelinformationprocessingprocess,thatis,thereareseveralparallelinformationtransmissionpathwaysfromtheretinatothecenter,andthesepathwayshavedifferentdestinations.Responsiblefordifferentinformationprocessingfunctions.Therefore,asinglecellitselfdoesnotrepresentacompletesense.Thesynthesisofcellactivitiesindifferentareasofthevisualcenterreflectstherecognitionofacompleximage,andeachareacellonlyextractsacertainkindofspecialinformation:shape,color,movement,etc..

Theprocessingofothervisualinformation(suchascolor,depth,etc.)inthevisualcenterisstillpoorlyunderstood.Cellsthatspecificallyrespondtoacertaincolororacertaindepthhavebeenfoundinthevisualcortex.However,theinformationisstillfragmentary,andmorein-depthresearchisneededinordertothoroughlyunderstandthemechanismofvision.

Humaneyescanseeobjectsclearlybecausethelightemittedbytheobjectisrefractedbytheintraocularrefractivesystem(includingcornea,aqueoushumor,lens,andvitreous)andimagedontheretina.Thephotoreceptorcellsontheretina—-Conecellsandrodcellscanconvertthevisualinformationcontainedinlightstimulationintoneuralinformation,whichistransmittedtothevisualcenterofthebrainviatheopticnervetoproducevision.Therefore,visualphysiologycanbedividedintotheprocessofimaginganobjectontheretina,andtheprocessofhowthephotoreceptorcellsoftheretinatransformtheimageintoanerveimpulse.

Objectimageformation

Theimagingprincipleoflightpassingthroughtheintraocularrefractivesystemisbasicallysimilartotheimagingprincipleofacameraandaconvexlens.Accordingtoopticalprinciples,thelightemittedorreflectedbyanobjectfromsixmeterstoinfinityinfrontoftheeyesisclosetoparallellight,andtherefractionsystemofthenormaleyecanformaclearimageontheretina.Ofcourse,thehumaneyecannotseeanydistantobjectsclearly.Thisisbecausethelightoftoofarobjectsistooweakortheimageontheretinaistoosmalltobefelt.Whenthelightemittedorreflectedbytwoobjectpointsentersthepupilandtheimageformedbytherefractionofthelensfallsonthesamephotoreceptorcell,itcannotbedistinguished.Thephotoreceptorcellhasacertainsize,soitsdensityislimited.Therefore,thehumaneyehasacertainresolution.Thisresolutionischaracterizedbytheparameterminimumangularresolution.Ingeneral,thenormalangularresolutionofthehumaneyeis1ˊ.Thelightemittedbyobjectsclosertotheeyewillnotbeparallelraysbutdivergentraysofdifferentdegrees.Theyareimagedbehindtheretinathroughtherefractionsystem,sotheycanonlycauseablurredimageoftheobject.Ontheotherhand,innormaleyes,nomatterfarornearobjects,aclearimagecanbeformedontheretinathroughtherefractionsystem.Thisisduetotheadjustmenteffectofthenormalhumaneye.Theadjustmentoftheeyeismainlyadjustedbychangingtheshapeofthelens,whichisachievedthroughneuralreflexes.Whentheblurredvisualimageistransmittedtothevisualareaof​​thecerebralcortexthroughthenerve,itcancausethedownwardimpulsetobetransmittedtotheparasympatheticnucleusofthemidbrainoculomotornerve,andtotheciliarymusclethroughtheciliarynerve,whichcausesthecircularmuscletocontract,causingtheciliaryconnectinglensThezonulesareloose.Duetotheelasticityofthelensitself,itprotrudesforwardandbackward,andtherefractivepowerincreases,sothattheradiatedlightcanbefocusedforwardandimagedontheretina(Figure12-5).Theclosertheobjectistotheeyeball,thegreaterthedegreeofradiationofthelightreachingtheeyeball,andthegreaterthedegreeofconvexityofthelens.Conversely,whenviewingdistantobjects,thelensconvexitydecreases.Theadjustmentabilityofthehumaneyelensgraduallyweakenswithage.Thepotentialchangesproducedbythemaincellsaretransmittedtotheganglioncellsthroughthebipolarcells,andthenthenervefibers(opticalnerve)sentbytheganglioncellsaretransmittedtothevisualcenterintheformofactionpotentialstoproducevision.Theconductionpathwayis:theopticnerveconductsahalf-crossattheopticchiasm(thefiberfromthenasalsideoftheretinacrossestotheoppositeside,whilethefiberfromthetemporalsidedoesnotcrossandstilladvancesonthesameside),andthecrossandnon-crossfibersoneachsideoftheeyearecomposedoffibers.Ononesideoftheoptictract,theoptictractreachesthelateralgeniculatebodyatthebackofthethalamus.Aftertheneuronisreplaced,itsfibersascendthroughtheinternalcapsuleandreachtheoccipitalvisualcenterofthebrain(Figure12-6).

