生理學雙語-感官-1課件

第九章感覺器官1第一節(jié)感受器的一般生理一、感受器的定義和分類

感受器(receptor)：分布在體表或各種組織內(nèi)部的能夠感受機體內(nèi)外環(huán)境變化的特殊結構或裝置。2高度分化的感受細胞

+

非神經(jīng)性附屬結構

（光感受細胞、毛細胞）

（折光系統(tǒng)、傳音系統(tǒng)）感覺器官視覺器官—眼聽覺器官—耳{3115.1-116.1Informationabouttheinternalandexternalenvironmentreaches

theCNSviaavarietyofsensoryreceptors.

Thesereceptorsaretransducersthatconvertvariousformsofenergyintheenvironmentintoactionpotentialsinneurons.

5Thecharacteristicsofthesereceptors,thewaytheygenerateimpulsesinafferentneurons,andthegeneralprinciplesor“l(fā)aws”thatapplytosensationwillbedisscussing.6SENSEORGANS&RECEPTORSItisworthnotingthattheterm"receptor"isusedinphysiologytorefernotonlytosensoryreceptorsbutalso,inaverydifferentsense,toproteinsthatbindneurotransmitters,hormones,andothersubstanceswithgreataffinityandspecificityasafirststepininitiatingspecificphysiologicresponses.7

Thesensoryreceptormaybepartofaneuronoraspecializedcellthatgeneratesactionpotentialsinneurons.Thereceptorisoftenassociatedwithnonneuralcellsthatsurroundit,formingasenseorgan.82、根據(jù)感受器接受刺激的性質(zhì)電磁感受器（光、熱等）機械感受器（動脈壓力、肌梭等）化學感受器（血O2）{10thecutaneoussensesarethosewithreceptorsintheskin;andthevisceralsensesarethoseconcernedwithperceptionoftheinternalenvironment.Painfromvisceralstructuresisusuallyclassifiedasavisceralsensation12(3)Interoceptors內(nèi)感受器whichareconcernedwiththeinternalenvironment;and(4)Proprioceptors本體感受器,

whichprovideinformationaboutthepositionofthebodyinspaceatanygiveninstant.

141516二、感受器的一般生理特性（一）適宜刺激(adequatestimulus):各種感受器都有自己最敏感（感覺閾值最?。┑拇碳?，這一刺激形式稱為該感受器的適宜刺激。17

118.5-120.3117.2-118.3Theformsofenergyconvertedbythereceptorsinclude,forexample,mechanical(touch-pressure),thermal(degreesofwarmth),electromagnetic(light),andchemicalenergy(odor,taste,andO2contentofblood).18Theadequatestimulus

fortherodsandconesintheeye,forexample,islight.Receptorsdorespondtoformsofenergyotherthantheiradequatestimulus,butthethresholdforthesenonspecificresponsesismuchhigher.20Pressureontheeyeballwillstimulatetherodsandcones,forexample,butthethresholdofthesereceptorstopressureismuchhigherthanthethresholdofthepressurereceptorsintheskin.21

各種感受器把作用于它們的各種刺激形式，轉(zhuǎn)變成為感受細胞的電反應（感受器電位，receptorpotential）和相應的傳入神經(jīng)末稍上的電位變化（發(fā)生器電位，generatorpotential），兩者都是局部電位。感受器電位和發(fā)生器電位最終引起該感受器的傳入神經(jīng)纖維上產(chǎn)生動作電位。

23Theproblemofhowreceptorsconvertenergyintoactionpotentialsinthesensorynerveshasbeenthesubjectofintensivestudy.

Inthecomplexsenseorganssuchasthoseconcernedwithvision,hearing,equilibrium,andtaste,thereareseparatereceptorcellsandsynapticjunctionsbetweenreceptorsandafferentnerves.

24GeneratorPotentialsRecordingelectrodescanbeplacedonthesensorynerveasitleavesapaciniancorpuscle環(huán)層小體andgradedpressureappliedtothecorpuscle.Whenasmallamountofpressureisapplied,anonpropagateddepolarizingpotentialresemblinganEPSPisrecorded.Thisiscalledthegeneratorpotential.Asthepressureisincreased,themagnitudeofthereceptorpotentialincreases.

