生理学双语-感官-1课件

第九章感觉器官1第一节感受器的一般生理一、感受器的定义和分类

感受器(receptor)：分布在体表或各种组织内部的能够感受机体内外环境变化的特殊结构或装置。2高度分化的感受细胞

+

非神经性附属结构

（光感受细胞、毛细胞）

（折光系统、传音系统）感觉器官视觉器官—眼听觉器官—耳{3115.1-116.1Informationabouttheinternalandexternalenvironmentreaches

theCNSviaavarietyofsensoryreceptors.

Thesereceptorsaretransducersthatconvertvariousformsofenergyintheenvironmentintoactionpotentialsinneurons.

5Thecharacteristicsofthesereceptors,thewaytheygenerateimpulsesinafferentneurons,andthegeneralprinciplesor“laws”thatapplytosensationwillbedisscussing.6SENSEORGANS&RECEPTORSItisworthnotingthattheterm"receptor"isusedinphysiologytorefernotonlytosensoryreceptorsbutalso,inaverydifferentsense,toproteinsthatbindneurotransmitters,hormones,andothersubstanceswithgreataffinityandspecificityasafirststepininitiatingspecificphysiologicresponses.7

Thesensoryreceptormaybepartofaneuronoraspecializedcellthatgeneratesactionpotentialsinneurons.Thereceptorisoftenassociatedwithnonneuralcellsthatsurroundit,formingasenseorgan.82、根据感受器接受刺激的性质电磁感受器（光、热等）机械感受器（动脉压力、肌梭等）化学感受器（血O2）{10thecutaneoussensesarethosewithreceptorsintheskin;andthevisceralsensesarethoseconcernedwithperceptionoftheinternalenvironment.Painfromvisceralstructuresisusuallyclassifiedasavisceralsensation12(3)Interoceptors内感受器whichareconcernedwiththeinternalenvironment;and(4)Proprioceptors本体感受器,

whichprovideinformationaboutthepositionofthebodyinspaceatanygiveninstant.

141516二、感受器的一般生理特性（一）适宜刺激(adequatestimulus):各种感受器都有自己最敏感（感觉阈值最小）的刺激，这一刺激形式称为该感受器的适宜刺激。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

各种感受器把作用于它们的各种刺激形式，转变成为感受细胞的电反应（感受器电位，receptorpotential）和相应的传入神经末稍上的电位变化（发生器电位，generatorpotential），两者都是局部电位。感受器电位和发生器电位最终引起该感受器的传入神经纤维上产生动作电位。

23Theproblemofhowreceptorsconvertenergyintoactionpotentialsinthesensorynerveshasbeenthesubjectofintensivestudy.

Inthecomplexsenseorganssuchasthoseconcernedwithvision,hearing,equilibrium,andtaste,thereareseparatereceptorcellsandsynapticjunctionsbetweenreceptorsandafferentnerves.

24GeneratorPotentialsRecordingelectrodescanbeplacedonthesensorynerveasitleavesapaciniancorpuscle环层小体andgradedpressureappliedtothecorpuscle.Whenasmallamountofpressureisapplied,anonpropagateddepolarizingpotentialresemblinganEPSPisrecorded.Thisiscalledthegeneratorpotential.Asthepressureisincreased,themagnitudeofthereceptorpotentialincreases.

2627Whenthemagnitudeofthegeneratorpotentialisabout10mV,anactionpotentialisgeneratedinthesensorynerve.Asthepressureisfurtherincreased,thegeneratorpotentialbecomesevenlargerandthesensorynervefiresrepetitively.28(三）编码作用(coding)把刺激所包含的环境条件变化的信息转移到动作电位的序列和组合中。30感觉强度：神经纤维上冲动频率高低参与信息传输神经纤维数目多少感觉的性质

决定于传入冲动所到达的高级中枢的部位3132不同重量的触压刺激是在单一传入纤维上引起的冲动频率的改变

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

Italsoraisesthequestionofhowitispossibletotellwhetherthetouchislightorheavy.35DoctrineofSpecificNerveEnergiesThesensationevokedbyimpulsesgeneratedinareceptordependsuponthespecificpartofthebraintheyultimatelyactivate.

Therefore,whenthenervepathwaysfromaparticularsenseorganarestimulated,thesensationevokednomatterhoworwherealongthepathwaytheactivityisinitiated.

