Introductiontothevisualorgans
Thevisualorgansarethereceptorsforhumansandanimalstoperceiveexternalthingsthroughtheactionoflight.Lightactsonthevisualorganstoexcitethesensorycells,andtheinformationisprocessedbythevisualnervoussystemtoproducevision.Throughvision,humansandanimalsperceivethesize,brightness,color,movementandstaticofexternalobjects,andobtainvariousinformationthatisimportanttothesurvivalofthebody.Atleast80%ofexternalinformationisobtainedthroughvision.Visionisthemostimportantthingforhumansandanimals.feel.Thevisualsystemofvertebratesusuallyincludestheretina,relatednervepathwaysandnervecenters,aswellasvariousaccessorysystemsnecessaryforitsfunctions.
Auxiliarysystems
Theseauxiliarysystemsmainlyinclude:extraocularmuscles,whichcanmaketheeyeballmoveinalldirections;therefractivesystemoftheeye(cornea,lens,etc.),whichguaranteesexternalobjectsAclearimageisformedontheretina.
Organcomposition
Composition
Theperipheralsensoryorganthatcausesvisionistheeye.Thevisualorganconsistsoftheretinacontainingphotoreceptorcellsandtherefractivesystemasanaccessorystructure.composition.Thesuitablestimulusforthehumaneyeiselectromagneticwaveswithawavelengthof380-780nm;inthisvisiblespectrum,thehumanbraincandistinguishthedifferentbrightnessandcoloroftheretinaimagebyreceivingincominginformationfromtheretina,sothatthelightinthefieldofviewcanbeseenclearlyTheoutline,shape,color,size,distanceandsurfacedetailsofthereflectivematerialoftheobject.Variousobjectsinnature,characters,graphicsandotherimagesarereflectedinthehumanbrainthroughthevisualsystem.About95%ofalltheinformationobtainedbythehumanbraincomesfromthevisualsystem,sotheeyeisundoubtedlyoneofthemostimportantsensoryorgansinthehumanbody.
Functionalstructure
Inadditiontotheextraocularmusclesthatcontroleyemovement,thesclera,choroidandotherstructuresthatplayaroleinmaintenanceandnutrition,theeyeisdirectlyrelatedtotheproductionofvisualafferentinformationThefunctionalstructureistherefractivesystemlocatedonthemidlineoftheeyeballandtheretinalocatedatthebackoftheeyeball.Fromthecornea,theaqueoushumor,thelens,thevitreousbodytothefrontsurfaceoftheretina,therearesometransparentandnon-vascularizedtissues.Theyconstitutetherefractivesystemintheeye,whichrefractsthelightfromoutsidetheeye,andfinallytheimageisimagedontheretina.Theretinahasacomplexstructuresimilartonervetissue,whichcontainsrodsandconesthatarehighlysensitivetolightstimuli,whichcanconvertthevisualinformationcontainedinexternallightstimuliintoelectricalsignals,andperformpreliminaryprocessingintheretina.Itistransmittedtothebrainintheformofactionpotentialsofopticnervefibers.Therefore,todescribethefunctionoftheeye,wemustfirststudytheunlearnedcharacteristicsoftheintraocularrefractivesystem,andfigureouthowtheycanimageobjectsatdifferentdistancesontheretinaandthelimitsofformingclearobjectimages;secondly,itisnecessarytoclarifyhowtheretinaperformsretinalimaging.Transformationandcoding.
Peripheralsystem
Theeyeandretinaaresphericalinshape,surroundedbysclera.Thescleraisinfrontofthetransparentcornea.Behindthecorneaisthelens,whichisequivalenttothelensofacameraandisthemainrefractivesystemoftheeye.Theanteriorandposteriorchambersbetweenthelensandthecorneacontainaqueoushumor.Theentireeyeballbehindthelensisfilledwithagel-likevitreousbody,whichcanprovidenutrientstovarioustissuesoftheeyeandhelpmaintaintheshapeoftheeyeball.Ontheinnersurfaceoftheeyeball,thereisalayerofretinawithathicknessofonly0.3mm,whichistheperipheralpartoftheopticnervoussystem.Betweentheretinaandthescleraisthechoroidfullofbloodvessels,whichnourishestheretina.
Thecorneaandlensformtherefractivesystemoftheeye,whichmakesexternalobjectsformaninvertedimageontheretina.Thecurvatureofthecorneaisfixed,butthecurvatureofthelenscanbeadjustedbytheciliarymuscleviathesuspensoryligament.Whentheobservationdistancechanges,thefocallengthoftheentirerefractivesystemischangedthroughthechangeofthelenscurvature,soastoensurethattheexternalobjectsareclearlyimagedontheretina.Thisfunctioniscalledvisualadjustment.Whenthevisualaccommodationisabnormal,theobjectcannotbeclearlyimagedontheretina,andnearsightednessorhyperopiacanoccur.Atthistime,asuitablelensisneededtocorrectit.Betweenthecorneaandthelens,thepupilformedbytheirisactsasadiaphragm.Thepupilshrinkswhenilluminatedandexpandsinthedarktoadjusttheamountoflightenteringtheeye,whichalsohelpstoimprovetheimagingqualityoftherefractivesystem.Pupilandvisualadjustmentarecontrolledbytheautonomicnervoussystem.
Themovementoftheeyeballisrealizedbythesixextraocularmuscles.Thecoordinatedmovementofthesemusclesensuresthattheeyeballmovesfreelyinalldirectionsandmakesthelineofsightchangeasneeded.Theactivitiesoftheextraocularmusclesofthetwoeyesmustbecoordinated,otherwiseitwillcausedoubleretina(diplopia)orstrabismus.
Theretinaisalayerofnervetissuecontaininghundredsofmillionsofnervecells.Accordingtothecharacteristicsoftheshapeandlocationofthesecells,itcanbedividedintosixcategories,namelyphotoreceptors,horizontalcells,bipolarcells,andamacrinecells.,Ganglioncells,andinterreticularcellsnewlydiscoveredinrecentyears.Amongthem,onlythephotoreceptorissensitivetolight,andtheinitialbiophysicalandchemicalprocesstriggeredbylightoccursinthephotoreceptor.
Classifiedbyshape
Photoreceptorscanbedividedintotwocategoriesaccordingtotheirshapes,namelyrodcellsandconecells.Thephotoreceptorsintheretinaofnocturnalanimals(suchasmice)aredominatedbyrodcells,whilethedaytimeanimals(suchaschickens,squirrels,etc.)aredominatedbyconecells.Butmostvertebrates(includinghumans)haveboth.Inthehumanretina,thereareabout6to8millioncones,andthetotalnumberofrodsreachesmorethan100million.Theyappeartobedistributedintheretinaintheformofmosaics;theirdistributionisuneven.Inthefoveaareaofthemaculaoftheretina,therearealmostonlycones.Thisareahashighspatialresolution(visualacuity,alsocalledvision).Italsohasgoodcolorvision,whichisthemostimportantforvision.Intheareaoutsidethefovea,therearebothtypesofcells.Thefartherawayfromthefovea,themorerodcellsandthefewercones.Inthepartwheretheopticnerveleavestheretina(thenipple),thereisnophotoreceptor,whichformsablindspot.
Theneuralnetworkoftheretinaanditsinformationprocessing.Hundredsofmillionsofnervecellsintheretinaarearrangedin3layers,formingacomplexnetworkofinformationprocessingthroughsynapses,namelyphotoreceptorsandbipolarcells,andsynapsesbetweenhorizontalcells.Theouterreticularlayercomposedofbipolarcells,amacrinecells,andtheinnerreticularlayercomposedofsynapsesbetweenganglioncells.Afterthephotoreceptorsareexcited,theirsignalsaremainlytransmittedtotheganglioncellsthroughthebipolarcells,andthentothenervecenterthroughtheaxons(opticalnervefibers)ofthelatter.Butintheouterreticularlayerandtheinnerreticularlayer,thesignalismodulatedbyhorizontalcellsandamacrinecells.Thetransmissionofthissignalismainlyachievedthroughchemicalsynapses,buttherearealsoelectricalsynapses(gapjunctions)betweenphotoreceptorsandhorizontalcells,linkingtheinteractionswitheachother.