Otherphenomena

1.Visionreferstotheabilityofvisualorganstofinelydistinguishtheshapeofobjects.

2,thefieldofviewfieldofviewreferstotherangethatasingleeyecanseewhenitisfixedinfrontoftheeye.Thevisualfieldofclinicalexaminationisofcertainsignificanceforthediagnosisofcertaindiseasesoftheretinaandopticnerve.

3.Darkadaptationandlightadaptation.Whenpeopleenteradarkroomfromabrightplace,theycannotseeclearlyatfirst.Afteracertainperiodoftime,thevisioninthedarkplaceisgraduallyrestored,whichiscalleddarkadaptation.Onthecontrary,whengoingfromadarkplacetoastronglight,Ifeeladazzlinglightatfirst,andIcan'tseethings.Icanonlywaitforawhilebeforemyvisioncanberestored.Thisiscalledbrightadaptation.Theoccurrenceofdarkadaptationisrelatedtotheincreasedresynthesisandabsoluteamountofphotosensitivepigmentsintheretina.Fromadarkplacetoastronglight,thedazzlinglightsensationcausedisduetotherapiddecompositionofrhodopsinaccumulatedinadarkplaceunderbrightlight.Therestorationofthevisionlatershowsthattheconecellshaverestoredtheirphotosensitivefunction.

Retina-related

Eyeandretina

Theeyeissphericalandsurroundedbysclera.Thescleraisconnectedtothetransparentcorneainthefront.Behindthecorneaisthelens,whichisequivalenttothelensofacameraandisthemainrefractivesystemoftheeye.Theanteriorandposteriorchambersbetweenthelensandthecorneacontainaqueoushumor.Theentireeyeballbehindthelensisfilledwithcolloidalvitreous,whichcanprovidenutrientstovarioustissuesoftheeyeandhelpmaintaintheshapeoftheeyeball.Ontheinnersurfaceoftheeyeball,thereisalayerofretinawithathicknessofonly0.3mm,whichistheperipheralpartoftheopticnervoussystem.Betweentheretinaandthescleraisthechoroidfullofbloodvessels,whichnourishestheretina.

Thecorneaandlensformtherefractivesystemoftheeye,whichmakesexternalobjectsformaninvertedimageontheretina.Thecurvatureofthecorneaisfixed,butthecurvatureofthelenscanbeadjustedbytheciliarymuscleviathesuspensoryligament.Whentheobservationdistancechanges,thefocallengthoftheentirerefractivesystemischangedthroughthechangeofthelenscurvature,soastoensurethattheimageofexternalobjectsontheretinaisclear.Thisfunctioniscalledvisualadjustment.Whenthevisualaccommodationisabnormal,theobjectcannotbeclearlyimagedontheretina,andnearsightednessorhyperopiacanoccur.Atthistime,asuitablelensshouldbeusedtocorrectit.

Betweenthecorneaandthelens,thepupilformedbytheirisactsasadiaphragm.Thepupilshrinkswhenilluminatedandexpandsinthedarktoadjusttheamountoflightenteringtheeye,whichalsohelpstoimprovetheimagingqualityoftherefractivesystem.Pupilandvisualadjustmentarecontrolledbytheautonomicnervoussystem.

Themovementoftheeyeballisrealizedbythesixextraocularmuscles.Thecoordinatedmovementofthesemusclesensuresthattheeyeballmovesfreelyinalldirectionsandmakesthelineofsightchangeasneeded.Theactivitiesoftheextraocularmusclesofthetwoeyesmustbecoordinated,otherwiseitwillcausedoubleretina(diplopia)orstrabismus.