36Thisprinciple,firstenunciatedbyMullerin1835,hasbeengiventhenameofthedoctrineofspecificnerveenergies.

Forexample,ifthesensorynervefromapaciniancorpuscleinthehandisstimulatedbypressureattheelboworbyirritationfromatumorinthebrachialplexus臂丛,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幂函数moreaccuratelydescribesthisrelation.Inotherwords,R=KSAwhereRisthesensationfelt,Sistheintensityofthestimulus,and,foranyspecificsensorymodality感觉通道,KandAareconstants.43Thefrequencyoftheactionpotentialsgeneratedbyastimulustoasensorynervefiberisalsorelatedtotheintensityoftheinitiatingstimulusbyapowerfunction.

44CurrentevidenceindicatesthatintheCNStherelationbetweenstimulusandsensationislinear;consequently,itappearsthatforanygivensensorymodality,therelationbetweensensationandstimulusintensityisdeterminedprimarilybythepropertiesoftheperipheralreceptorsthemselves.45（四）适应现象（adaptation)适应：刺激仍持续作用于感受器，而传入神经上冲动频率下降的现象。4647快适应：皮肤触觉感受器慢适应：肌梭、颈动脉窦压力感受器

{48AdaptationWhenamaintainedstimulusofconstantstrengthisappliedtoareceptor,thefrequencyoftheactionpotentialsinitssensorynervedeclinesovertime.Thisphenomenonisknownasadaptation.

49Thedegreetowhichadaptationoccursvarieswiththetypeofsenseorgan(Figurebelow).Touchadaptsrapidly,anditsreceptorsarecalledphasicreceptors.Ontheotherhand,thecarotidsinus,themusclespindles,andthereceptorsforcold,pain,andlunginflationadaptveryslowlyandincompletely;thereceptorsinvolvedaretermedtonicreceptors.505152第二节视觉器官

引起视觉的感觉器官是眼，人眼的适宜刺激是波长370-740nm的电磁波；在人脑获得的全部信息中，大约有70－95%来自视觉系统。5354144.1-3TheprincipalstructuresoftheeyeareshowninFigureabove.Theouterprotectivelayeroftheeyeball,thesclera〕巩膜,ismodifiedanteriorlytoformthetransparentcornea角膜,throughwhichlightraysentertheeye.55Insidethescleraisthechoroid脉络膜,alayerthatcontainsmanyofthebloodvesselswhichnourishthestructuresintheeyeball.Liningtheposteriortwo­thirdsofthechoroidistheretina,视网膜theneuraltissuecontainingthereceptorcells.565758一、眼的折光系统及其调节

（一）

眼的折光系统

正确视觉形成的首要条件是：外界物体能够在视网膜上形成准确而清晰的物像。这由眼的折光系统完成。59眼的折光系统可视为凸透镜。外界的物体发出或反射的光线经过这个折光系统，聚焦在视网膜，形成清晰物象。6061（二）

眼的调节

凡是位于眼前方6m以远的物体，它们发出或反射出的光线在到达眼的折光系统时已近于平行，平行光线通过眼的折光系统，不需要调节就可以聚焦在视网膜形成清晰物像。62而来自较6m为近的物体的光线是呈辐射状的，在折射后的成像将在视网膜之后，因而物像模糊，引起模糊的视觉形象。但正常眼在看近物也十分清楚，这是由于眼在看近物时已进行了调节，使进入眼内的光线经历较强的折射，结果也能成像在视网膜上。人眼的调节亦即折光能力的改变，主要是靠晶状体形状的改变；这是神经反射。

636465人眼看近物的能力，即晶状体的调节能力是有一定限度的。眼的最大调节能力可用它所能看清物体的最近距离来表示，这距离称为近点。调节反射进行时，除晶状体的变化外，同时还出现瞳孔的缩小和两眼视轴向鼻中线的会聚。66149.2-151.1THEIMAGE-FORMINGMECHANISMTheeyesconvertenergyinthevisiblespectrumintoactionpotentialsintheopticnerve.Thewave­lengthsofvisiblelightrangefromapproximately

397nmto723nm.

67Theimagesofobjectsintheenvironmentarefocusedontheretina.Thelightraysstrikingtheretinageneratepotentialsintherodsandcones.