Thetransmissionpathwaysoftherodcellsignalandtheconecellsignalintheretinaarerelativelyindependent,untiltheganglioncellsconverge.Intheouterreticularlayer,horizontalcellsreceivesignalsfromphotoreceptorsinawiderangeandinteractwithbipolarcellsatsynapses.Inaddition,thehorizontalcellsalsomodulatethesignalintheformoffeedbacktothephotoreceptors.Thesignalsfromthebipolarcellsintheinnerreticularlayeraretransmittedtotheganglioncells,andtheamacrinecellsconnecttheneighboringbipolarcells.Theconfluenceofrodandconesignalsmayalsooccurinamacrinecells.Thesignalofthephotoreceptorismainlytransmittedtotheinterneuronsbychangingtheamountofthetransmitterreleasedbythechemicalsynapse.
InformationprocessingofthevisualcenterTheinformationprocessedbytheretinalneuralnetworkistransmittedtothecenterbytheaxonsofganglioncells-opticnervefibers.
Becauseoftheintersectionoftheopticnerve,theleftlateralgeniculatebodyandcortexareconnectedtothetwolefthalvesoftheretina,sotheyarerelatedtotherighthalfofthevisualfield;therightlateralgeniculatebodyandrightcortexThesituationistheopposite.Thelateralgeniculatebodyandcortexononesidereceiveinformationinputfrombotheyes,andeachsideisrelatedtotheoppositehalfofthevisualworld.Whendifferentpartsofthevisualpathwayaredamaged,correspondingvisualfielddefectswillappear,whichisofgreatsignificanceinclinicaldiagnosis.
Visualinformationundergoesfurtherprocessingatalllevelsofthevisualcentralpathway.Thelateralgeniculatebodyisjustarelaystationforvisualinformationtransmission.
Refractionsystem
Therefractivesystemoftheeyeanditsadjustment
Whenlightentersthesinglesphericalrefractivebodycomposedofanothermediumfromair,itentersthematerialTherefractiondependsontheradiusofcurvatureRoftheinterfacebetweenthesubstanceandtheairandtherefractiveindexn2ofthesubstance;iftherefractiveindexoftheairisn1,therelationshipisn2R/(n2-n1)=F2(1)
F2iscalledtherearmainfocallengthorthesecondfocallength(thefocallengthontheairsideisthefrontmainfocallengthorthefirstfocallength),whichreferstothedistancefromtherefractionsurfacetotherearmainfocalpoint,andcanrepresenttherefractivepowerofthisrefraction.Anothermethodcanbeusedtoexpresstherefractivepowerofarefractingbody,thatis,themainfocallengthisexpressedinm(meters),andthenthereciprocalofthisvalueistaken.Thelatteriscalledthediopteroftherefractingbody;Themainfocallengthofacertainlensis10cm,whichisequivalentto0.1m,andtherefractivepowerofthelensis10powers(10D).Itisusuallystipulatedthatthepowerofaconvexlensisapositivevalue,andthepowerofaconcavelensisanegativevalue.
Themainfocallengthisthemostimportantopticalparameterofarefractingbody,fromwhichthepositionoftherefractedimageformedbyanobjectatanypositioncanbecalculated.Takingathinlensasanexample,iftheobjectdistanceαisknown,theimagedistancebcanbecalculatedbythefollowingformula:
1/a+1/b=1/F2(2)
Itcanbeseenfromequation(2)thatwhentheobjectdistanceatendstoinfinity,1/aapproacheszero,so1/biscloseto1/F2,thatis,theimagedistancebisalmostequaltoF2;thisisInotherwords,whenanobjectisinfinitelyfarawayfromaconvexlens,itsimagingpositionwillbeatthebackmainfocusposition.Itisalsonotdifficulttoseethatforobjectswhoseobjectdistanceislessthaninfinity,theirimagedistancebisalwaysgreaterthanF2,thatis,theywillbeimagedfartherthanthemainfocus.Theabovetwoconclusionsareveryimportantforunderstandingtherefractiveimagingabilityoftheeye.
Inaddition,accordingtotheprincipleofoptics,thepositionofthemainfocusisthepositionwheretheparallellightraysarecondensedintoapointafterbeingrefracted.Thisconclusionisconsistentwiththefirstconclusionmentionedabove.Thesurfaceofeachobjectcanbeconsideredtobecomposedofcountlessluminouspointsorreflectivepoints,andthelightemittedbyeachpointisdivergent;onlywhenthedistancebetweenthesepointsandthecorrespondingrefractingsurfacetendstobeinfinite,Thelightraysreachingtherefractingsurfacefromthesepointscanbeclosetoparallel,soaftertheyarerefracted,theyconvergeintoapointonthesurfacewherethemainfocusislocated,andtheentiresubstancereachesonesurfacetoformanobjectimage.Ofcourse,theconceptofinfinitepassageitselfdeterminesthatitisanimpossibleposition.Infact,forthehumaneyeandgeneralopticalsystems,thelightfromeachlightpointofanobjectbeyond6mcanberegardedasnearlyparallel.Therefore,itispossibletoformanobjectimageonthesurfacewherethemainfocusislocated.
Opticalcharacteristicsoftherefractivesystemoftheeye
Whenusingtheaboveopticalprinciplestoanalyzetherefractivecharacteristicsoftheeye,thefirstdifficultyencounteredisthattheeyeballisnotathinlensorasinglesphericalrefractiveindex.Itisarefractionsystemcomposedofaseriesofrefractionbodieswithdifferentradiiandrefractiveindices.Obviously,thebackmainfocallengthofthehumaneye'srefractivesystemcannotbesimplycalculatedbyequation(1),butitsmostimportantrefractionoccursinthecornea,andmorecomplicatedcalculationsbasedongeometricalprinciplescanstilltracethelightpassingthroughtheeyeThepathofmultiplerefractingsurfaces,andthepositionofthebackmainfocusdeterminedbythesecombinedlensgroupsisobtained.Thecalculationresultsshowthatwhenthenormaladulteyeisinaquietstatewithoutadjustment,thepositionoftherearmainfocusofitsrefractivesystemisexactlythepositionofitsopticwindmembrane.Thisanatomicalrelationshipisveryimportantforunderstandingtherefractiveimagingcapabilitiesofnormaleyes.Itshowsthatallobjectslocated6minfrontoftheeyetoinfinity,accordingtoformula(2)orbecausethelightemittedorreflectedbythemisalmostparallelwhenreachingtherefractivesystemoftheeye,theycanbeformedontheretinaBasicallyclearimages,justlikethefilmplacedinthemainfocusofthecamera,youcantakeaclearperspective.Ofcourse,thehumaneyedoesnotunconditionallyseeanydistantobjects.Forexample,thehumaneyecanseethemoon(orotherfartherstars)andthelargershadowsonitssurface,butitcannotseethesmallerobjectsonthemoon.Orfeatures.Thereasonforthelatterlimitationisthatifthelightfromanobjectistooweak,ortheyarescatteredorabsorbedwhentheytravelthroughthespacevirgin,thenwhentheyreachtheretina,theyhavebeenweakenedtotheextentthattheyarenotenoughtoexcitethephotoreceptorcells,soitisimpossible.Beperceived;Inaddition,ifobjectsaretoosmallortheirdistancefromtheeyeistoolarge,thesizeoftheirformationontheretinawillbesmallerthanthelimitoftheretina'sresolvingpower,andthereforecannotbesensed.
Eyeadjustment
Iftherefractivepoweroftheeyeinaquietstateisjusttoimageobjectsbeyond6montheretina,thenthelightfromobjectscloserto6mwillbeofdifferentdegreesTheyareradial,andtheirimagingpositionafterrefractionwillbebehindthemainfocus,thatis,thepositionoftheretina;becausethelightisnotfocusedwhenitreachestheretina,theobjectimageisblurred,whichcanonlycauseablurredvisualimage.However,thenormaleyeisalsoveryclearwhenlookingatthenearobject.Thisisbecausetheeyehasadjusted(accommodation)whenlookingatthenearobject,sothatthelightenteringtheeyeundergoesstrongrefraction,andasaresult,theimagecanalsobeimagedontheretina.Theadjustmentofthehumaneye,thatis,thechangeinrefractivepower,ismainlydependentonthechangeintheshapeofthelens;thisisaneuroreflexactivity,andtheprocessisasfollows:Whenablurredvisualimageappearsinthevisualcortex,theresultingdownwardimpulseThecortex-midbraintractinthepyramidaltractreachesthemediannucleusofthemidbrain,thenreachestherelatednucleusthatsendsouttheparasympatheticpreganglionicfibersoftheoculomotornerve,andfinallyreachestheintraocularciliarymusclethroughtheciliaryganglion,makingitcirculateMusclecontractioncausesthesuspensoryligamentconnectedtothelenscapsuletorelax;thiscausesthelenstoprotrudeforwardandbackwardduetoitsownelasticity(theanteriorprotrusionismoreobvious),whichincreasesthetotalrefractivepoweroftheeyewhenitisquieter.Themoreradiantlightisfocusedinadvanceandcanalsobeimagedontheretina.Indicatesthechangeintheshapeofthelensbeforeandafteradjustment.Obviously,theclosertheobjectistotheeyeball,thegreaterthedegreeofdivergenceofthelightreachingtheeye,whichrequiresthelenstobecomemoreconvex.Whenadjustingthereflex,inadditiontothechangeofthelens,thereisalsotheshrinkageofthepupilandtheconvergenceofthenasalmidlineofthevisualaxisofthetwoeyes.Themeaningoftheformeristoreducetheamountoflightenteringtheeye(whentheobjectmovescloser,strongerlightwillarrive.Eyeball)andreducethesphericalaberrationandchromaticaberrationoftherefractivesystem;themeaningoftheconvergenceofthetwoeyesisthattheimageoftheobjectcanstillfallontheappropriatepositionoftheretinaofthetwoeyeswhenlookingatacloseobject.