Theretinaisalayerofnervetissuecontaininghundredsofmillionsofnervecells.Accordingtothecharacteristicsoftheshapeandlocationofthesecells,itcanbedividedintosixcategories,namelyphotoreceptors,horizontalcells,bipolarcells,andamacrinecells.,Ganglioncells,andinterreticularcellsnewlydiscoveredinrecentyears.Amongthem,onlythephotoreceptorissensitivetolight,andtheinitialbiophysicalandchemicalprocesstriggeredbylightoccursinthephotoreceptor.Thevertebrateretinaisinvertedduetoembryonicdevelopment,thatis,afterlightenterstheeyeball,itfirstpassesthroughthenetworkofnervecellsandfinallyreachesthephotoreceptor.However,thetransparencyofnervecellsissohighthatitdoesnotaffectthequalityoftheimage.

Neuralnetworkoftheretinaanditsinformationprocessing

Billionsofnervecellsintheretinaarearrangedinthreelayers,formingacomplexnetworkofinformationprocessingthroughsynapses.Thefirstlayerisphotoreceptors,thesecondlayerisinterneuroncells,includingbipolarcells,horizontalcellsandamacrinecells,andthethirdlayerisganglioncells.Thesynapsesbetweenthemformtwosynapticlayers,namelytheouterreticularlayercomposedofphotoreceptors,bipolarcells,horizontalintercellularsynapses,andbipolarcells,amacrinecells,andinter-ganglioncellsynapses.Innermeshlayer.Afterthephotoreceptorsareexcited,theirsignalsaremainlytransmittedtotheganglioncellsthroughthebipolarcells,andthentothenervecenterthroughtheaxons(opticalnervefibers)ofthelatter.Butintheouterreticularlayerandtheinnerreticularlayer,thesignalismodulatedbyhorizontalcellsandamacrinecells.Thetransmissionofthissignalismainlyachievedthroughchemicalsynapses,buttherearealsoelectricalsynapses(gapjunctions)betweenphotoreceptorsandhorizontalcells,linkingtheinteractionswitheachother.

Thetransmissionpathwaysoftherodcellsignalandtheconecellsignalintheretinaarerelativelyindependent,untiltheganglioncellsconverge.Thereisonlyonetypeofbipolarcellsthatreceivesignalsfromrodcells(rodbipolarcells),butbipolarcellsthatreceivesignalsfromconecellscanbedividedintotwotypes:trappedtypeandflattypeaccordingtothecharacteristicsoftheirsynapses.Cellshavedifferentfunctionalproperties.Intheouterreticularlayer,horizontalcellsreceivesignalsfromphotoreceptorsinawiderangeandinteractwithbipolarcellsatsynapses.Inaddition,thehorizontalcellsalsomodulatethesignalintheformoffeedbacktothephotoreceptors.Thesignalsfromthebipolarcellsintheinnerreticularlayeraretransmittedtotheganglioncells,andtheamacrinecellsconnecttheneighboringbipolarcells.Theconfluenceofrodandconesignalsmayalsooccurinamacrinecells.

Thesignalofthephotoreceptorismainlytransmittedtotheinterneuroncellsbychangingtheamountofthetransmitterreleasedbythechemicalsynapse.Theactivityofbipolarcellsandhorizontalcellsisstillintheformofgradedpotentialswithoutnerveimpulses.Buttheyarenolongerlikephotoreceptors,theyonlyreactwhenlightilluminatesacertainpointoftheretina,butspreadtoaregion,andtherangeoftheretinatheyfeelincreasessignificantly.Somehorizontalcellsevenrespondtolightonanypartoftheretina,whichindicatestheconvergenceofphotoreceptorsignalsindifferentspatiallocations.Itisparticularlyimportantthatthereceptivefieldofbipolarcellspresentsacertainspatialconfiguration.Somecellsdepolarizeinthecenterofthephotoreceptivefield,butthepolarityoftheresponseisreversedwhentheperipheralareaisilluminated—hyperpolarization;othercellshavetheoppositereactionpattern;horizontalcellsareinthiscenter-peripheralantagonismThetypeofreceptivefieldplaysanimportantrole.Thesetwotypesofcellsarecomparableinmorphologytotrappedandflatbipolarcells,respectively.