2627Whenthemagnitudeofthegeneratorpotentialisabout10mV,anactionpotentialisgeneratedinthesensorynerve.Asthepressureisfurtherincreased,thegeneratorpotentialbecomesevenlargerandthesensorynervefiresrepetitively.28(三）編碼作用(coding)把刺激所包含的環(huán)境條件變化的信息轉(zhuǎn)移到動作電位的序列和組合中。30感覺強度：神經(jīng)纖維上沖動頻率高低參與信息傳輸神經(jīng)纖維數(shù)目多少感覺的性質(zhì)

決定于傳入沖動所到達的高級中樞的部位3132不同重量的觸壓刺激是在單一傳入纖維上引起的沖動頻率的改變

33"CODING"OFSENSORYINFORMATIONTherearevariationsinthespeedofconductionandothercharacteristicsofsensorynervefibers,butactionpotentialsaresimilarinallnerves.Theactionpotentialsinthenervefromatouchreceptor,forexample,areessentiallyidenticaltothoseinthenervefromawarmthreceptor.34Thisraisesthequestionofwhystimulationofatouchreceptorcausessensationoftouchandnotofwarmth.

Italsoraisesthequestionofhowitispossibletotellwhetherthetouchislightorheavy.35DoctrineofSpecificNerveEnergiesThesensationevokedbyimpulsesgeneratedinareceptordependsuponthespecificpartofthebraintheyultimatelyactivate.

Therefore,whenthenervepathwaysfromaparticularsenseorganarestimulated,thesensationevokednomatterhoworwherealongthepathwaytheactivityisinitiated.

36Thisprinciple,firstenunciatedbyMullerin1835,hasbeengiventhenameofthedoctrineofspecificnerveenergies.

Forexample,ifthesensorynervefromapaciniancorpuscleinthehandisstimulatedbypressureattheelboworbyirritationfromatumorinthebrachialplexus臂叢,thesensationevokedistouch.

37Similarly,ifafineenoughelectrodecouldbeinsertedintotheappropriatefibersofthedorsalcolumnsofthespinalcord,thethalamus,orthepostcentralgyrusofthecerebralcortex,thesensationproducedbystimulationwouldbetouch.38Thisdoctrinehasbeenquestionedfromtimetotime,especiallybythosewhoclaimthatpainisproducedbyoverstimulationofavarietyofreceptors.However,theoverstimulation

hypothesishasbeenlargelydiscredited不足信的,andtheprincipleofspecificnerveenergiesremainsoneofthecornerstonesofsensoryphysiology.39ThelawofprojectionNomatterwhereaparticularsensorypathwayisstimulatedalongitscoursetothecortex,theconscioussensationproducedisreferredtothelocationofthereceptor.Thisprincipleiscalledthelawofprojection.Corticalstimulationexperimentsduringneurosurgicalproceduresonconsciouspatientsillustratethisphenomenon.

40Forexample,whenthecorticalreceivingareaforimpulsesfromthelefthandisstimulated,thepatientreportssensationinthelefthand,notinthehead.

41IntensityDiscriminationTherearetwowaysinwhichinformationaboutintensityofstimuliistransmittedtothebrain:byvariationinthefrequencyoftheactionpotentialsgeneratedbytheactivityinagivenreceptor,andbyvariationinthenumberofreceptorsactivated.

42Ithaslongbeentaughtthatthemagnitudeofthesensationfeltisproportionatetothelogoftheintensityofthestimulus(Weber-Fechnerlaw).Itnowappears,however,thatapowerfunction冪函數(shù)moreaccuratelydescribesthisrelation.Inotherwords,R=KSAwhereRisthesensationfelt,Sistheintensityofthestimulus,and,foranyspecificsensorymodality感覺通道,KandAareconstants.43Thefrequencyoftheactionpotentialsgeneratedbyastimulustoasensorynervefiberisalsorelatedtotheintensityoftheinitiatingstimulusbyapowerfunction.