Impulsesinitiatedintheretinaareconductedtothecerebralcortex,wheretheyproducethesensationofvision.68Parallellightraysstrikingabiconvex双凸透镜lens(Figurebelow)arerefractedtoapoint(principalfocus主焦点)behindthelens.

Thedistancebetweenthelensandtheprincipalfocusistheprincipal

focaldistance焦距.

69物70Forpracticalpurposes,lightraysfromanobjectthatstrikealensmorethan6m(20ft)awayareconsideredtobeparallel.71Theraysfromanobjectcloserthan6marediverging发散andarethereforebroughttoafocusfartherbackontheprincipalaxisthantheprincipalfocus.

Biconcavelensescauselightraystodiverge.72Accommodation

调节：Whentheciliarymuscle睫状肌isrelaxed,parallellightraysstrikingtheopticallynormaleyearebroughttoafocusontheretina.73Aslongasthisrelaxationismaintained,raysfromobjectscloserthan6mfromtheobserverarebroughttoafocusbehindtheretina,andconsequentlytheobjectsappearblurred.

74Theproblemofbringing

divergingraysfromcloseobjectstoafocusontheretinacanbesolvedbyincreasingthecurvature曲率of

lens

orrefractivepowerofthelens晶状体.

75Inmammals,theproblemissolvedbyincreasingthecurvatureofthelens.Theprocessbywhichthecurvatureofthelensisincreasediscalled

accommodation.

76

Atrest,thelensisheldundertensionbythelensligaments（suspensoryligamentoflens悬韧带）.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瞳孔缩小的意义：（1）减少入眼光量（2）减小球面像差和色像差83球面像差和色像差

简单透镜中间区域的焦距与周边区域不同,物像变形失真，这称为球面像差。

如一般的简单透镜一样，物体红色部分光线比蓝色部分折光程度较小，这种差异可使物象失真，为色像差。84（三）简化眼和视敏度

从实用角度人们设计了一些和正常眼在折光效果上相同、但更为简单的等效光学模型，称为简化眼。可用来分析眼的成像情况和进行其他计算。利用简化眼可以测定人的视力（视敏度)，即人眼能看清的最小的视网膜像的大小。

85折光指数=1.33386

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

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

88visualacuityisthedegreetowhichthedetailsandcontoursofobjectsareperceived.

89（四）非正视眼

眼结构功能异常,使平行光线不能在未调节的眼的视网膜上成清晰像，则称为非正视眼，其中包括近视、远视和散光。90近视

91近点比正常眼还要近。92151.3-152.2Inmyopia

近视,theanteroposteriordiameteroftheeyeballistoolong.9394Myopiaissaidtobegeneticinorigin.However,inexperimentalanimalsitcanbeproducedbychangingrefractionduringdevelopment.

95Thus,theshapeoftheeyeappearstobedeterminedinpartbytherefractionpresentedtoit.Inyoungadulthumanstheextensivecloseworkinvolvedinactivitiessuchasstudyingacceleratesthedevelopmentof

myopia.96Thisdefectcanbecorrectedbyglasseswith

biconcavelenses

双凹透镜,whichmakeparallellightraysdivergeslightlybeforetheystriketheeye.97远视

98近点距离较正常人为大。99100Insomeindividuals,theeyeballisshorterthannormalandtheparallelraysoflightarebroughttoafocusbehindtheretina.Thisabnormalityiscalledhyperopia.101Sustainedaccommodation,evenwhenviewingdistantobjects,canpartiallycompensateforthedefect,buttheprolongedmusculareffortistiringandmaycauseheadachesandblurringofvision.102Theprolongedconvergence会聚ofthevisualaxesassociatedwiththeaccommodationmayleadeventuallytosquint斜视

(strabismus).

103Thedefectcanbecorrectedbyusingglaseswithconvexlenses凸透镜,whichaidtherefractivepoweroftheeyeinshorteningthefocaldistance.104散光

如果折光面在某一方位上曲率半径变小，而在与之相垂直的方位上曲率半径变大，不同方位的光线在眼内不能同时聚焦，造成物像变形和视物不清。散光可用适当的柱面镜纠正，后者的特点正是互相垂直方位上具有不同的曲率半径，由此抵消眼球折光面改变对视觉的影响。