Responsetolight
Thepupilreferstotheopeninginthemiddleoftheiris,whichistheportalforlighttoentertheeye;itshrinksinbrightlight,
Theorganofvision(rightSideeyeballhorizontallybrokenpatternpicture)Scatteredinthedarkplace.Theirisiscomposedofmultipleunitsofsmoothmuscle;aroundthepupilisthecircularmusclelayer,whichisinnervatedbytheparasympatheticnervefibersintheoculomotornerve,whichshrinksthepupilwhencontracted,soitisalsocalledpupillarysphincter;Itisinnervatedbytheascendingsympatheticnervefibersintheneckanddilatesthepupilswhencontracted,soitisalsocalledmydriaticmuscle.Thesizeofthepupilcancontroltheamountoflightenteringtheeye.Thediameterofordinarypeople'spupilscanvarybetween1.5-8.0mm.Assumingthatthepupildiametercanbeincreasedby5timeswhenapersonentersadarkroomfromabrightplace,thepupil'slight-receivingareashouldbeincreasedby25times;visiblepupilchangeshavetheeffectofkeepingtheamountoflightenteringtheeyerelativelyconstantunderdifferentlightingconditions.However,theintensityofthestrongsunlightinthedarkroomisactuallyreducedbyabout1milliontimes,sothechangeofpupilsizealoneisfarfromenoughtokeeptheamountoflightenteringtheeyeconstant.Infact,thehumaneyereliesondifferentphotoreceptorcellsintheretinatoreceivelightstimulationunderdifferentbrightnessconditions.Therodcellsthatworkindarklightaremoresensitivetolightthantheconesthatworkinbrightlight.Thecellsaremuchlarger,sowhenyoulookatsomethinginthedark,youonlyneedtoincreasetheamountoflightenteringtheeye.Itcanbeseenthatitismeaningfultoadjusttheamountoflightenteringtheeyebychangingthepupilsize.Theresponseofpupilsizetochangesinlightintensityisakindofneuralreflex,calledpupilreflextolight.Thereceptorthatcausesthisreflexistheretina,andtheafferentfibersareintheopticnerve,butthispartofthefibersdoesnotreachthelateralgeniculatebodyafterenteringthecenter,andchangesneuronsintheanteriorregionofthemidbrain,andthentotheipsilateralandoppositesides.Onthesideoftheoculomotornucleus,theefferentfibersaremainlytheparasympatheticfibersintheoculomotornerve,andtheeffectorsarealsomainlythepupillarymuscles.
Thecharacteristicofthepupil’sresponsetolightisthebilateralnatureoftheeffect,thatis,ifthelightisononeeye,inadditiontothedilatedpupilsoftheilluminatedeye,thepupilsalsodiminishwhentheyarenotexposedtolight.Iamthereflectionoflightformutualinductance.Inclinicalpractice,abnormalitiessuchasdisappearanceofpupilresponsetolight,disparitybetweenleftandrightpupil,disappearanceofmutualinductivepupilreaction,etc.aresometimesseeninclinicalpractice.Itisoftentheresultofdamagetoacertainpartofthereflectionarcrelatedtothesereflections.Therefore,theabnormalpupilresponsecanbeborrowed.Helpinthelocalizationanddiagnosisofneuropathy.Aqueoushumorandintraocularpressurereferstothefluidthatfillstheanteriorandposteriorchambersoftheeye.Itscompositionissimilartothatofplasma,buttheproteincontentismuchlowerthanthatofplasma,whilethecontentofHCO3-exceedsthatofplasma;therefore,thetotalosmoticpressureofaqueoushumorItisalsohigherthanplasma.Aqueoushumorisgeneratedinthechoroidalplexusoftheciliarybody.Afteritisgenerated,itenterstheanteriorchamberfromtheposteriorchamberthroughthepupil,thenentersthescleralvenoussinusfromtheanteriorchamberangleatthejunctionofthescleraandthecornea,andfinallyentersthevenoussystem.Aqueoushumorisconstantlybeinggeneratedandreturningtotheincomevein,makingitflowcontinuouslybetweentheposteriorchamberandtheanteriorchamber.Accordingtothemeasurement,theproductionrateofaqueoushumorisabout2mm3perminuteinnormaltime.Becauseofthedynamicbalancebetweenitsproductionandrecovery,theamountofaqueoushumorintheeyeiskeptconstant,andthevolumeoftheanteriorandposteriorchambersoftheeyeisalsoconstant.Relativelyconstant,sothehydrostaticpressure(thatis,intraocularpressure)alsoremainsrelativelystable.ThenormalvalueofintraocularpressureinChineseadultsis2.27-3.2kPa(17-24mmHg),withanaverageof2.67kPa(17-24mmHg).Therelativestabilityofintraocularpressureisimportantformaintainingthenormalopeningandrefractivepoweroftheeyeball,especiallythecornea.Whentheeyeballispierced,itmaycausethelossofaqueoushumor,droptheintraocularpressure,causetheeyeballtodeform,andthecorneacannotmaintainthenormalcurvature.Thetotalrefractivepowerofthehumaneyehasacertainrelationshipwiththeintraocularrefractivebody,butthemostimportantrefractionoccursattheinterfacebetweentheairandthecornea.Thisaccountsforabout80%ofthetotalrefractivepower.Therefore,changesinthecurvatureandshapeofthecorneawillsignificantlyaffecttherefractivepoweroftheeyeandseverelyaffectvision.Aqueoushumoralsoplaysanourishingroleintheavasculartissuesittouches,suchasthecorneaandlens.Aqueouscirculatorydisorderscancauseexcessiveintraocularpressure,whichisclinicallycalledglaucoma,whichcancausemetabolicdisordersofthecornea,lens,andiris.Inseverecases,itcancausecornealopacityandlossofvision.Themechanismofaqueoushumorformationiscurrentlynotfullyunderstood.
Itisgenerallybelievedthat,exceptforthecapillariesinthechoroidalplexusoftheciliarybody,passivefiltration(similartotheformationoftissuefluidatthearterialendofgeneralcapillaries)allowsthewaterandsaltsintheplasmatopenetratethebloodvesselwallInadditiontotheformationofaqueoushumor,thereareactiveprocessesinvolved,otherwiseitwillbedifficulttoexplainwhyaqueoushumorhashigherconcentrationsofHCO3-andothersaltionsthanthoseinplasma.Itisprovedbyhistochemicalmethodthatciliaryepithelialcellscontainmorecarbonicanhydrase.ThefunctionofthisenzymeistorapidlygenerateCO2andH2OproducedintheprocessofcellmetabolismanddissociateHCO3-,whichpassesthroughthemembrane.Theactivetransportprocessaboveenterstheaqueoushumor,resultinginahighconcentrationintheaqueoushumor.ThenegativepotentialandhighosmoticpressurecausedbythishighconcentrationcanfurtherpromotetheNaandwatermoleculesintheplasmatoentertheaqueoushumor.Clinically,carbonicanhydraseinhibitors(suchasacetazolamide)canbeusedtoreducetheproductionofaqueoushumorandlowerintraocularpressure.Themechanismofactionisrelatedtotheaforementionedmechanismofaqueoushumorproduction.
