Effectcells
Thehumaneyeretinaiscomposedofcountlessphotosensitivecells.Thephotosensitivecellsaredividedintorod-shapedcellsandcone-shapedcellsaccordingtotheirshape.Thereare7millioncone-shapedcells,mainlyThecentralareaoftheretinathatisconcentratedonthepupiliscalledthemacula.Therearenorod-shapedcellshere.Thefartherawayfromthemaculararea,thefewercone-shapedcellsandthemorerod-shapedcells.Intheareaclosetotheplusmargin,almostallrod-shapedcellsarepresent.Rod-shapedcellscanonlybelight-sensitive,notcolor,butthelight-sensitivesensitivityisextremelyhigh,whichis10,000timesthatofcone-shapedcells.Conecellscanbothlightandcolor.Thetwohaveacleardivisionoflabor:undertheactionofstronglight,theyaremainlyplayedbyconecells,sointhedaytimeorinabrightenvironment,thesceneseenhasbothasenseofbrightnessandasenseofcolor.Thiskindofvisioniscalledphotopic(Ordayvision).Undertheactionoflowlight,therod-shapedcellsactmainly,sointhedarkorlowlightenvironment,thescenesseenareallgrayandblack,withonlythesenseoflightanddarkness,andnosenseofcolor.
Pyramidalcellsandrod-shapedcellsareconnectedtotheopticnervethroughthebipolarmeridiancell,andtheopticnervecellleadstothebrainthroughtheopticmeridianfiber.Theopticnervegathersatapointintheretina.Thispointhasnophotosensitivecellsandiscalledablindspot.
Brightnessadaptation
Theeyescanadjustthepyramidalcellsandcolumnarcellsappropriatelytothebrightnesschangesoftheoutsideworldtomakethemhaveappropriatesensitivity.Thisphenomenoniscalled"brightnessadaptation".Theadaptationtothedarkenvironment(thebrightnessinthefieldofviewisabove0.01cd/m2,suchasdaytime,outdoors)ismainlybecausetheconecellsareinworkingcondition.Therefore,thedarkadaptationprocessisslow,whilethelightadaptationprocessismuchfaster.
Nightblindness
Nightblindnesspatientshavedifficultyseeingindarkconditionsduetothelackofphotosensitivechemicals(rhodopsin)intherod-shapedcells.Inaddition,intheretinaofsomecircadiananimals,thereareonlycone-shapedcells,butnorod-shapedcells.Circadiananimalscangenerallydistinguishcolors.Inmostbirdsitisdiurnalvision.Intheretinaofnightvisionanimals,thereareonlyrod-shapedcellsandnocone-shapedcells.Nightvisionanimalsaregenerallycolorblind.
Diettherapy
VitaminAisacomponentofrhodopsinthatiscomposedoflow-light-receivingvisualcells.Rhodopsiniscomposedofopsinand11-cis-retinal.,Isrelatedtoscotopicvision.
AnimalliverisrichinvitaminA.ThehumanbodylacksvitaminA,whichaffectstheabilityofdarkadaptation,suchaschildrenwithstuntedgrowth,dryskin,dryeyes,nightblindnessandsoon.
TheminimumdailyrequirementofvitaminAfornormaladultsisabout3500internationalunits(0.3microgramvitaminAisequivalentto1internationalunit),andchildrenareabout2000to2500internationalunits.
Darkvisionbats
Foralongtime,peoplethinkthatbatsarenocturnalanimalswithdegradedeyes.Itisanindisputablefactthatbatsare"blind"becausetheymainlyuseecholocationtodeterminetheirpositionwhencapturinginsects..However,academicianZhangYapingoftheKunmingInstituteofZoology,ChineseAcademyofSciencesandhisdoctoralstudentShenYongyidiscoveredthattheoldworldfruitbatsdidnothaveecholocationcapabilities.Theymainlyreliedonvisionandsmelltofindfood,andtheireyeswere"developed".
TheresearchteamsuccessfullyclonedtheRH1genethatcontrolstheformationofrodcellsintheretina,whichdominatethescotopicreceptors.ScientistshavefoundthattherodcellsofbothinsectivorousbatswithdegradedeyesandOldWorldfruitbatswithdevelopedeyesallexpresstheRH1gene,indicatingthateventhoseinsectivorousbatswithhighlydegradedeyesstillhavedarkvision.
FurtheranalysisofthegenesequencebyZhangYaping’sresearchteamfoundthatthegenehasconvergedevolutionbetweenfruitbatsandinsectivorousbatswhoseeyeshavenotdegenerated,whilelong-wingedbatsandlong-wingedbatswithdegradedeyeshaveoccurred.Thechrysanthemumbathasalsoundergoneconvergentevolution.Thisresultrevealsthatafterthedifferentiationofbats,itispossiblethatsomespeciestendtorelymoreonvisionandhavedevelopedeyes;whileotherstendtorelylessonvisionandtheireyesdegenerate,leadingtomultipleconvergentevolutionsoftheRH1geneinbats.Thisiswhypeoplethinkthatbatshavenovision.