44CurrentevidenceindicatesthatintheCNStherelationbetweenstimulusandsensationislinear;consequently,itappearsthatforanygivensensorymodality,therelationbetweensensationandstimulusintensityisdeterminedprimarilybythepropertiesoftheperipheralreceptorsthemselves.45（四）適應現(xiàn)象（adaptation)適應：刺激仍持續(xù)作用于感受器，而傳入神經(jīng)上沖動頻率下降的現(xiàn)象。4647快適應：皮膚觸覺感受器慢適應：肌梭、頸動脈竇壓力感受器

{48AdaptationWhenamaintainedstimulusofconstantstrengthisappliedtoareceptor,thefrequencyoftheactionpotentialsinitssensorynervedeclinesovertime.Thisphenomenonisknownasadaptation.

49Thedegreetowhichadaptationoccursvarieswiththetypeofsenseorgan(Figurebelow).Touchadaptsrapidly,anditsreceptorsarecalledphasicreceptors.Ontheotherhand,thecarotidsinus,themusclespindles,andthereceptorsforcold,pain,andlunginflationadaptveryslowlyandincompletely;thereceptorsinvolvedaretermedtonicreceptors.505152第二節(jié)視覺器官

引起視覺的感覺器官是眼，人眼的適宜刺激是波長370-740nm的電磁波；在人腦獲得的全部信息中，大約有70－95%來自視覺系統(tǒng)。5354144.1-3TheprincipalstructuresoftheeyeareshowninFigureabove.Theouterprotectivelayeroftheeyeball,thesclera〕鞏膜,ismodifiedanteriorlytoformthetransparentcornea角膜,throughwhichlightraysentertheeye.55Insidethescleraisthechoroid脈絡膜,alayerthatcontainsmanyofthebloodvesselswhichnourishthestructuresintheeyeball.Liningtheposteriortwo-thirdsofthechoroidistheretina,視網(wǎng)膜theneuraltissuecontainingthereceptorcells.565758一、眼的折光系統(tǒng)及其調(diào)節(jié)

（一）

眼的折光系統(tǒng)

正確視覺形成的首要條件是：外界物體能夠在視網(wǎng)膜上形成準確而清晰的物像。這由眼的折光系統(tǒng)完成。59眼的折光系統(tǒng)可視為凸透鏡。外界的物體發(fā)出或反射的光線經(jīng)過這個折光系統(tǒng)，聚焦在視網(wǎng)膜，形成清晰物象。6061（二）

眼的調(diào)節(jié)

凡是位于眼前方6m以遠的物體，它們發(fā)出或反射出的光線在到達眼的折光系統(tǒng)時已近于平行，平行光線通過眼的折光系統(tǒng)，不需要調(diào)節(jié)就可以聚焦在視網(wǎng)膜形成清晰物像。62而來自較6m為近的物體的光線是呈輻射狀的，在折射后的成像將在視網(wǎng)膜之后，因而物像模糊，引起模糊的視覺形象。但正常眼在看近物也十分清楚，這是由于眼在看近物時已進行了調(diào)節(jié)，使進入眼內(nèi)的光線經(jīng)歷較強的折射，結果也能成像在視網(wǎng)膜上。人眼的調(diào)節(jié)亦即折光能力的改變，主要是靠晶狀體形狀的改變；這是神經(jīng)反射。

636465人眼看近物的能力，即晶狀體的調(diào)節(jié)能力是有一定限度的。眼的最大調(diào)節(jié)能力可用它所能看清物體的最近距離來表示，這距離稱為近點。調(diào)節(jié)反射進行時，除晶狀體的變化外，同時還出現(xiàn)瞳孔的縮小和兩眼視軸向鼻中線的會聚。66149.2-151.1THEIMAGE-FORMINGMECHANISMTheeyesconvertenergyinthevisiblespectrumintoactionpotentialsintheopticnerve.Thewave-lengthsofvisiblelightrangefromapproximately

397nmto723nm.

67Theimagesofobjectsintheenvironmentarefocusedontheretina.Thelightraysstrikingtheretinageneratepotentialsintherodsandcones.