Photosensitivesystem
Lightfromexternalobjects,throughtherefractivesystemintheeye,formsanimageontheretina,whichisapreconditionforthephotoreceptorcellsintheretinatobestimulated.Theretinalimagealsohasaninternalimageinthephysicalcategory,whichcanbeeasilyexplainedbytheprincipleofgeometricoptics.Itisnotinprincipledifferentfromtheobjectimageformedbyexternalobjectsonthenegativethroughthelensgroupofthecamera;However,thefinal"image"formedbythevisualsysteminsubjectiveconsciousnessisasubjectiveimpressionbelongingtothecategoryofconsciousnessorpsychology,whichisfinallyformedincentralstructuressuchasthecerebralcortexbyneuralinformationfromtheretina.Asthereceptorphysiology,thefocusisonhowtheretinaconvertsphysicalimagesintonervesignalsontheopticnervefibers,andhowthesequenceandcombinationofthesesignalsincludetheimageoftheretina,thatis,theinformationcontentprovidedbyexternalobjects.Itshouldbepointedoutthatalthoughtheprogressofvisualresearchisrelativelyfast,itisonlypreliminary.
Structuralfeatures
Thethicknessoftheretinaisonly0.1-0.5mm,butthestructureisverycomplicated.Themainpartofitcomesfromtheforebrainvesiclesinontogenesis,soitbelongstoaneurologicalstructureinwhichcellsareconnectedtoeachotherthroughsynapses.Classicalhistologydividestheretinaintotenlayers,butaccordingtothemaincelllevel,itissimplifiedtofourlayers.Fromthesideclosetothechoroid,theoutermostlayeroftheretinaisthepigmentcelllayer;thesourceofthislayerisnotneuraltissueThebloodsupplyalsocomesfromthechoroidside,whichisdifferentfromtheotherlayersoftheretinathatreceivebloodsupplyfromtheinnersurfaceoftheretina;clinicallyseenretinaldetachmentoccursbetweenthislayerandotherlayers.Thepigmentcelllayerisnotunimportantforthecauseofvision.ItiscontainedinmelaninparticlesandvitaminA,andithasnourishmentandprotectionforthephotoreceptorcellsadjacenttoit.Theprotectiveeffectisthatinadditiontothepigmentlayerthatcanshieldthescatteredlightfromthesideofthesclera,thepigmentcellscanextendpseudopod-likeprotrusionswhentheretinaisilluminatedbystronglight,andcoattheoutersegmentoftherodcells,makingthemisolatedfromeachotherandlessaffectedbyothersources.Thelightstimulation;onlyunderdarklightconditions,theoutersegmentoftherodisexposed;thisactivityofthepigmentepitheliumiscontrolledbydopaminereceptorsonthemembrane.Theinnersideofthislayeristhephotoreceptorcelllayer.Inhumansandmostmammaliananimals,photoreceptorcellsaredividedintorodsandcones.Theybothcontainspecialphotosensitivepigmentsandaretruephotoreceptorcells.Rodsandconescanbedividedintofourpartsinmorphology,whicharecalledoutersegment,innersegment,cellbody,andendfootfromtheoutsidetotheinside;theoutersegmentisthepartwherethephotosensitivepigmentisconcentrated,whichplaysanimportantroleinthelight-sensitivetransduction.effect.Thedifferenceintheformationofrodsandconesisalsomainlyintheoutersegment.Theyaredifferentinappearanceandcontaindifferentphotosensitivepigments.Theoutersegmentoftherodisalongrod,andtheoutersegmentoftheconeisacone.Bothphotoreceptorcellsaresynapticallyconnectedtothebipolarcellsinthebipolarcelllayerthroughtheendfoot,andthebipolarcellsgenerallycommunicatewiththeganglioncellsintheganglioncelllayer.Inadditiontothislongitudinalcell-to-cellconnectionintheretina,thereisalsoahorizontalconnection,suchashorizontalcellsbetweenthephotoreceptorcelllayerandthebipolarcelllayer,andamacrinecellsbetweenthelargebipolarcelllayerandtheganglioncelllayer.;Theprotrusionsofthesecellsstretchhorizontallybetweenthetwolayersofcells,whichcantransmitinformationinthehorizontaldirection,makingitpossiblefortheretinatoinfluenceeachotherindifferentareas;theseamacrinecellscanalsodirectlytransmitsignalstotheganglioncells.Inrecentyears,ithasbeendiscoveredthatthereisakindofinterreticularcellintheretina.Itscellbodyislocatedbetweenthebipolarcelllayerandtheganglioncelllayer,buttheprotrusionsextendtothephotoreceptorcelllayerandthebipolarcelllayer.Ifthephotoreceptorcellpassesthroughthebipolarcelltotheganglioncell,itisregardedastheinitialstageofvisualinformation.Inadditiontotheusualchemicalsynapsesintheretina,therearealsoalargenumberofelectricalsynapses.Itcanbeseenthattheretinaisalsothesameasthenervetissue.Therearecomplexconnectionsbetweencellsatalllevels.Aftervisualinformationisinitiallyconvertedintoelectricalsignalsatthephotoreceptorcelllayer,itwillundergosomeprocessinginthecomplexneuralnetworkoftheretina.Andchange,whentheactionpotentialsequenceoftheopticnervefiberistransmittedtothecenterasthefinaloutputsignaloftheretina,theyarealreadypreliminaryprocessedandprocessedinformation.
BlindspotThenerveaxonsemittedfromtheganglioncelllayerfirstaggregateintoabundleonthesurfaceoftheretina,andthenitpenetratestheretinaandprotrudesoutoftheeyeballatthebackoftheeye.Thisformstheopticnerveheadonthesurfaceoftheretina.Intheareaofthenipple,thereisactuallynospecificcellstructureoftheretina,sothelightorthecomponentsoftheretinalimagefallingtherewillnotbeabletobeperceived,soitiscalledablindspot.Theopticnerveheadsonbothsidesareabout3mmfromthenasalsideofthecenterofthemaculaorfoveaintheretina.Butundernormalconditions,theblindspotononesidecanbecompensatedbythecontralateralvisionduetotheuseoftwoeyestoseeobjects,andpeoplearenotawareofanon-visualareaintheirfieldofvision.Theexistenceofblindspotscanbeprovedbyspeciallydesignedmethods.
Accordingtotheresearchonthestructureandfunctionoftheretina,itisbelievedthattherearetwolight-sensitivetransductionsystemsintheretinaofhumansandmostvertebrates.Itiscomposedofrodcellsandtheirassociatedbipolarcellsandganglioncells.Theyarehighlysensitivetolightandcanfeellightstimulationinadimenvironmenttocausevision,buttheyhavenocolorvision.Itcanonlydistinguishbetweenlightanddark;andtherecanonlybearoughoutlinewhenlookingatobjects,andtheaccuracyispoor.Thisiscalledtherodsystemorthelatelightperceptionsystem;theotheriscomposedofconecellsandtheirrelatedtransfercells.,Theyarelesssensitivetolight,andcanonlybestimulatedunderstronglightregulationssimilartodaylight,buttheycandistinguishcolorswhenlookingatobjects,andtheycanseethedetailsandcontoursoftheobjectsurfaceclearly,withhighresolutionAbility,thisiscalledtheconesystemordaylightsystem(theaforementionedmeasurementofvisualacuityisactuallythemeasurementofthevisualacuityoftheconesystem).
Themainbasisforprovingtheexistenceofthesetworelativelyindependentphotosensitive-transductionsystemsis:
①Thespatialdistributionofrodsandconesinthehumanretinaisuneven,Theclosertotheperipheryoftheretina,themorerodcellsandthefewerconecells;theclosertothecenteroftheretina,thefewerrodcellsandthemoreconecells;inthecentralfoveaofthemacula,allphotoreceptorcellsareopticConewithoutrodcells;Correspondingtotheabove-mentionedcelldistribution,humanvisionischaracterizedbythehighestvisualacuityandcolorvisionofthefoveainbrightlight,andpoorercentralvisioninthedark;onthecontrary,theperipheralpartoftheretinaisCanfeelthestimulationoflowlight,butatthistimethereisnocolorvisionandpoorclarity.
②Whenthetwokindsofphotoreceptorcellsandbipolarcellsandganglioncellsforminformationtransmissionpathways,thereisacertaindegreeofconvergencebetweenlevels,butthisconvergenceisrelativelysmallintheconesystem.Inthefovea,youcanevenseethataconecellisincontactwithonlyonebipolarcell,andthisbipolarcellisonlyincontactwithaganglioncell.Thiskindof"single-lineconnection"withlowornoconvergence,Obviouslytheconesystemhasastructuralbasisforhighfineresolution;incontrast,intherodsystem,multiplephotoreceptorcellsarecommonlyassociatedwiththesamebipolarcell,andmultiplebipolarcellsareinthesameganglion.Convergentarrangementofcellconnections;intheperipheryoftheretina,upto250rodcellscanbeseenconvergingintooneganglioncellviaafewbipolarcells;inthiscase,ofcourse,youcannotexpectsuchafeelingThesystemhashighfineresolutioncapability.However,suchapolymerizationsystemhasastrongabilitytosummultipleweakstimuli.
