Chapter7TheVisualSystemContentSensoryReceptorandSenseOrgansFocusingontheRetinaFunctionoftheRetinaReference–TextBookP21-22P121-140P410–416P423-442P572–576P613-639Reference–CourseWebsiteSection1SensoryReceptorandSenseOrgansPart1SensoryReceptorsspecializednervecellsthattransduceenergyintoneuralsignals“mode”specific“LawofSpecificNerveEnergies”:sensorymessagesarecarriedonseparatechannelstodifferentareasofthebraindetectasmallrangeofenergylevelsEye:400-700nMEar:20-20,000HzTastebuds:specificchemicalsSpectrumoftheElectromagneticWaveSomaticSensoryNerveEndingsFreeNerveEndingsdendritesinterspersedamongothercells/tissuespain,temperature,touchEncapsulatedNerveEndingsdendriteswithspecialsupportingstructuresmechanoreceptorsandproprioceptorsClassificationofReceptors:LocationExternoceptorsLocatedonthebodysurfaceorspecializedtodetectexternalstimuliPressure,pain,temp,touch,etc.Visceroceptorwithininternalorgans,detectinternalstimuliBloodpressure,pain,fullness.ProprioceptorsLimbandbodypositionandmovement.inthejointsandmusclesinthevestibularstructuresandthesemicircularcanalsoftheinnerear.ClassificationofReceptors:ModalitiesMechanoceptiveDetectsstimuliwhichmechanicallydeformthereceptor;Pressure,vibration,touch,sound.ThermoceptiveDetectschangesintemperature;hot/coldNociceptive(pain)DetectsdamagetothestructuresPhotoreceptorsDetectlight;vision,retinaloftheeyeChemoceptiveDetectchemicalstimuliCO2andO2intheblood,glucose,small,tasteClassificationofReceptors:ComplexitySimplereceptorsUsuallyasinglemodifieddendriteGeneralsenseTouch,pressure,pain,vibration,temperatureComplexityHighmodifieddendrites,organizedintocomplexstructuresear,eye.SpecialsensesVision,hearing,smell,tasteWhichreceptor?SequenceofEventsinaReceptorStimulusReceptorProteinActivatedEnzymeCascade(insomecases)ReceptorIonChannelsopened(orclosed)ReceptorCurrentReceptorPotentialModulatedImpulseFrequencyinSecondOrderNeuronBasicFunctionReceptionAmplificationTransductionTransmissionModulatedImpulseFrequencyinReceptorCellAxonModulatedTransmitterReleasefromReceptorCellPart2PropertiesoftheReceptorsAdequateStimulusTransductionAdaptationEncoding1.AdequateStimulusThetypeofstimulusthereceptorishighlysensitiveReceptor:speciallydesignedforonekindofstimulusThelowestthresholdInsensitivetootherstimulation2.TransductionAprocessbywhichanenvironmentalstimulusbecomesencodedasasequenceofnerveimpulsesinanafferentnervefiberTransducesensoryenergyintoneural(bioelectrical)energyReceptorpotentials:Changesinthetransmembranepotentialofareceptorcausedbythestimulus.GeneratorPotential:Areceptorpotentialthatisstrongenough(reachesthreshold)togenerateanactionpotentialThestrongerthesitmulus(abovethreshold)themoreAPsarefiredoveragiventimeperiod;translatedbytheCNSasastrongsensationReceptorPotentialandGeneratorPotential3.AdaptationThesensoryreceptoradapttoanyconstantstimulusafteraperiodoftimePhasicreceptorsquicklyadapt.Mostexteroreceptors

Tonicreceptorsadaptslowlyornotatall.MostvisceroceptorsPhasicandTonicReceptorsPhasicReceptoralertustochangesinsensorystimuliceasepayingattentiontoconstantstimuliTonicReceptorusefulinsituationsrequiringmaintainedinformationaboutastimulus.4.EncodingThequalityofthestimulusisencodedinthefrequencyoftheactionpotentials.StretchReceptors:Weakstretchcauseslowimpulsefrequencyonneuronleavingreceptor.Strongstretchcauseshighimpulsefrequencyonneuronleavingreceptor.