Impulsesinitiatedintheretinaareconductedtothecerebralcortex,wheretheyproducethesensationofvision.68Parallellightraysstrikingabiconvex雙凸透鏡lens(Figurebelow)arerefractedtoapoint(principalfocus主焦點)behindthelens.

Thedistancebetweenthelensandtheprincipalfocusistheprincipal

focaldistance焦距.

69物70Forpracticalpurposes,lightraysfromanobjectthatstrikealensmorethan6m(20ft)awayareconsideredtobeparallel.71Theraysfromanobjectcloserthan6marediverging發(fā)散andarethereforebroughttoafocusfartherbackontheprincipalaxisthantheprincipalfocus.

Biconcavelensescauselightraystodiverge.72Accommodation

調(diào)節(jié)：Whentheciliarymuscle睫狀肌isrelaxed,parallellightraysstrikingtheopticallynormaleyearebroughttoafocusontheretina.73Aslongasthisrelaxationismaintained,raysfromobjectscloserthan6mfromtheobserverarebroughttoafocusbehindtheretina,andconsequentlytheobjectsappearblurred.

74Theproblemofbringing

divergingraysfromcloseobjectstoafocusontheretinacanbesolvedbyincreasingthecurvature曲率of

lens

orrefractivepowerofthelens晶狀體.

75Inmammals,theproblemissolvedbyincreasingthecurvatureofthelens.Theprocessbywhichthecurvatureofthelensisincreasediscalled

accommodation.

76

Atrest,thelensisheldundertensionbythelensligaments（suspensoryligamentoflens懸韌帶）.Becausethelenssubstanceismalleable（延展性的）andthelenscapsulehasconsiderableelasticity,thelensispulledintoaflattenedshape.

77Whenthegazeisdirectedatanearobject,theciliarymusclecontracts.Thisrelaxesthelensligaments,sothatthelensspringsintoamoreconvex

凸起的shape.78NearPointThedegreetowhichthelenscurvaturecanbeincreasedis,ofcourse,limited,andlightraysfromanobjectveryneartheindividualcannotbebroughttoafocusontheretinaevenwiththegreatestofeffort.79Thenearestpoint

totheeyeatwhichanobjectcanbebroughtintoclearfocusbyaccommodationiscalled

thenearpointofvision.

80TheNearResponseInadditiontoaccommodation,thevisualaxesconvergeandthepupilconstrictswhenanindividuallooksatanearobject.Thisthree-partresponse-accommodation,convergenceofthevisualaxes,andpupillaryconstrictioniscalledthe

nearresponse.8182瞳孔縮小的意義：（1）減少入眼光量（2）減小球面像差和色像差83球面像差和色像差

簡單透鏡中間區(qū)域的焦距與周邊區(qū)域不同,物像變形失真，這稱為球面像差。

如一般的簡單透鏡一樣，物體紅色部分光線比藍色部分折光程度較小，這種差異可使物象失真，為色像差。84（三）簡化眼和視敏度

從實用角度人們設計了一些和正常眼在折光效果上相同、但更為簡單的等效光學模型，稱為簡化眼。可用來分析眼的成像情況和進行其他計算。利用簡化眼可以測定人的視力（視敏度)，即人眼能看清的最小的視網(wǎng)膜像的大小。

85折光指數(shù)=1.33386

151.2Figureaboveisadiagramofsucha"reduced"or"schematic"eye.Allotherraysenteringthepupilfromeachpointontheobjectarerefractedandbroughttoafocusontheretina.

87Iftheheightoftheobject(AB)anditsdistancefromtheobserver(Bn)areknown,thesizeofitsretinalimagecanbecalculated,becauseAnBandanbinFigureabovearesimilartriangles.TheangleAnBisthevisualangle.

88visualacuityisthedegreetowhichthedetailsandcontoursofobjectsareperceived.

89（四）非正視眼

眼結構功能異常,使平行光線不能在未調(diào)節(jié)的眼的視網(wǎng)膜上成清晰像，則稱為非正視眼，其中包括近視、遠視和散光。90近視

91近點比正常眼還要近。92151.3-152.2Inmyopia

近視,theanteroposteriordiameteroftheeyeballistoolong.9394Myopiaissaidtobegeneticinorigin.However,inexperimentalanimalsitcanbeproducedbychangingrefractionduringdevelopment.

