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Ganglion Cells



Introduction

Ganglioncelllayer:Locatedintheinnermostlayeroftheretina,itiscomposedofmultipolarganglioncells.Itsdendritesaremainlyconnectedwithbipolarcellsandcanalsopassthroughapodocytes.Lateralconnection;itsaxonsextendtotheopticnerveheadandpassthroughthelaminatoformtheopticnerve.

Transmittingsignals

Thereareseveraldifferenttypesofganglioncellsthattransmitbrightnesssignals,andtheyareexcitedbybipolarcells,horizontalcells,andapodocytesindifferentsituations.Asmallnumberofganglioncellsmainlyrespondtotheintensity(brightness)ofthelightthathitsthephotoreceptors.Aslongasthebrightnessishigh,thefrequencyofimpulsesfromthesecellsisalwayshigherthanthenaturalfrequencyoffiringimpulses.Itisthesignalsemittedbythesecellsthatletthebrainknowthetotalbrightnessofthesceneitsees.

Transmitthesignalaboutthecontrastinthevisualscene-intheprocessoflateralinhibition,mostganglioncellshardlyrespondtotheactuallevelofbrightnessinthescene,butonlyreacttotheedgeofthecontrastinthescene.Becausethisisthemainwaytheformofthesceneistransmittedtothebrain,thefollowingexplainshowthisprocessoccurs.

Whenuniformlighthitstheentireretina(thatis,whenallphotoreceptorsareequallyexcitedbytheincidentlight),thecontrasttypeganglioncellsareneitherexcitednorinhibited.Thisisbecausethesignalstransmitteddirectlyfromthephotoreceptorsthroughthebipolarcellsareexcitatory,whilethesignalstransmittedfromthesurroundingphotoreceptorsthroughthehorizontalcellsareinhibited.Thesetworeactionsoffseteachother.Onthecontrary,ifthereisacontrastedge,thephotoreceptorsononesideoftheedgeareexcited,andthephotoreceptorsontheothersideoftheedgearenotexcited,thereisnodirectoffsetbytheexcitatorysignalofthebipolarcellandthehorizontalcellinhibitorysignal.Theresultisthattheganglioncellsonthebrightsideareexcited,whilethedarksideissuppressed.

Thebalanceprocessoftheexcitatorysignalandtheinhibitorysignalisexactlythesameasthelateralinhibitionduringthetransmissionofvariousothersensorysignals.Thisprocessisamechanismusedbythenervoussystemtoenhancecontrast.

Receptivefield

Twotypesofganglioncells:ON-centeredtype,OFF-centeredtype

ON-centeredtype

WhenWhenalightspothitsthecenterofthereceptivefield,thefiringofthisganglioncellsuddenlyincreases.(Thedistributedareaisacirclewiththecenterofthefieldofviewasthedot.ThisareaiscalledtheONarea);outsidethissmallcenter,thereisanannularrangearoundthem.Inthisarea,ifthesamesmalllightspotisusedtostimulateit,theoppositeeffectwilloccur.Ifthelightspotfallscompletelyontheringarea,thebackgroundemissionstopscompletely.ThisareaiscalledtheOFFarea.Fromthispointofview,ON-centeredcellsbelongtothe“light-givingtype(thelightspotisgiveninthecenterofthefieldofviewtoproduce[ON])”.ForON-centeredganglioncells,thelightenergyfalling(giving)intheONareaisSee,thelightfalling(giving)intheOFFzoneisinvisible.

OFF-centeredtype

Whenthelightspotisremovedfromthecenterofthereceptivefield,itwillemitstrongly.(ThesmallcircularareaemittedistheONarea).Onthecontrary,ifthelightisinthecenterofthefieldofview,itstopsemitting.Outsidethissmallcentralcirculararea,thereisanannularrangearoundthem.Inthisring-shapedarea,ifthesmalllightspotisremoved,thedistributionwillstop.ThisareaiscalledtheOFFarea.Fromthispointofview,OFF-centeredcellsbelongtothe“light-extractedtype(thelightspotinthecenterofthefieldofviewisremovedtoproduceemission,andthelightwillstopemitting[OFF])".ForOFF-centeredganglioncells,ThelightevacuatedintheONareacanbeseen,andthelightevacuatedintheOFFareaisinvisible.Inthesameway,thelightfallingintheONareaisinvisible,andthelightfallingintheOFFareacanbeseen.

Visualreceptivefieldreferstoacertainareaorrangeontheomentum.Whenitisstimulated,itcanactivatetheactivityoftheinterlayernervecellsthatareconnectedtothevisualsystemandthisarea.Thisareaontheomentumisthereceptivefieldofthesenervecells.Thereceptivefieldofaganglioncellisonlyintheareaconnectedtoitontheretina.Towhatextentanervenodeisconnectedtotheretina,thelargestreceptivefieldisthesize.

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