Introduction
(Figure1)Theperipheralpartthateffectivelyaffectstheexcitementofasensorycelliscalledthereceptivefieldoftheneuron.
Skinreceptivefield
Thesensoryfieldofskinsensoryperipheralneuronsistherangethatmanyreceptorsinnervatedbythenervefibersofasensoryneuroncanrespondontheskinsurface.Anopticnervecellisconnectedtomanyvisualreceptors,andthereceptivefieldinnervatedbyanopticnervefibercanbefoundontheretina.Thisvisualreceptivefieldcangenerallybeexpressedbythesizeoftheviewingangle.Asforthereceptivefieldoftheauditorysystem,itisequivalenttotheresponsefieldofeachauditoryneurontosoundsofdifferentfrequencies.
Therearemanyrelaynucleifromtheperipheralneuronsofthesensorysystemtotheneuronsinthesensoryareaofthecerebralcortex.Sinceasinglesensormaybeassociatedwithmanysensorynervefibers,sensoryinformationistransmittedthroughmanysensorynervefibersthatemitdifferentimpulsesofdifferentspatialandtemporaltypes,thatis,encoded.Itisnowclearthatneuronscanbefunctionallydividedintothreetypes:excitability,postsynapticinhibition,andpresynapticinhibition.Varioussensoryinformationiscontinuouslyre-modifiedinspaceandtimeasitpassesthroughthecentersatalllevels,thatis,re-encoding.Thisistheessenceofthecentralnervoussystemasasensoryinformationprocessingmechanism.Accordingtotheabovereasons,thesizeandnatureofthereceptivefieldsofsensorynervecellsabovethesecondlevelaredifferentfromthoseofthefirst-levelnervecells,andtheirreceptivefieldsarecomposedofdistinctexcitatoryfieldsandsurroundinginhibitoryfields..
Specificexamples
Undertheconditionsofbrightadaptation,thecat'sretinalganglioncellscanprovethatthecenterofmanyofitsreceptivefieldsistheexcitatoryfield,anditssurroundingissurroundedbytheinhibitoryfield,formingApproximatelyconcentricdoublestructure,therangeofthecentralexcitingfieldis0.5°~2.0°(0.125~2.0mm).Whenlightstimulationisappliedtothecentralpart,someganglioncellsdischargewhenlightisgiven(ontype),andsomearedischargedwhenlightisremoved(offtype).Ifthecenterofthereceptivefieldison-type,itssurroundingsareoff-type;ifthecenterofthereceptivefieldisoff-type,itssurroundingsareon-type.Therefore,undertheconditionsofbrightadaptation,thereceptivefieldofthecat'sretinalopticganglioncellscanbedividedintotwocategories:oneislight-central/light-external-peripheral;theotherislight-extracted-center/light-external.Whenlightirradiatesthecenterandouterboundaryofthereceptivefield,on-offdischargecanbegenerated,butwhenthecenterandouterperipheryofthereceptivefieldareirradiatedatthesametime,itdoesnotrespondatall.Thiscanreflectthatthecentralandperipheralpartsofthesusceptormayhaveacomplementaryeffect(Figure2).
Differentreceptivefields
Thereceptivefieldsofretinalganglioncellsarenotthesamedependingonthedegreeofanimalevolution.Eveniftheyareallvertebrates,thereceptivefieldsofdifferentspeciesarenotthesame.Forexample,thereceptivefieldsoftheopticganglioncellsofcatsandmonkeysarecomposedofexcitatoryfieldsandinhibitoryfieldsthatexistinconcentriccircles,whilethereceptivefieldsofopticganglioncellsoffrogs,rabbits,pigeons,etc.,exceptfortheabove-mentionedconcentriccirclesOutsideofthereceptivefieldofform,therearestillfeaturesthatcanonlyreacttomovingobjectsortospecialgraphics.Forexample,inthereceptivefieldoftheopticganglioncellofarabbit,whentheganglioncellisirradiatedwithalightspotandthedischargeoftheganglioncellisobserved,onlyanelectricalresponse(on-offtype)occurswhenthelightisappliedorwithdrawn.However,itisworthnotingthatwhenthelightspotmovesfromtoptobottominthereceptivefield,adischargeoftwopulsescanberecorded;whilewhenthelightspotmovesfrombottomtotop,79pulsescanberecorded.Whenthelightspotmovesdiagonallybetweentheupperandlowerdirections,thenumberofdischargescausedisjustinbetween.Therefore,itcanbesaidthatthemovingdirectionofthelightspotthatcausesthemaximumdischargefrombottomtotopisthemostsuitabledirectionofmotion;thedirectionfromtoptobottomisthedirectionofnoresponse.Theselectivityofthisdirectionisnotonlyduetotheexcitementfieldandthereceptivefield.Theasymmetryofthespatialarrangementoftheinhibitoryfieldislikelytobecontrolledbytheneuralstructureofthereceptorcellsrelatedtothesequentialorder.Experimentshaveprovedthatthemaximumresponsecanalsobeobtainedwhenthearrangedlightspotsaresequentiallylitinthemostsuitabledirectionofmovementwithoutmovingthelightspots.Onthecontrary,ifthelightpointsaresequentiallylitintheoppositedirection,thedischargereactionisalmostinvisible.