95Thus,theshapeoftheeyeappearstobedeterminedinpartbytherefractionpresentedtoit.Inyoungadulthumanstheextensivecloseworkinvolvedinactivitiessuchasstudyingacceleratesthedevelopmentof

myopia.96Thisdefectcanbecorrectedbyglasseswith

biconcavelenses

雙凹透鏡,whichmakeparallellightraysdivergeslightlybeforetheystriketheeye.97遠視

98近點距離較正常人為大。99100Insomeindividuals,theeyeballisshorterthannormalandtheparallelraysoflightarebroughttoafocusbehindtheretina.Thisabnormalityiscalledhyperopia.101Sustainedaccommodation,evenwhenviewingdistantobjects,canpartiallycompensateforthedefect,buttheprolongedmusculareffortistiringandmaycauseheadachesandblurringofvision.102Theprolongedconvergence會聚ofthevisualaxesassociatedwiththeaccommodationmayleadeventuallytosquint斜視

(strabismus).

103Thedefectcanbecorrectedbyusingglaseswithconvexlenses凸透鏡,whichaidtherefractivepoweroftheeyeinshorteningthefocaldistance.104散光

如果折光面在某一方位上曲率半徑變小，而在與之相垂直的方位上曲率半徑變大，不同方位的光線在眼內(nèi)不能同時聚焦，造成物像變形和視物不清。散光可用適當?shù)闹骁R糾正，后者的特點正是互相垂直方位上具有不同的曲率半徑，由此抵消眼球折光面改變對視覺的影響。

105Astigmatism散光isacommonconditioninwhichthecurvatureofthecorneaisnotuniform.Whenthecurvatureinonemeridian角膜經(jīng)線

isdifferentfromthatinothers,lightraysinthatmeridianarerefractedtoadifferentfocus,sothatpartoftheretinalimageisblurred.106Astigmatismcanusuallybecorrectedwithcylindriclensesplacedinsuchawaythattheyequalizetherefractioninallmeridians經(jīng)線..107老年人晶狀體彈性睫狀肌收縮力調(diào)節(jié)力視近物模糊矯正：看近物時，戴凸透鏡。4、老視（老花眼）108

Thenearpointrecedesthroughoutlife,slowlyatfirstandthenrapidlywithadvancingage.Thisrecessionisdueprincipallytoincreasinghardnessofthelens,witharesultinglossofaccommodation.109Bythetimeanormalindividualreachesage40-45,thelossofaccommodationisusuallysufficienttomakereadingandcloseworkdifficult.

110Thiscondition,whichisknownasPresbyopia老視,canbe

correctedbywearingglasseswithconvexlenses

凸透鏡.111二、瞳孔和瞳孔對光反應

瞳孔在亮光處縮小，在暗光處散大。瞳孔的大小可以控制進入眼內(nèi)的光量。112瞳孔和瞳孔對光反應瞳孔大小隨光照強度而變化的反應稱為瞳孔對光反射?；ジ行詫夥瓷洌喝绻庹盏氖且粋妊劬r，另一側瞳孔也縮小，稱~。113Whenlightisdirectedintooneeye,thepupilconstricts(pupillarylightreflex).Thepupiloftheothereyealsoconstricts(consensuallightreflex同(互)感性光反射).

114Theopticnervefiberscarrytheimpulsesinitiatingthesepupillaryresponses.115房水和眼內(nèi)壓

房水循環(huán)障礙時會造成眼內(nèi)壓過高。

116144.4-145.1Aqueoushumor房水,aclearliquidwhichnourishesthecorneaandlens,isproducedintheciliarybodybydiffusionandactivetransportfromplasma.Itflowsthroughthepupilandfillstheanteriorchamberoftheeye.117Itisnormallyreabsorbedintoavenouschannelatthejunctionbetweentheiris虹膜andthecornea(anteriorchamberangle).