TimeMembranepotentialFrequencyCodeSummaryTheexternal&internalenvironmentsaremonitoredbysensoryreceptors.Eachtypeofreceptorisexcitedmosteffectivelybyonlyonemodalityofstimulusknownastheadequatestimulus.Thestimulusisconvertedintoanelectricalpotential.Stimuliaredetectedaseitherstaticordynamicevents.Theintensity&durationofthestimulusisfrequencycodedasburstsofactionpotentialsintheprimaryafferentnerve.Section2FocusingontheRetinaTheimagesofobjectsintheenvironmentarefocusedontheretina.Part1.PrincipleofOpticsLightraysarebent(refracted)whentheypassfromonemediumintoamediumofadifferentdensity.Parallellightraysstrikingabiconvexlensarerefractedtoapoint(principalfocus)behindthelens.Theprinciplefocusisonalinepassingthroughthecentersofacurvatureofthelens,attheprincipalfocaldistance.Forpracticalpurpose,lightraysfromanobjectthatstrikealensmorethan20ft(6m)awayareconsideredtobeparallel.Theraysfromanobjectcloserthan20ftaredivergingandarethereforebroughttoafocusfartherbackontheprincipalaxisthantheprincipalfocus.Biconcavelensescauselightraystodiverge.Part2.EmmertropiaTherefractivesystemofthehumaneye:cornea,aqueoushumor,crystallinelens,andvitreoushumor.Whenlightcomingfromanobjectisbroughttoafocus,animageisformed.Emmertropia:parallelraysoflightarefocusedtoanimageontheretina.normalhumaneyePart3.AccommodationEmmertropia:ciliarymusclerelaxduringseeingobjectfartherthan6.objectscloserthan6marebroughttoafocusbehindtheretina,theobjectsappearblurred.Problemsolvedbyaccommodationnearobjectsarebroughttoasharpfocusontheretina(1)AccommodationoflensIncreasebulging(refraction)oflensViacontractionofciliarymuscle,relaxesthesuspensoryligaments(parasympatheticfibers)Thesolidlines:thelens,iris,andciliarybodyatrestthedottedlines:duringaccommodation.FocusingMusclesworkingLensmoresphericalFocusnearMusclesrelaxedLenslesssphericalFocusfarNearPointThepowerofaccommodationislimitedThenearestdistanceoftheeyeatwhichanobjectcanseendistinctlythevisualaccommodationisatamaximumDeclineintheamplitudeofaccommodationinhumanwithadvancingageDiopter:屈光度(2)PupillaryreflexReducestheamountoflightenteringtheeyeRestrictsthelighttothecentralpartofthelensformoreaccuratevision(3)ConvergenceofeyeballsViewingnearobjectcausesbotheyestomoveinwardMovetheimagesonthecorrespondingpositionontheretinaofthetwoeyesPart4.ErrorofRefractionCausedbyshapeofeyepoweroflensFarsightednesslesscommoneyetooshortand/orlenstooweaklightfocusesbehindretinalcorrectwith“convex”lenstoaddpowerFARSIGHTEDNESS(HYPEROPIA)UNCORRECTEDCORRECTEDNearsightednessmorecommoneyeistoolongand/orlensistoopowerfullightfocusesinfrontofretinacorrectwith“concave”lenstoreducepowerNEARSIGHTEDNESS(MYOPIA)UNCORRECTEDCORRECTEDAstigmatism:abnormalcurvatureofthecorneaLightfromverticalandhorizontaldirectiondonotfocusesinthesamepointcorrectwith“cylindrical”lenstocompensateOldsightedness(Presbyopia)ThecrystallinelenstendstohardenandthecapsuleitselfbecomeslesselasticwithageThenearpointofdistinctvisionmovesfurtherandfurtherawayfromtheeyewithage.ThefarpointisnormalMaybecompensatedbyplacingaconverginglensinfrontoftheeye.Thelossinpowerofaccommodationismostsignificantanddramaticbetweentheagesof40and50Section3FunctionoftheRetinaPart1.Rodsystemandconesystemrodsystem

consistsofrodsandsubsequentbipolarcellsandganglioncellsconesystem

composedofconesandsubsequentbipolarcellsandganglioncells.Distributionoftheconesandrodsontheretina.Rodslocatedmainlyinperipheryofretinaresponsiblefornightvisiondetailnotdetectedseeblack,white,andgray(nocolor)severalrodsshare1bipolarand1ganglioncellrodvisionlacksdetail,but,bycombiningtheirefforts,groupsofrodsallowustoseeinlowlightConeslocatedmainlyinfoveaworkbestinbrightlightenableustoseefinedetailresponsibleforcolorvisioneachconehasitsownbipolarandganglioncellthisallowsustoseedetailbutbrightlightisneededConesseedetailbutrequirebrightlightRodsseeinlowlightbutlackdetailevidencethattwophotoreceptorsystemofretinaexitthenocturnalanimalshaveapreponderanceofrodsthediurnalanimalshaveapreponderanceofconesintheirretinaThevisualpigmentintherodsisonlyrhodopsinTherearethreeclassesofconesintheretinaPart2TransductionofLightEnergybyRodCell1.PhotochemicalReactionandMetabolismofRhodopsinRhodopsin:thevisualpigment(light-sensitivepigment).combinationofaproteinpart,scotopsin(opsin),acarotenoidpigment,11-cisretinal.Rhodopsin:cisformoftheretinalbindwithscotopsin.11-cisretinalRetinal–LightSensitivePigment

11-cis-Retinal-All-trans-RetinalLightDark11-cisretinal(bentshapeform)all-transretinal(straightchainform)LightDarkconvertingtheall-transretinalinto11-cisretinaloccursunderthedarkenvironmentrequiresmetabolicenergycatalyzedbytheretinalisomerase.Underthedimlight,the11-cisand11-transkeepdynamicbalancerhodopsin

11-cisretinal+opsinall-transretinal+opsinisomerase11-cisretinalAll-transretinalopsinopsinRetinal–derivedfrom

RetinoicAcidorRetinol

or-Carotene（胡蘿卜素）VitaminAandRetinal（視黃醛）All-transretinolisoneformofvitaminAVitaminAispresentbothinthecytoplasmoftherodsinthepigmentlayeroftheretinavitaminAisalwaysavailabletoformnewretinalwhenneeded.severevitaminAdeficiency-Nightblindness.2.ReceptorPotentialofRods：

HyperpolarizationPart3ColorVisionPhotochemistryofColorVisionbyConesThreekindsofproteinsonthephotopigmentsSensitivetothreekindsoflightTrichromaticTheoryVisiblespectrum:380-760nm

(nmisabillionthofameter)TrichromaticTheoryofColorVisionOccursatthereceptorlevelEachconeiscoatedbyoneof3photopig