③Fromthecharacteristicsofanimalsystems,someanimalsthatonlymoveindaylight,suchasreptilesandchickens,havenorodsintheretinabutonlyconecells.Otheranimalsthatonlymoveatnight,suchasgroundsquirrelsandowls,haveonlyrodsandnoconesintheirretina.
④Therodcellscontainonlyonekindofphotosensitivepigment,namelyRhodopsin(rhodopsin),whiletheconecellsaredifferentduetotheabsorptionspectrumcharacteristicsofthephotosensitivepigmentcontainedinit.Therearethreetypes,whichareconsistentwiththefactthattherodsystemhasnocolorvisionandtheconesystemhascolorvision.
Photosensitivemechanism
Someonehasextractedacertainpurityofthephotosensitivepigment,rhodopsin,fromtheretina,whichisredinthedark;experimentscanalsoprovethattheextractedTheabsorptionspectrumofthisphotosensitivepigmenttolightofdifferentwavelengthsisbasicallyconsistentwiththesensitivitycurveofnightlightperceptiontodifferentpartsofthespectrum.Thisfactisveryimportant,becausesincetheintensityofthephotochemicaleffectoflightonacertainphotosensitivepigmentisexactlythesameasthevisualintensitycausedbythelight,itisahintthattheformermaybethebasisofthelatter.ThephotochemicalreactionandmetabolismofrhodopsinThemolecularweightofrhodopsinisabout27-28kd.Itisakindofproteinthatbindstoit.Itconsistsofaproteincalledopsinandamoleculecalledopsin.Itiscomposedofthechromophoreofretinal(retnal).Thepeptidechainsequenceofopsinhasbeenclarified,anditspeptidechainhas7segmentsofalpha-helixsegmentsthatpassthroughthemembranestructureandaremainlycomposedofhydrophobicaminoacids,whichhaveasimilarstructuretogeneralcellmembranereceptors.RetinaldehydeisderivedfromvitaminA,whichisanunsaturatedalcoholthatcanbeoxidizedtoretinalundertheactionofanenzymeinthebody.Thepurifiedrhodopsinhasthestrongestabilitytoabsorblightof500nmwavelengthinthesolution.Thisisthesameastheblue-greenlightregion(equivalenttonear500nmwavelength)inthehumaneyeunderlowlightconditions.Green)isconsistentwiththefacts,indicatingthatthehumanscoopiaisdirectlyrelatedtothephotochemicalreactionoftherhodopsincontainedintherodcells.Thebrightestareasinthevisualabsorptioncurveofrhodopsinfordifferentpartsofthespectrumandthestrongestpartofrhodopsinabsorptionarebothnearthewavelengthof500nm.Rhodopsinisrapidlydecomposedintoopsinandretinalwhenexposedtolight,whichisamulti-stagereaction.Thedecompositionoccursfirstlybecausetheretinalmoleculeundergoesachangeinmolecularconformationwhenilluminated,thatis,itisoriginally11-cis(amorecurvedconformation)intherhodopsinmolecule,butbecomesfullwhenilluminated.Inversion(arelativelystraightmolecularconformation).Thischangeintheconformationoftheretinalmoleculewillresultinachangeintheconformationoftheopsinmolecule.Theactivityofamorecomplexsignaltransmissionsysteminducestheappearanceofreceptorpotentialsintherodcells.Accordingtocalculations,theabsorptionofonelightquantumbyrhodopsinisenoughtochangethemolecularstructureofretinal,resultinginthefinaldecompositionofrhodopsinintoopsinandretinal.Somestagesofrhodopsindecompositionareaccompaniedbythereleaseofenergy,butthisdoesnotseemtobethedirectcauseoftheevokedreceptorpotential.
Rhodopsindecomposedinbrightplacescanbere-synthesizedindarkplaces,thatis,itisareversiblereaction,andtheequilibriumpointofthereactiondependsontheintensityoflight.Thefirststepintheresynthesisofrhodopsinisthatall-transretinalischangedto11-cisretinal,whichwillsoonrecombinewithopsin.Inaddition,thevitaminAstoredinthepigmentcelllayeroftheretinaisalsoall-trans.Theycanalsobecome11-cisunderenergyconsumption,entertherodcells,andthenoxidizeto11-cis.Retinaldehydeisinvolvedinthesynthesisandsupplementationofrhodopsin;however,thisprocessisslowerandisnotanimmediatefactorthatpromotestheresynthesisofrhodopsinsanctions.Whenpeopleseethingsinthedark,theyactuallyhaveboththedecompositionofrhodopsinandthesynthesisofit.Thisisthebasisforpeopletocontinuouslyseethingsinthedark;thedarkerthelight,themoretheprocessoffullbloomexceedstheprocessofdecomposition,theretinaThenumberofrhodopsininthesyntheticstateisalsohigher,whichalsomakestheretinamoresensitivetoweaklight;onthecontrary,whenpeopleareinbrightlight,thedecompositionofrhodopsinisenhanced,andthesynthesisprocessisweak,whichmakestheretinamoresensitive.Muchoftherhodopsinisinadecomposedstate,sothattherodcellsalmostlosetheabilitytosenselightstimulation;humanvisioninbrightlightiscompletedbyanotherphotosensitivesystemthatislesssensitivetolightstimulation,thatis,thecone.Asystemisnotenoughtobestimulatedunderlowlight,andwhentherhodopsinintherodcellsismoredecomposedunderthestronglightsystem,theconesystembecomesthesensorysystemforstronglightstimulationinstead.Intheprocessofrhodopsinandresynthesis,apartofretinalisconsumed,whichultimatelydependsonvitaminAthatentersthebloodcirculationfromfood(aconsiderablepartofitisstoredintheliver)tosupplementit.Long-termintakeofvitaminAwillaffectpeople’svisionindarkplacesandcausenightblindness.
Chaoweistructureandproductionofreceptorpotential
Chaoweistructureandproductionofreceptorpotentialintheoutersegmentofrodcells
Theoutersegmentofthephotoreceptorcellisthekeypartforphoto-electricconversion.Theoutersegmentoftherodcellhasaspecialsuperpowerstructure.Intheoutersegment,thereisverylittlecytoplasminthemembrane,andmostofitisoccupiedbysomeneatlyoverlappinglayereddisc-likestructures.Thisdisciscalledtheopticdisc.Eachopticdiscisaflatcapsule.Thestructureofthecapsulemembraneissimilartothatofthecellmembrane.Ithasagenerallipidbilayerstructure,butmostoftheproteinsembeddedinitarerhodopsin,whichiscontainedinrodcells.Infact,almostalloftherhodopsinisconcentratedintheopticdiscmembrane.Thenumberofopticdiscsvariesgreatlyamongrodcellsofdifferentanimals.Therearenearlyathousandofthemintheoutersegmentofeachrodcellinhumans;eachopticdisccontainsabout1millionrhodopsinmolecules.Suchastructureisobviouslybeneficialtomakethelightquantumenteringtheretinahaveagreaterchanceofencounteringrhodopsinmoleculesintheoutersegment.
Someoneusedintracellularmicroelectrodetechnologytostudythechangesinthepotentialdifferencebetweentheinnerandoutersegmentsoftherodcellsbeforeandafterlight.Theresultsfoundthatwhentheretinaisnotirradiated,therestingpotentialoftherodcellsishigherthannormal.Thecellsaremuchsmaller.TheanalysisshowsthatthisiscausedbyaconsiderablenumberofNachannelsintheopenstateandcontinuousNainflowintheoutermembranewhenthereisnolight,whilethecontinuousmovementoftheNapumpintheinnermembranemovesNaoutofthemembrane.,SoastomaintaintheNabalanceinsideandoutsidethemembrane.Whentheretinaisexposedtolight,thepotentialonbothsidesoftheoutersegmentmembranecanbeseentochangetemporarilytothedirectionofhyperpolarization.Itcanbeseenthattheoutersegmentmembraneisinconsistentwiththegeneralcellmembrane.Itisdepolarizedinthedarkorwithoutlight.Thetransmembraneelectrolysischangestohyperpolarizationwhenitisstimulatedbylight.Therefore,thereceptorpotentialoftherodcells(thesameistruefortheconecells)appearsasahyperpolarizedslowpotential.Itisspecialamongallthegeneratorsorsensorpotentialsthathavebeenstudied.Theyaregenerallyexpressedasatemporarydepolarizationofthemembrane.