Obstructionofthisoutletleadstoincreasedintraocularpressure.118Increasedintraocularpressuredoesnotcause

glaucoma

青光眼,however,increasedpressuremakesglaucomaworse,andtreatmentisaimedatloweringthepressure.119三、視網(wǎng)膜與視覺

（一）視網(wǎng)膜的結構特點

視網(wǎng)膜的結構十分復雜。主要的細胞層次簡化為四層：120色素細胞層：內(nèi)含色素顆粒和VitA，起營養(yǎng)及保護感光細胞的作用。感光細胞層：視錐細胞和視桿細胞，感光換能并產(chǎn)生感受器電位。雙極細胞層：傳導感受器電位。神經(jīng)節(jié)細胞層：產(chǎn)生并傳導動作電位121145.2-146.4147.1-148.1Theretinaisorganizedintenlayersandcontainstherodsandcones,whicharethevisualreceptors,plusfourtypesofneurons:bipolarcells,ganglioncells,horizontalcells,andamacrine無長突細胞

cells.122123

Therodsandcones,whicharenexttothechoroid,脈絡膜synapsewithbipolarcells,andthebipolarcellssynapsewithganglioncells.124Theaxonsoftheganglioncellsconvergeandleavetheeyeastheopticnerve.

125126Sincethereceptorlayeroftheretinarestsonthepigmentepithelium色素上皮nexttothechoroid脈絡膜,lightraysmustpassthroughtheganglioncellandbipolarcelllayerstoreachtherodsandcones.

127Thepigmentepithelium

色素上皮absorbslightrays,preventingthereflectionofraysbackthroughtheretina.Suchreflectionwouldproduceblurringofthevisualimages.128Theopticnerveleavestheeyeandtheretinalbloodvesselsenteritatapoint3mmmedialtoandslightlyabovetheposteriorpoleoftheglobe.129Thisregionisvisiblethroughtheophthalmoscope

眼底鏡astheopticdisk

視神經(jīng)乳頭.Therearenovisualreceptorsoverlyingthedisk,andconsequentlythisspotisblind(theblindspot).130131Neartheposteriorpoleoftheeye,thereisayellowishpigmentedspot,themaculalutea

黃斑.Thismarksthelocationofthefoveacentralis中央凹.132Inthefoveacentralis,theconesaredenselypacked,andeachsynapsestoasinglebipolarcellwhichinturnsynapsesonasingleganglioncell,providingadirectpathwaytothebrain.133

Consequently,thefoveaisthepointwherevisualacuityisgreatest.Whenattentionisattractedtoorfixedonanobject,theeyesarenormallymovedsothatlightrayscomingfromtheobjectfallonthefovea.134Thearteries,arterioles,andveinsinthesuperficiallayersoftheretinanearitsvitreoussurfacecanbeseenthroughtheophthalmoscope.

135Sincethisistheoneplaceinthebodywherearteriolesarereadilyvisible,ophthalmoscopicexaminationisofgreatvalueinthediagnosisandevaluationofdiabetesmellitus,糖尿病hypertension,andotherdiseasesthataffectbloodvessels.

136137138視桿細胞視錐細胞暗光覺-光高度敏感明暗視覺，無色覺模糊視覺中央凹少周緣部多視桿-雙極-節(jié)細胞聯(lián)系中的明顯會聚晝光覺-光敏感度低顏色視覺精細視覺黃斑中央凹多，周緣部少視錐-雙極-節(jié)細胞的單線聯(lián)系(二).視網(wǎng)膜光學感受器139140141

152.3-5Thephotoreceptormechanism

GenesisofElectricalResponsesThepotentialchangesthatinitiate

actionpotentialsintheretina

aregeneratedbythe

actionoflightonphotosensitive

感光性的compoundsintherodsandcones.142Whenlightisabsorbedbythesesubstances,theirstructurechanges,andthischangetriggersasequenceofeventsthatinitiatesneuralactivity.