105Astigmatism散光isacommonconditioninwhichthecurvatureofthecorneaisnotuniform.Whenthecurvatureinonemeridian角膜经线

isdifferentfromthatinothers,lightraysinthatmeridianarerefractedtoadifferentfocus,sothatpartoftheretinalimageisblurred.106Astigmatismcanusuallybecorrectedwithcylindriclensesplacedinsuchawaythattheyequalizetherefractioninallmeridians经线..107老年人晶状体弹性睫状肌收缩力调节力视近物模糊矫正：看近物时，戴凸透镜。4、老视（老花眼）108

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

110Thiscondition,whichisknownasPresbyopia老视,canbe

correctedbywearingglasseswithconvexlenses

凸透镜.111二、瞳孔和瞳孔对光反应

瞳孔在亮光处缩小，在暗光处散大。瞳孔的大小可以控制进入眼内的光量。112瞳孔和瞳孔对光反应瞳孔大小随光照强度而变化的反应称为瞳孔对光反射。互感性对光反射：如果光照的是一侧眼睛时，另一侧瞳孔也缩小，称~。113Whenlightisdirectedintooneeye,thepupilconstricts(pupillarylightreflex).Thepupiloftheothereyealsoconstricts(consensuallightreflex同(互)感性光反射).

114Theopticnervefiberscarrytheimpulsesinitiatingthesepupillaryresponses.115房水和眼内压

房水循环障碍时会造成眼内压过高。

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、视网膜与视觉

（一）视网膜的结构特点

视网膜的结构十分复杂。主要的细胞层次简化为四层：120色素细胞层：内含色素颗粒和VitA，起营养及保护感光细胞的作用。感光细胞层：视锥细胞和视杆细胞，感光换能并产生感受器电位。双极细胞层：传导感受器电位。神经节细胞层：产生并传导动作电位121145.2-146.4147.1-148.1Theretinaisorganizedintenlayersandcontainstherodsandcones,whicharethevisualreceptors,plusfourtypesofneurons:bipolarcells,ganglioncells,horizontalcells,andamacrine无长突细胞

cells.122123

Therodsandcones,whicharenexttothechoroid,脉络膜synapsewithbipolarcells,andthebipolarcellssynapsewithganglioncells.124Theaxonsoftheganglioncellsconvergeandleavetheeyeastheopticnerve.

125126Sincethereceptorlayeroftheretinarestsonthepigmentepithelium色素上皮nexttothechoroid脉络膜,lightraysmustpassthroughtheganglioncellandbipolarcelllayerstoreachtherodsandcones.

127Thepigmentepithelium

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

眼底镜astheopticdisk

视神经乳头.Therearenovisualreceptorsoverlyingthedisk,andconsequentlythisspotisblind(theblindspot).130131Neartheposteriorpoleoftheeye,thereisayellowishpigmentedspot,themaculalutea

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

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

135Sincethisistheoneplaceinthebodywherearteriolesarereadilyvisible,ophthalmoscopicexaminationisofgreatvalueinthediagnosisandevaluationofdiabetesmellitus,糖尿病hypertension,andotherdiseasesthataffectbloodvessels.

136137138视杆细胞视锥细胞暗光觉-光高度敏感明暗视觉，无色觉模糊视觉中央凹少周缘部多视杆-双极-节细胞联系中的明显会聚昼光觉-光敏感度低颜色视觉精细视觉黄斑中央凹多，周缘部少视锥-双极-节细胞的单线联系(二).视网膜光学感受器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、视杆细胞的感光换能机制

视紫红质由视蛋白和视黄醛组成。

147153.1-155.4PhotosensitiveCompoundsThephotosensitivecompoundsintheeyesofhumansandmostothermammalsaremadeupofaproteincalled

anopsin

视蛋白,andretinenel

视黄醛.148RhodopsinThephotosensitivepigmentinthe

rods

iscalledrhodopsinorvisualpurple.Rhodopsinhasapeaksensitivitytolightatawavelengthof505nm.149Inthedark,theretinene1inrhodopsinisinthe11­cis顺式configuration.Theonlyactionoflightistochangetheshapeoftheretinene,convertingittotheall-trans反式isomer异构体(Figure8-17).150151Thisinturnalterstheconfigurationoftheopsin

视蛋白,andtheopsinchangeactivatestheassociatedGprotein,whichinthiscaseiscalledtransducin转导蛋白.152TheGprotein

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

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

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

158

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

160161162163五与视觉有关的基本现象

（一）暗适应和明适应

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）视野

单