1.Overview:
1.Itisakindoflight-stimulatedcellinthefirstlevelneuronsoftheretina.Thecellsaredistributedatadistanceof5-6mmfromthecentralfoveainaring-shapedbelt,withatotalofabout125millioncells.Themorphologicalfeatureofthiscellisthatitprotrudestowardtheendofthepigmentlayertoformaspecialrod-shapedphotoreceptor;theprotoplasmicprotrusionoftheotherpoleformsasynapsewiththebipolarcellofthesecondlevelneuron.Therod-shapedbodyofrodcellsiselongated,about40-60μminlength,andabout2μmindiameter.Itiscomposedoftwoinnerandoutersections:theoutersectioniscylindrical,hasstrongrefractivepower,hashorizontalstripes,andcontainsapigmentcalledrhodopsin.Thisisabindingproteinthatcanbedecomposedintoopsinandretinalwhenexposedtolight(VitaminAaldehyde)isdecomposedintovitaminAthroughtheactionofenzymes.VitaminAcanre-synthesizerhodopsininthedark.Therod-shapedinnersegmentisslightlythicker,granular,richincytoplasm,andslightlylessrefractive.Thejointbetweentheinnerandoutersectionsisanarrowneckwithfibrilbundlesextendingfromtheinnersectiontotheoutersection.Theinnersegmentcontainsmitochondria,endoplasmicreticulum,andRNAparticles.Thecellbodyisslightlyenlarged.Thenucleusissmallanddeeplystained,andthereislittlecytoplasmaroundthenucleus.Themainfunctionofrodcellsistosenselowlight(starlight,moonlight,etc.),andvitaminAisanimportantcomponentoftheirfunctionalstructure.Therefore,theylackvitaminA,reducetheabilitytosenselowlight,anddevelopnightblindness.Therodcellsofnocturnalanimalsarewelldeveloped.
2.Itisthecellsintheretinathatareclosetothepigmentcelllayerandarestimulatedbyweaklight,thatis,theycanrecognizeblackandwhiteobjectsunderweaklight.Mostlydistributedontheperipheryofthefovea,thecellsareelongated,withalengthofabout40-60micronsandadiameterof2microns.Theyarecomposedoftheinnersegment,outersegment,nucleus,andterminalfoot.Theoutersegmentiscylindricalwithstrongrefractivepowerandhorizontalstripes.Itcontainsrhodopsin.Itisabindingproteinthatcanbedecomposedintoopsinandretinal(alsocalledvitaminAaldehyde)undertheactionoflight.ThelattercanbereducedtovitaminAundertheactionofreductase.ItcanbeseenthatvitaminAistherawmaterialofrhodopsin.IfvitaminAisinsufficient,itwillaffectthesynthesisofrhodopsinandthenormalprogressofitsphotochemicalreaction,causingnightblindness.Intheinnersegment,apartfromthefibrilbundles,therearemainlyalargenumberoflongmitochondria(producingdevices),vesiclesoftheendoplasmicreticulum,andRNAparticles.