Themechanismbywhichtheabsorptionofphotonscauseshyperpolarizedelectricalreactionsintheoutersegmentmembranehasbeenbasicallyunderstood.Thisistheabsorptionofphotonsbyrhodopsinasthereceptor,whichcausesthedeconstructionofopsinmoleculesandstimulatesthem.Seacucumbershaveanintermediarycalledtransducin(transducin)Ctintheopticdiscmembrane.ThelatterisamemberoftheG-proteinfamilystructurally.Theresultofitsactivationistoactivatenearbyphosphodiesterase,thuscausingexternalThecGMPinthecytoplasmofthesegmentisdecomposedinalargeamount,andthedecompositionofcGMPinthecytoplasmmakesthecGMPboundtotheoutersegmentmembranewhenitisnotstimulatedbylighttobedecomposedbythedissociationofthemembrane,andtheexistenceofcGMPonthemembraneispreciselythis.TheconditionsfortheopeningofchemicallygatedNachannelsinthemembranereducethecGMPonthemembraneandtheopeningofNachannels.Asaresultofthelight,thehyperpolarizedsensorpotentialwehaverecordedappears.Itisestimatedthatwhenarhodopsinisactivated,about500transferinscanbeactivated;althoughtransferinsactivatephosphodiesteraseonetoone,anactivatedphosphodiesterasetakesapproximatelyonesecondItcandegrademorethan4,000cGMPmolecules.Duetothebiologicalamplificationeffectoftheenzymesystem,itcanbeexplainedwhytheeffectofalightquantumcancausetheclosureofalargenumberofchemicallygatedNachannelsontheoutermembrane,causingahyperpolarizedelectricitythatissufficientforthehumanvisualsystemtoperceive.Variety.
Theoutersegmentoftherodcellandtheentirerodcellhavenoabilitytogenerateactionpotentials.Thesensorpotentialcausedbythelightstimulationontheoutersegmentmembranecanonlyreachitsfinalfootwithanelectrotonicspread.Partofitaffectsthereleaseoftransmittersoutsidetheendpoint(equivalenttotheaxonend).
Transducedvision
Theoutersegmentoftheconesystemalsohasadisc-likestructuresimilartorodcells,andcontainsspecialphotosensitivepigments,butmoleculesThenumberissmall.Itisknownthatmostvertebrateshavethreedifferentconepigments,eachofwhichexistsinadifferentconecell.Thethreeconepigmentsallcontainthesame11-cis-retinal,butthemolecularstructureoftheopsinisslightlydifferent.Itseemsthattheslightdifferenceinthemolecularstructureofopsindetermineswhichwavelengthoflighttheretinalmoleculecombinedwithitismostsensitiveto.Therefore,therearerhodopsininrodcellsandthreedifferenttypesofoptics.Thedifferencebetweenconepigments.Whenlightactsontheoutersegmentoftheconecells,thesuper-receptorpotentialsimilartothatoftherodcellsalsooccursonbothsidesoftheiroutersegmentmembrane,asthefirststepofphoto-electricconversion.Itisbelievedthatthetransductionmechanismoftheoutersegmentofconecellsissimilartothatofrodcells.
Animportantfeatureofconecellfunctionisitsabilitytodistinguishcolors.Colorvisionisacomplexphysical-psychologicalphenomenon.Thedifferenceincolorismainlythesubjectiveimpressionofthehumanbraincausedbylightofdifferentwavelengthsactingontheretina.Thehumaneyecangenerallydistinguishsevencolorsofred,orange,yellow,green,cyan,blue,andpurpleonthespectrum,andeachcolorcorrespondstoacertainwavelengthoflight;butacarefulinspectioncanrevealthatthehumaneyealoneThereareactuallynolessthan150differentcolorsinthespectrum,whichmeansthataslongasthewavelengthlengthincreasesordecreaseswithintherangeofthevisiblespectrum,itcanbedistinguishedintodifferentcolorsbythevisualsystem.Obviously,itisimpossibletoimaginethattherearehundredsofconecellsorphotosensitivepigmentsintheretinathatrespondtolightofdifferentwavelengths.ButphysicshasknownfromNewton'stimeorearlierthatacolormaynotonlybecausedbyacertainfixedwavelengthoflight,butalsobythemixingoftwoormoreotherwavelengthsoflight.Forexample,rotatingthesevencolorsoflightonthespectrumontheso-calledNewtoniancolorwheelcancauseawhitesensationinthehumaneye;usingred,green,andbluelight(notthepigmentsofthesethreecolors)forpropermixingcancausethespectrumAllthefeelingofanycolor.Thislatterphenomenonisparticularlyimportant;thisso-calledthree-primarycolormixingprinciplehasnotonlybeenwidelyusedincolorphotography,colortelevision,etc.,butalsousedtoexplaintheprincipleofcolorvision.Asearlyasthebeginningofthelastcentury,Young(1809)andHelmholtz(1824)proposedthetheoryofthreeprimarycolorsofvision.Itwasassumedthattherearethreekindsofconecellsorcorrespondingthreekindsofconecellsthatareparticularlysensitivetored,red,andbluelightintheretina.Akindofphotosensitivepigment,anditisassumedthatwhenthelightonthespectrumandthelightbetweenthesethreeactsontheretina,theselightscanstimulatethetwoconecellsorphotosensitivepigmentswithsimilarsensitivewavelengthstodifferentdegrees,soInthecenter,itcausesthesensationofothercolorsbetweenthetwoprimarycolors.Thetheoryofvisualthreeprimarycolorsusestheassumptionofsimplerbiologicalperceptionstructuretoexplainthecomplexcolorvisionphenomenon,whichisgenerallyacceptedbymostpeople.However,attemptstofindthesamekindofconecellsorphotosensitivepigmentsintheexperimenthavebeenunsuccessfulforalongtime.Theopticalmicroscopeandtheelectronmicroscopecannotfindanydifferenceinthestructureoftheconecells.Atthesametime,itisnotpossibletoseparatethedifferentconephotosensitivepigmentsbygeneralchemicalmethods.
Duetotheadvancementofexperimentaltechnology,thehypothesisthattherearethreeconecellsintheretinathatareparticularlysensitivetolightofdifferentwavelengthshasbeenconfirmedbymanyexcellentexperiments.Forexample,somepeopleusenomorethanasingleconediameterThesmallmonochromaticlightbeamswereexaminedonebyoneanddrawninthebody(theinitialexperimentwascarriedoutinanimalssuchasgoldenmalesandsalamanders,andlaterinhumans).Itwasfoundthatallthedrawncurveswerenomorethanthreetypes.Theyrepresentthreetypesofconecellswithdifferentspectralabsorptioncharacteristics.Onetypehasanabsorptionpeakoutsideof420nm,onetypeisoutside531nm,andtheothertypeisoutside558nm,whichisalmostexactlyequivalenttothewavelengthsofblue,green,andredlight,andTheabovehypothesisofthethree-primary-colortheoryofvisionisconsistent.Themethodofrecordingthereceptorpotentialofasingleconecellwithamicroelectrodehasalsoobtainedsimilarresults,thatis,themagnitudeofthehyperpolarizedreceptorpotentialcausedbydifferentsinglebeamsisdifferentindifferentcones,andthepeakappearanceisconsistentYuthetheoryofthreeprimarycolors.
Thetheoryofthreeprimarycolorsanditsexperimentalbasiscangenerallyexplainthepossiblepathogenesisoftheso-calledcolorblindnessandcolorweaknessencounteredinclinicalpractice.Protanopiaisalsocalledfirstcolorblindness,whichisbelievedtobecausedbythelackofconecellsthataresensitivetolongerwavelengthsoflight;therearealsogreenblindness,alsocalledsecondcolorblindness,andblueblindness,alsocalledthirdprimarycolorblindness,whichmaybecausedbyDuetolackofcorrespondingspecialconecells.Protanopiaanddeuteranopiaaremorecommon,andtheyarenotdistinguishedclinicallyasred-greenblindness;blueblindnessisveryrare.Thecolorofacolorblindpatientcannotonlydistinguishgreen,butalsocannotdistinguishbetweenredandgreen,greenandblue,etc.Somepeoplewithabnormalcolorvisionareonlylessabletorecognizeacertaincolor,thatis,theyarenotduetolackofacertainkindofconecells,buttheresultofthelatter'sweakerresponseabilitythannormalpeople.Thissituationisdifferentfromtherealone.Colorblindnessiscalledcolorweakness.Exceptforaverysmallnumberofcolorblindnesscanbecausedbyacquireddiseasesoftheretina,thevastmajorityaredeterminedbygeneticfactors.