143Theeyeisuniqueinthatthereceptorpotentialsofthephotoreceptorsintheretinaare

local,gradedpotentials,anditisonlyintheganglioncellsthatactionpotentialsaregenerated.144

Theresponsesoftherods,cones,andhorizontalcellsarehyperpolarizing,andtheresponsesofthebipolarcellsareeitherhyperpolarizingordepolarizing,

145whereasamacrinecellsproducedepolarizingpotentialsthatmayactasgeneratorpotentialsforthepropagatedspikesproducedin

theganglioncells.146四、視桿細胞的感光換能機制

視紫紅質(zhì)由視蛋白和視黃醛組成。

147153.1-155.4PhotosensitiveCompoundsThephotosensitivecompoundsintheeyesofhumansandmostothermammalsaremadeupofaproteincalled

anopsin

視蛋白,andretinenel

視黃醛.148RhodopsinThephotosensitivepigmentinthe

rods

iscalledrhodopsinorvisualpurple.Rhodopsinhasapeaksensitivitytolightatawavelengthof505nm.149Inthedark,theretinene1inrhodopsinisinthe11-cis順式configuration.Theonlyactionoflightistochangetheshapeoftheretinene,convertingittotheall-trans反式isomer異構體(Figure8-17).150151Thisinturnalterstheconfigurationoftheopsin

視蛋白,andtheopsinchangeactivatestheassociatedGprotein,whichinthiscaseiscalledtransducin轉(zhuǎn)導蛋白.152TheGprotein

bindGTP.ThiscausessomeNa+channelstoclose.Thisproducesthehyperpolarizingpotential.153Thecascadeofreactionsoccursveryrapidlyandamplifiesthelightsignal.Theamplificationhelpsexplaintheremarkablesensitivityofrodphotoreceptors;

thesereceptorsarecapableofproducingadetectableresponsetoaslittleasonephoton光子oflight.154Afterretinene1isconvertedtotheall-transconfiguration,itseparatesfromtheopsin(bleaching

漂白).someoftheretinene1isreducedtovitaminA1,andthisinturnreactswithopsintoformrhodopsin.155EffectofVitaminDeficienciesontheEyeInviewoftheimportanceofvitaminAinthesynthesisofretinene1,ItisnotsurprisingthatavitaminosisA維生素A缺乏癥producesvisualabnormalities.Amongthese,oneoftheearliesttoappearisnightblindness(nyctalopia夜盲癥).156155.3-5ImageFormationInasense,theprocessingofvisualinformationintheretinainvolvestheformationofthreeimages.157Thefirstimage,formedbytheactionoflightonthephotoreceptors,ischangedtoasecondimageinthebipolarcells,andthisinturnisconvertedtoathirdimageintheganglioncells.

158

Intheformationofthesecondimage,thesignalisalteredbythehorizontalcells,andintheformationofthethird,itisalteredbytheamacrinecells.159Thereislittlechangeintheimpulsepatterninthelateralgeniculatebodies,sothethirdimagereachestheoccipitalcortex.

160161162163五與視覺有關的基本現(xiàn)象

（一）暗適應和明適應

164

159.2-163.2Ifapersonspendsaconsiderablelengthoftimeinbrightlylightedsurroundingsandthenmovestoadimlylightedenvironment,the

retinasslowlybecomemoresensitivetolightastheindividualbecomes"accustomedtothedark."165Thisdeclineinvisualthresholdisknownas

darkadaptation.Itisnearlymaximalinabout20minutes,althoughthereissomefurtherdeclineoverlongerperiods.166167Thereareactuallytwocomponentstothedarkadaptationresponse(Figurebelow).Thefirstdropinvisualthreshold,rapidbutsmallinmagnitude,isknowntobeduetodarkadaptationoftheconesbecausewhenonlythefoveal,rod-freeportionoftheretinaistested,thedeclineproceedsnofurther.168Intheperipheralportionsoftheretina,afurtherdropoccursasaresultofadaptationoftherods.169170Ontheotherhand,whenonepassessuddenlyfromadimtoabrightlylightedenvironment,thelightseemsintenselyandevenuncomfortablybrightuntiltheeyesadapttotheincreasedilluminationandthevisualthresholdrises.

171Thisadaptationoccursoveraperiodofabout5minutesandiscalled

lightadaptation.172(二）視野

單