Thereceptivefieldoflateralgeniculatebodynervecellshasbeenprovedtobebasicallythesameasthereceptivefieldconfirmedbyopticganglioncellsincatsandmonkeys,exceptthattheperipheralpartofthelateralgeniculatebodyreceptivefieldinhibitsthecentralpartTheeffectisrelativelystrong.Iftheentireretinaisirradiated,thedischargeresponseisalmostimpossibletoprove.Theauditoryresponsefield,whichisequivalenttothereceptivefield,hasprovedthatthecochlearnervenucleusisthewidestincats.Whenitreachesthetrapezoid,itsresponsefieldnarrows,andtheresponsefieldoftheinferiorcolliculusauditorynervecellsbecomesnarrower.Thereasonisthatdifferentdegreesofsuppressionfieldscanappearonboththetreblesideandthebasssideoftheresponsefield.Ithasbeenprovedthattheresponsefieldtothemedialgeniculatebodyisthenarrowestpathintheprocessofauditoryinformationtransmission,andtheybelievethatsuchexperimentalresultsmeanthattheaudioanalysisfunctionislikelytocompleteitsmissioninthemedialgeniculatebody.
Sensoryimpulse
Thesensoryimpulseistransmittedtotherelevantsensoryareasofthecerebralcortexintheformofradiationandaggregationtoproduceperception.Asfarasthereceptivefieldofneuronsinthesensoryareaofthecerebralcortexisconcerned,ithasitsowninherentcharacteristics.Takingvisionasanexample,thereceptivefieldsofneuronsinthevisualfieldofthecerebralcortex(area17)areelongated.Itsexcitementfieldandinhibitionfieldareseparatedbyaline,theyareincontactwitheachotherandparalleltothelongaxisofthereceptivefield.Inthisway,whenthelinearstimulus(fissurelight,blackline,lightanddarkboundaryline,etc.)movesparalleltothereceptivefielddirectionaxis(ie,thelongaxisofthereceptivefield),themostobviousresponsewillbeproduced.Thisshowsthatthereceptivefieldofneuronsinthevisualareahaspositionspecificity.Thisisthebasicfeaturedifferentfromthereceptivefieldofthelateralgeniculatebody,andsomepeoplecallthisreceptivefieldthesimplereceptivefield.
Otherreceptivefields
Inadditiontothesimpletypesmentionedabove,thereceptivefieldsofneuronsinthevisualareaalsohavecomplexreceptivefieldsandsuper-complexreceptivefields.Theboundarybetweentheexcitatoryfieldandtheinhibitoryfieldofthereceptivefieldofthesecellsisnotclear.Thetwoareintertwined,anditisgenerallydifficulttoaccuratelycontrolwithasmalllightspot.Irradiatingtheentireretinadoesnotcauseadischargeresponse.Forexample,whenapatternwithaclearboundarybetweenlightanddarkisirradiatedwithoutalightspot,itcanbeclearlyseenthatwhenthelight-darkinterfacemovestoacertaindirection,anobviousdischargeresponseofthecorticalcellwillbeproduced.Thedifferencebetweenthecomplexreceptivefieldandthesimplepurereceptivefieldisthatitdoesnotdetectlinesinspecificpositionsandspecialorientationsinthefieldofview,buttodetectlinesegmentsinvariousdirections.Thismaybebecauseitdoesnothavestrictlocalfeatures.Sothinkthatithastheabilitytorespondtographics.Asforthesuper-complexreceptivefield,thedifferencebetweenitandthecomplexreceptivefieldliesinthestimulationofthelightanddarklinesegmentsreceivedbythecomplexreceptivefield.Nomatterhowlongthelineis,itisaneffectivestimulusthatcancausecelldischarge;Thestimulusreactedbythecomplexreceptivefieldmustbespecificallylimitedatbothendsofthestimulus,thatis,thestimuluslinesegmentmusthaveoneorbothendsinthereceptivefieldtobecomeastimulus,oronlywhenthereisaspecialcurveandothershapeatoneendCausesthecell'sdischargeresponse.
Comparedwiththereceptivefieldofneuronsintheauditoryareaofthecerebralcortex,thereceptivefieldofneuronsinthemedialgeniculatebodynotonlyhasnotbecomenarrow,buthaswidenedagain.Somepeoplethinkthatthismaybethatafterthemedialgeniculatebodyhasbasicallycompletedtheaudioanalysis,thecerebralcortexcellshavecompletedtheperceptionofthesoundtoneandthechoiceofthedirectionofthetone,andthecomprehensiveprocessingoftheinformationinputtothecortexhasacloserelationshipwiththeperceptionoftimbre.