Althoughthetheoryofthreeprimarycolorssatisfactorilyexplainsmanycolorvisionphenomenaandthecausesofcolorblindness,andhasbeenexperimentallyconfirmedatthelevelofphotoreceptivecells,itcannotexplainallofthem.Colorvisionphenomena,suchascolorcontrast,areanexample.Tryputtingasmallbluepieceofpaperonayelloworothercoloredbackground.Youwillfeelthatthebluepieceofpaperplacedonayellowbackgroundisparticularlyblue.Atthesametime,youfeelthatthebackgroundismoreyellowthanwhenthebluepieceofpaperisnotplaced(innorthernChina).OntheLoessPlateau,whenthewindinthespringcausesyellowdusttoobscurethesun,itwillfeelthatthelightoftheordinarysun-shakinglampbecomesbluer).Thisphenomenoniscalledcolorcontrast,andyellowandbluearecalledcontrastingcolorsorcomplementarycolors.Thecolorcontrastphenomenononlyappearsbetweencontrastingcolors,notbetweenanytwocolors.Therearestillcontrastingcolorpairs:redandgreen,andblackandwhite.Accordingtocolorcontrast,itisnoteasytousethetheoryofthreeprimarycolorstocompletevisualphenomena.Almostatthesametimewhenthetheoryofthreeprimarycolorswasproposed,anothertheoryofcolorvisionappeared,calledthetheoryofcontrastingcolors.Thistheoryproposesthattherearethreesubstancesintheretina,eachofwhichreactsoppositelytoasetofcontrastingcolorstimuli.Asmentionedearlier,theresearchconductedattheconelevelinrecentyearsisconducivetothetheoryofthreeprimarycolorsbutnottothetheoryofcontrast.However,someexperimentsperformedoncellsinotherlayersoftheretinaconformtothespeculationofthetheoryofcontrast.Forexample,microelectrodestudiesconductedongoldfishlevelcellsshowthatthesecells,unlikerodsandcones,canhavehyperpolarizedtransmembranepotentialchangesaswellasdepolarization.Whenstimulatedwithavarietyofdifferentcolorsoflight,itwasfoundthatsomehorizontalcellsshowedthelargestdepolarizationresponsewhenstimulatedbyyellowlight,andthelargesthyperpolarizationresponsewhenstimulatedbybluelight;Therearesimilardifferentresponsestoredandgreenstimuli.Thesephenomenaareconsistentwiththetheoryofcontrastingcolors.Itseemspossiblethatthetwocolorvisiontheories,whicharebasedonpartialcolorvisionphenomena,arebothpartiallycorrect.Attheconelevel,differentcolorsoflightcancausethreedifferentconecellstoproducehyperpolarizedelectricityofdifferentsizes.Changesarecoded;butatthelevelofhorizontalcellsorotherlevelsofcells(includingsomecentralneurons),theinformationisre-encoded.Differentcolorpairscanbecodedbytheoppositeformofelectricalresponseofthesamecelltothecontrastingcolorsofeachother..Theabovefactsshowthatthecauseofcolorvisionisaverycomplicatedprocess,whichrequirestheparticipationofmulti-levelneuralcomponentsfromretinalconestocorticalneurons.
Informationprocessing
Theelectricalsignalsgeneratedbyrodsandconesneedtobetransmittedthroughacomplexcellnetworkintheretina,andfinallynervefiberscanbegeneratedbyganglioncellsItistransmittedtothecenterintheformofactionpotentials.Becausethearrangementandconnectionofvariouscellsintheretinaareverycomplicated,therearemanykindsofchemicalsubstancesrelatedtothetransmissionofinformationbetweencells(exceptforthecommontransmittersinthegeneralnervoussystem,togetherwiththevariousneuropeptidesfoundintheretina,Thetotalnumberhasreachedmorethan30kinds),sovisualinformationmustundergovariouschangeswhenitistransmittedfromphotoreceptorcellstoganglioncells;thisisactuallytheinitialprocessingofvisualinformationbytheretinaitself,whichisconstructedinspecificcellsandchemicalstructuresoftheretina.Accordingtocertainrulesinthenetwork,buttheunderstandingoftheserulesisstillverysuperficial.Whatcanbepreliminarilyconfirmedisthatbipolarcells,horizontalcellsandmostamacrinecells,likethetwophotoreceptorcells,havenoabilitytogenerateactionpotentials(butsomeamacrinecellscangenerateactionpotentials);thesethreeThedifferencebetweenthesecellsandphotoreceptorcellsisthatundertheinfluenceofthepreviouscells,theycanproducebothsuper-modifiedslowpotentialsanddepolarizedslowpotentials(equivalenttothepostsynapticmembraneofgeneralneurons).IPSpandEPS).Alltheseslowpotentialscanonlybeusedfortheexpansionofelectrotonicity,affectingthechangesinthereleaseofpresynapticmembranetransmitters,causingslowpotentialchangesinthenextlevelofcells(includingelectricalsynapticinteractions);onlywhenWhensuchaslowpotentialistransmittedtotheganglioncellbody,becausethelatterhastheabilitytogenerateactionpotentials,whenthesumofthetwotypesofslowpotentialsmakestherestingmembranepotentialoftheganglioncelldepolartothethresholdpotentiallevel,Onlythenwillan"all-or-nothing"actionpotentialbegenerated,whichistransmittedtothecenterasthefinaloutputsignaloftheretina.Onlyganglioncellscangenerateactionpotentials.Althoughthedetailsoftheinformationprocessingintheretinaarenotveryclear,itisbettertotreattheretinaasa"blackbox"incybernetics.Onlyitsinput(equivalenttothelightactingontheretina)Stimulus)anditsinput(equivalenttotheactionpotentialsequencesentbytheopticnerve)arecomparedandanalyzed,sothatthefinalresultofretinalinformationprocessingandcodingcanalsobeinitiallyunderstood.Thefirstthingtorememberisthefactthatthetotalnumberoffibersintheopticnerve(thatis,thetotalnumberofganglioncells)isonly1%ofallphotoreceptorcells.Thissimplefactsufficestoexplainthattheopticnervecannottransmitthelightofthephotoreceptorcellsintheretinathroughitsfibers"point-to-point"(exceptforafewconecellsinthefovea);therefore,mostofthesignalstransmittedbytheopticnervefibersareonlyItcanbedeterminedbymultiplephotoreceptorcellsandthereforecontainsmoreinformation.
Themethodofstimulatingthecat’sretinawithasmalllightspotandrecordingtheactionpotentialofasingleopticnervefiberatthesametimeshowsthattheganglioncellsthatemitopticnervefiberscanberoughlydividedintothreetypes,namelyX-,Y-andW-cell.ThecharacteristicofX-andY-cellsisthattheybothhavea"center-peripheralreceptivefield"thatisroughlyconcentric;thereceptivefieldofaganglioncellherereferstoaspecificareaontheretina,whichmaybestimulatedwhenthelatterisstimulated.Maketheganglioncellreact;thereceptivefieldofthesetwokindsofcellsiscomposedoftwoparts;whenthelightactsonthecentralpartofthereceptivefield,theganglioncelldischargeincreases,andwhenthelightactsontheretinainacertainrangearoundthecenterInsomecases,thedischargeoftheganglioncellisreduced.Thisisatypeofcenter-peripheralreceptivefield.Thisiswhenthecentralpartofthereceptivefieldreceiveslightstimulation,thenumberofdischargesoftheganglioncelldecreases,andwhentheperipheralpartisstimulated,itisonthecontraryCausedbytheincreaseofthecelldischarge,thiskindofganglioncellcanbecalledthecentrallight-withdrawingresponsecell.