Inamacrinecells,therearesomepulse-likeresponsesatthebeginning,buttheyarestilldominatedbygradedpotentials.Theresponseofganglioncellstolightiscompletelyimpulsive,andthecentral-peripheralanteriorreceptivefielddevelopsmorecompletely.Thereceptivefieldofhigheranimalganglioncellsisusuallyconcentricandconsistsoftwoparts:thecenterandthesurroundingarea.Somecellswillhaveaseriesofpulseswhenilluminatingthecentralareaof​​theirreceptivefield.Thestrongerthelight,thehigherthepulsefrequency.Whentheperipheralareaisilluminated,thespontaneouspulseofthecellwillbeinhibited.Thiskindofcellisoftencalledlight-Centralcell.Forotherso-calledlight-extracted-centralcells,whenthecentralareaof​​thereceptivefieldisirradiated,notonlydoesnotappearpulses,butspontaneouspulsesaresuppressed,butaseriesofpulsessuddenlyappearafterthelightstops.Ifthelightismovedtotheouterperipheralarea,thereactionpatternistheopposite.Ifthelightirradiatesallthereceptivefields,theganglioncellsoftenhavenoresponseoronlyaweakresponse;whileonadarkbackgroundalightspotfilledwiththecentralareaof​​thereceptivefield(forlight-centralcells)orabrightbackgroundlightisfullofreceptivefieldsThedarkspotinthecentralarea(forthelight-exhausted-centralcell)causesthestrongestresponseofthecell.

Theappearanceofcentral-peripheralanteriorreceptivefieldmarksanimportantstageofvisualinformationprocessing.Themostimportantfunctionofvisionistodistinguishimages,andanyimageis,inthefinalanalysis,acombinationofdifferentlightanddarkparts.Whenthephotoreceptordetectsthepresenceoflight,itrequiresaneuralmechanismtospecificallyprocessthecontrastinformation.Thecentral-peripheralanteriorreceptivefieldisanimportantmanifestationofthisneuralmechanism.

Colorperceptionisanotherimportantaspectofvision.Althoughthecolorinformationisencodedbythreedifferentsignalsofred,green,andblueatthelevelofphotoreceptors,thesethreesignalsarenotasassumedbythethree-colortheory,andeachistransmittedtothebrainbydedicatedlines.Inthehorizontalcell,thesignalsofdifferentcolorsconvergeinaspecificway.Forexample,somecellsaredepolarizedwhenirradiatedwithredlight,butthereactionpolaritychangestohyperpolarizationwhenirradiatedwithgreenlight.Othercellsreactintheoppositepattern.Similarly,therearecellsthatreacttothegreen-bluecolor.Althoughothernervecellsintheretinareactindifferenttypes(eitherhierarchicalpotentials,ornerveimpulses),theyallrespondtocolorsignalsinaantagonisticmanner.Inganglioncells,thisformofantagonisticreactionismorecomplete,andmanyofthesecellsarealsoantagonisticinspatialresponse.Forexample,thereisaso-calleddouble-jacketcellthatrespondstolightwhenirradiatingthecentralareaof​​thereceptivefieldwithredlight,andwithdrawinglightwhenirradiatingtheareaaroundthereceptivefield;theresponsetogreenlightistheopposite.Thistypeofcodingformensuresthatthesignalsofdifferentphotoreceptorswillnotbeconfusedduringthetransmissionprocess.Thiswayisexactlywhatanothertheoryofcolorvisionassumes-thecolortheory.Therefore,withthedeepeningoftheunderstandingofobjectivelaws,thethree-colortheoryandtheJiejie-colortheoryhavebeendialecticallyunifiedonanewlevel.

Thecellbodiesoftheinterreticularcellsarearrangedatthesamelevelastheamacrinecells,andtheirprotrusionsextendwidelyinthetwosynapticlayers.Theyreceivesignalsfromamacrinecellsandfeedthembacktohorizontalcells.Thiscentrifugalfeedbackpathwayiscombinedwiththemainpathwayofphotoreceptor→bipolarcell→ganglioncelltotransmitinformationtotheheart,makingtheretinaacompleteone.NeuralNetworks.

