3.17-3.21 Visual System The Eye

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1、3.17-3.21VisualSystem:TheEyeIReviewthegrossanatomyofthehumaneye(seeFigure11.12)II.FormationofopticalimagesA.refraction:“bending”oflightthatoccursattheinterfaceofdifferentopticalmedia1.most(~80%)ofrefractionoccursaslightpassesthroughthecornea2.sharpfocusingofimagesat

2、variabledistancesfromtheeyerequiresanadjustablelens,byaprocesscalledaccommodationB.accommodation1.dynamicchangesintherefractivepowerofthelensa.whenthelensisflat…leastrefractivepower(farvision)b.whenlensisrounded…mostrefractivepower(nearvision)2.controloflensshape(se

3、eFigure11.2)a.lensisanelasticstructurethattendstoroundupb.thelensisattachedtothecircularciliarymusclebyanarrayofzonulefibers;therestingtensionexertedbythezonulefibersissufficienttopullthelensintoaflattenedpositionc.tofocusonanearobject,thelensmustbecomemorerounded,w

4、hichisaccomplishedbythecontractionoftheciliarymuscle(i)contractionofthecircularciliarymusclebringstheattachmentpointsofthezonulefibersclosertothelens(ii)thisrelievestensiononthelensandthelens’sintrinsicelasticityallowsittoroundup3.adjustmentsofpupilsizealsocontribut

5、etofocusingofimagesontheretinaa.becauseopticalaberrationsaregreatestaslightpassesthroughtheedgesofthelens,thepupilminimizestheseerrorsbyrestrictingthepassageoflighttothecenterofthelensb.averysmallpupilseverelylimitstheamountoflightenteringtheeye,whichmaybedetrimenta

6、lunderconditionsofdimilluminationc.theneuralcontroloverpupilsize(seeFigure12.2)providesforaoptimizationofthesetwocompetingconsiderationsundervaryingconditionsofilluminationIII.InnerlayeroftheeyeA.comprisestwotissuesderivedfromthedevelopingdiencephalon(seeFigure11.4)

7、1.neuralretina(seebelow)Phototransduction2.pigmentepithelium(seeFigure11.6)a.tissuecontainingthepigment,melanin黑色素(albinoindividualslackthispigmentandarehighly“lightsensitive”)b.reducesthebackscatteringoflightc.absorbsphotoreceptordiskmembraneandrecyclesphotopigment

8、d.photoreceptorsbecomedysfunctionalandeventuallynonfunctionalifseparatedfromthepigmentedepithelium(e.g.,retinaldetachment)分区01.大脑科学的第1页B.b