BothX-andY-cellscanhavetheabovetwotypesofreceptivefields.ThedifferencebetweenthemisthatthereceptivefieldofX-cellsissmall,andtheresponsetostimuliiscontinuous;thereceptivefieldofY-cellsislarger,andtheresponseistemporalandhasanon-linearrelationshipwiththestimulus.Inmonkeyexperiments,itwasalsofoundthatdifferentX-cellsresponddifferentlytodifferentwavelengthsoflight,butarenotsensitivetochangesintheintensityoflight;whileY-cellsaresensitivetochangesintheintensityoflight,butarenotsensitivetochangesinthewavelengthoflight..AsforW-cells,theyhaveamuchlargerreceptivefieldthantheformertwo.Theyeitherdischargewhenlightisstimulatedorwhenlightiswithdrawn,buttheyrespondslowlytothestimulusandgenerallydonothavetheoppositeperipheralproperties.Fieldofview.Fromthemorphologicalpointofview,X-andY-cellsmainlyreceiveinputfrombipolarcells,whileWcellsmainlyreceiveamacrinecells.
Thenerveimpulseoneachopticnervefiberdoesnotsimplymeanthatacertainpartoftheretinaisexposedtolightornolight.TakingtheaboveX-asanexample,itsstrongestdischargeoccursinthecentralpartofitsreceptivefield.Whenthereisnolightontheouterperiphery(orviceversa);ifthecenterandouterperipheryareexposedtolightatthesametime,thedischargeofthecellsinthissectionwillnotchangeorincreaseonlyslightly.Inaddition,therearefactsthattheretinalimagehasbeendecomposedintodifferent"pixels"afterbeingprocessedbytheretina.Forexample,theresidencesofthewavelengthsoftheimagetransmittedbythenodularcellcentertransmititsdifferentbrightness.Thiskindofdecompositionofperceptioninformationintoitsconstituent“elements”,andthen“parallel”transmissionandprocessingintheperceptionpathway,isoftenencounteredintheresearchofvarioussensoryfunctionsofthebrain,buthumanvisionisalsodifferentfromothersensations.Similarly,themostcomplexinformationprocessingandprocessingtakesplaceinthecenter,especiallyitshigh-levelparts.
Otherphenomena
Darkadaptationandlightadaptation
Whenapersonentersadarkroomfromabrightplace,hecannotseeanythingatfirst.Afteracertainperiodoftime,hisvisualsensitivityItgraduallyincreasesthemainbodyandrestoresthevisioninthedark.Thisiscalleddarkadaptation.Onthecontrary,whenyoucomefromthedarkplacetothebrightplace,youfeeladazzlinglightatfirst,youcan'tseetheobjectclearly,andyoucanonlyrestoreyourvisionafterawhile.Thisiscalledbrightadaptation.
Darkadaptationisaprocessinwhichthehumaneye'ssensitivitytolightgraduallyincreasesinthedark.Atdifferenttimesafterenteringthedarkroom,continuouslymeasurethehumanvisualthreshold,thatis,measuretheintensityofthelightstimulusthatthehumaneyecanjustperceive.Itcanbeseenthatthisthresholdgraduallydecreases,thatis,theprocessofvisualsensitivitygraduallyincreasinginthedark.Generally,withinthefirst7minutesafterenteringthedarkroom,thereisaperiodofsignificantdeclineinthethresholdvalue,andthentheamountofthethresholdvaluesignificantlydecreases;about25-30minutesafterenteringthedarkroom,thethresholdvaluedropstothelowestpointandstabilizesInthisstate.Themechanismofdarkadaptationisrelatedtotheincreaseinthere-synthesisofphotosensitivepigmentsintheretinainthedark,thusincreasingtheamountofpigmentsintheundecomposedstateintheretina.Accordingtoanalysis,thefirststageofdarkadaptationismainlyconsistentwiththeincreaseinthesynthesisofconecytochromes;thesecondstage,themaincomponentofdarkadaptation,isrelatedtotheenhancedsynthesisofrhodopsininrodcells.
Mingadaptsquicklyandtakesaboutoneminutetocomplete.Thedazzlinglightperceptionismainlyduetothefactthatthesyntheticrhodopsinaccumulatedinthedarkisquicklydecomposedwhenitentersthebrightplace,becauseitssensitivitytolightishigherthanthatofthephotosensitivepigmentintheconecells;Aftertherodcytochromeisrapidlydecomposed,theconecytochrome,whichislesssensitivetolight,canbephotosensitiveinthebrightlightenvironment.
Fieldofview
Singleeyefixedlystaresatthefrontwithoutmoving,andtherangethattheeyecanseeatthistimeiscalledthefieldofview.Themaximumlimitofthevisualfieldshouldbeexpressedbythesizeoftheanglebetweenitandthevisualaxis(whenasingleeyeislookingatacertainpointintheoutsideworld,theimageofthispointisexactlyinthefoveaoftheretina,andtheimaginarylineconnectingthesetwopointsisthevisualaxis).Underthesamelightingconditions,thesizeofthefieldofviewmeasuredwithdifferentcolortargetsisdifferent,thewhitefieldofviewisthelargest,followedbyyellow-blue,redagain,andthegreenfieldofviewisthesmallest.Itisassumedthatthesizeofthefieldofviewisnotonlyrelatedtothedistributionrangeofvariousphotoreceptorcellsintheretina,butalsolacksasatisfactoryexplanationindetails.Inaddition,becausethefacialstructureblocksthelineofsight,italsoaffectstheshapeofthevisualfield.Forexample,thevisualfieldonthetemporalsideoftheaveragepersonislarger,andthevisualfieldonthenasalsideissmaller.Theclinicianchecksthefieldofvision,usingaspecialperiscope,andusingdifferentcoloredoptotypestocheck,thepurposeistounderstandthegeneralphotosensitiveabilityoftheretina,andsometimescanbeusedtofindalargerrangeofretinopathy.Certaindiseasesoftheretina,opticnerve,orvisualconductionpathwayhavespecialformsofvisualfielddefects,whicharemeaningfulindiagnosis.
Electroretinogram
Aguideelectrodeisplacedincontactwiththecornea,andtheotherelectrodeisplacedontheforeheadasareferenceelectrode.Whentheretinaisextensivelystimulatedwithlight,itcanbeAseriesofelectricalchangesarerecordedonthemeasuringinstrument,whichiscalledanelectroretinogram.Theelectroretinogramisdifferentinnaturefromtheelectricalphenomenarecordedbythesamemicroelectrodeinasingleretinalcellcomponent;theelectroretinogramisacomprehensivereflectionofmultipleelectricalresponsesofvariouscomponentsintheentireretinawhenexposedtoawiderangeoflight.Itusuallyconsistsofthreewavesnameda,b,andc.Accordingtoexperimentalanalysis,theawaveismainlyderivedfromthesensorpotentialofphotoreceptorcells;thebwavehasalargeramplitude,whichismainlyrelatedtotheactivityofcellssuchasbipolarcells;thecwaveisgentleandhasalongduration,whichmayberelatedtotheactivityofthepigmentcelllayer.Sometimeswhenthelightisremoved,therewillbeanotherwaveontheslowandcontinuouscwave,calledthedwave,thecauseofwhichisstillunclear.Althoughelectroretinogramiseasytomeasureandtrace,itreflectsthespecificityofretinalfunctionalstatusorpathologicalchanges.Atpresent,onlyafewdiseaseshavespecialelectroretinogramchanges,soitisoflittleclinicalsignificance.
Binocularvisionandstereovision
Theeyesofhumansandhighermammalsareallinfrontoftheface,andtheimagesofsimilarpartsofthevisualfieldofthetwoeyeswhenlookingatobjectsaretransmittedthroughtheirownuniqueneuralpathways.Tothecenter,butnormalpeopleonlyproduceasenseof"thing"inthesubjectiveobservationsense.Theprerequisiteforthetwoeyestoseeobjectsandproduceonlyonevisualimageisthatthelightfromthesamepartofthesubstanceshouldbeimagedonthecommensuratepointsoftheretinaonbothsides.Forexample,themaculasofthetwoeyesarecommensuratepoints;whenthetwoeyeslookatasmallblackspotontheculturewall,duetotheadjustmentoftheextraocularmuscles,thisspotisexactlyimagedonthemaculasofbotheyes.Onlyonepointis"seen"invision;atthistime,ifyougentlypushtheoutsideoftheeyeballwithyourhandtoshiftthevisualaxisoftheeyeslightly,thentheblackdotimageontheretinaoftheeyewillmoveawayfromthemacula.,Fallingonapointthatisnotcommensuratewiththeimageoftheoppositesideoftheretina,soyouwillfeelthattherearetwoblackspotsonthewall,thisisthephenomenonofdoublevision.Obviously,outsidethemacula,thetemporalretinaofoneeyeandthenasalretinaoftheothereyearesymmetricalwitheachother;andthenasalretinaofoneeyeisalsosymmetricalwiththetemporalretinaoftheotherjustright.