Thephotoreceptortransductioneffectoftheretina

Asmentionedabove,thereisaphotoreceptorcelllayerintheretina,andthephotoreceptorcellsofhumansandmostvertebrateshaverodcells.Andcones.Photoreceptorcellscansynapticcontactwithbipolarcellsthroughtheterminalfoot,andthebipolarcellscontactganglioncells.Theprotrusionsgeneratedbytheganglioncellsaggregateintobundlesonthesurfaceoftheretina,andthenpassthroughthechoroidandscleratoformtheopticnerve.Theeyeballpassesthroughtheopticcanalintothecranialcavity,andconnectstothediencephalonviatheopticchiasm.

Theimageoftheobjectfallingontheretinafirstlycausesaphotochemicalreaction,andthephotosensitivematerialhasbeenextractedfromtheretina.Thesesubstancesarepurple-redinthedark,butquicklyfadeandturntowhitewhenexposedtolight.Ifyouputafrogorarabbitinadarkroom,keeptheanimalfacingthebrightwindowforacertainperiodoftime,thenimmediatelyremovetheeyeballandremovetheretinafromthelight,andtreattheretinawithappropriatechemicalssuchasalum,andyoucanfindthattheanimal’sretinahasanimageofthewindow.Thelight-transmittingpartofthewindowiswhite,andthewindowframeisdarkred.Alltheseindicatethephotochemicalreactionofthephotosensitivematerialontheretinaundertheactionoflight.Inalargenumberofstudiesonphotoreceptorcells,rodcellshavebeenstudiedmoreclearly.Thephotosensitivesubstanceofrodcellsiscalledrhodopsin,whichisformedbythecombinationofopsinandretinal.RetinaldehydeisconvertedfromvitaminA.Rhodopsinisrapidlydecomposedintoopsinandretinalunderlight.Atthesametime,itcanbeseenthattherodcellshavereceptorpotentials,whichthencausetheactivitiesofotherretinalcells.

Rhodopsinisdecomposedinbrightplacesandcanbere-synthesizedindarkplaces.Whenpeoplelookatthingsinthedark,theyactuallyhaveboththedecompositionandsynthesisofrhodopsin.Thedarkerthelight,themorethesynthesisprocesssurpassesthedecompositionprocess,whichisthebasisforpeopletoconstantlyseematterinthedark.Onthecontrary,undertheactionofstronglight,thedecompositionofrhodopsinisenhanced,synthesisisreduced,andtherhodopsinintheretinaisgreatlyreduced,sothesensitivitytoweaklightisreduced.Therefore,rodcellsaresensitivetolowlight,whichisrelatedtoscotopicvisionatdusk.Duringthedecompositionandresynthesisofrhodopsin,apartofretinalwillbeconsumed,whichismainlysupplementedbyvitaminAintheblood.IfvitaminAislacking,itwillaffectpeople'svisioninthedarkasnightblindness.

Theconecellsalsocontainspecialphotosensitivepigments.Itiscalledrhodopsin.Accordingtothestudyoflight-sensitivepigmentsinavarietyofanimalcones,itisbelievedthattheyarealsoacombinationofretinalandopsin.

Therearethreetypesofconecellsorcorrespondingphotosensitivepigmentsthatareparticularlysensitivetored,green,andbluelightintheretina.Becausethethreecolorsofred,greenandblueareproperlymixed,itcancausethesensationofanycolorinthespectrum.Therefore,itisbelievedthatconecellsarerelatedtocolorvision.Colorblindnessmaybecausedbythelackofcorrespondingconecells.Thedifferenceinlightsensitivityofthethreetypesofconecellsisrelatedtotheirdifferentlight-sensitivesubstances.Thethreephotosensitivepigmentsarecomposedofretinalandopsin.Amongthem,retinalisbasicallythesame,whiletheopsinofthethreeisslightlydifferent.Thisdifferencemaybethereasonfortheirdifferentphotosensitivecharacteristics.