医学电子chapter4theoriginofbi

1Chapter4TheOriginofBiopotentials2BioelectricGivenadequatemonitoringequipment,theengineeroftodaycanrecordmanyformsofbioelectricphenomenawithrelativeease.Thesephenomenaincludethe„electrocardiogram(ECG,)„electroencephalogram(EEG,)„electroneurogram(ENG,),„electromyogram(EMG,)„electroretinogram(ERG,)34.1ELECTRICALACTIVITYOFEXCITABLECELLSBioelectricpotentialsareproducedasaresultofelectrochemicalactivityofacertainclassofcells,knownasexcitablecells,thatarecomponentsofnervous,muscular,orglandular(;)tissue.Electricallytheyexhibitarestingpotentialand,whenappropriatelystimulated,anactionpotential.4THERESTINGSTATE(CELL)Theindividualexcitablecellmaintainsasteadyelectricalpotentialdifferencebetweenitsinternalandexternalenvironments.Thisrestingpotentialoftheinternalmediumliesintherange–50to–100mV,relativetotheexternalmedium.5Thedistributionofionsacrossthecellmembraneandthestructureofthismembrane(PK,PNa,PCl)accountfortherestingpotential.67Thefactorsinfluencingtheflowofionsacrossthemembraneare:„Diffusiongradients,()„Theinwardlydirectedelectricfield,()„Membranestructure(availabilityofpores),and()„Activetransportofionsagainstanestablishedelectrochemicalgradient.()8THEACTIVESTATE9Anexcitablecellcanconductanactionpotentialwhenadequatelystimulated.Anadequatelystimulusisonethatbringsaboutadepolarizationinamembranethatissufficienttoexceedthethresholdpotentialofthemembraneandtherebyelicit(;)anall-or-noneactionpotentialthattravelsinanuntenanted()fashionataconstantconditionvelocityalongthemembraneoftheexcitablecell.Becauseofthesteadyrestingpotential,thecellmembraneissaidtobepolarized.10Alesseningofthemagnitudeofthispolarizationiscalleddepolarization;anincreasingInitsmagnitudeisreferredtoashyperpolarization.Furtherincreasesinintensityordurationofstimulusbeyondthatrequiredforexceedingthethresholdlevelproduceonlythesameresult.Theoriginoftheactionpotentialliesinthevoltage-andtime-dependentnatureofthemembranepermeabilitiestospecificions,notablyNa+andK+.11AllornoneThreshold121314151617Duringtheinitialportionoftheactionpotential,themembranecannotrespondtoanystimulus,nomatterhowintense.Thisintervalisreferredtoastheabsoluterefractoryperiod.Itisfollowedbytherelativelyrefractoryperiod.Theexistenceoftherefractoryperiodproducesanupperlimittothefrequencyatwhichanexcitablecellmayberepetitivelydischarged.„Forexample,anerveaxonhasanabsoluterefractoryperiodof1msandanupperlimitofrepetitivelydischargeoflessthan1000impulses/s.181920214.2VOLUMECONDUCTORFIELDSAfundamentalprobleminelectrophysiology()istheproblemofthesingleactivecellimmersedinavolumeconductor(asaltsolutionsimulatingthecompositionofbodyfluids).Astudyofthisproblemprovidesconsiderableinsightintoother,morecomplexvolume-fieldproblems,suchastheENG,EMG,ECG,andsoforth.224.3FUNCTIONALORGANIZATIONOFTHEPERIPHERALNERVOUSSYSTEMTHEREFLEXARC:„(1)Asenseorgan(),„(2)Asensorynerve(),„(3)TheCNS(CentralNervousSystem),„(4)Amotornerve(),„(5)Theeffectororgan,23exampleThesimplestexampleofthebehaviorofthereflexarcistheknee-jerkreflex,inwhichthepatellar()tendon()belowthekneeisgivenaslighttapthatstretchesspecializedlengthreceptors,calledmusclespindles(),withinthemuscleandsubsequentlyexcitesthem.24()(Dendrites):(Axon):25JUNCTIONALTRANSMISSION:Withinthereflexarcthereareintercommunicatinglinksbetweenneurons(neuro-neurojunctions)calledsynapses().Therearealsocommunicatinglinksbetweenneuronsandmusclefibers(neuromuscularjunctions)atasmallspecializedregionofthemusclefiberreferredtoastheend-plateregion.Thejunctionaltransmissionprocessineachofthesecasesiselectrochemicalinnature.()264.4THEELECTRONEUROGRAM(ENG,)27284.5THEELECTROMYOGRAM(EMG,)294.6THEELECTROCARDIOGRAM(ECG,)35LeadII394142ECGŠLead-FailDetectionŠBaselineRestorationŠPacemakerSpikeRejectionŠAmplifierProtectionŠIsolation4748Atrioventricular()block49Etopic()beat50Fibrillation()51524.7THEELECTRORETINOGRAM(ERG,)53爲54:††††††55,,,,;,,,,(,),,,56,,,,,57,,,,,,,.,,58,,,,,59Fundusoftheeye60ELECTRORETINOGRAM(ERG,)産EOGERGVEP61ERGERG爲„FERGŠFERGERGERGERGOPS„PERG„LERG62ERG63ERGclinicallyrecordedwiththeaidofanAg/AgClelectrodeembeddedinaspecialcontactlensusedastheexploringelectrode.(See,forexample,Strong,1973.)Thesaline-filledcontactlensisingoodcontactwiththecornea(),whichisverythinandinintimatecontactwiththeaqueoushumorandpassivefluidmediumoftheinnereye.Thecontactlensisusuallywell-toleratedbythesubjectandpermitslongexaminationswithoutdiscomfort.64爲産ü65爲66„„ü„爲„67ERG6869爲707172EOG73VEP代表第三神經元即神經節　細胞以上視資訊的傳遞狀況。其檢查的目的是用以推測自視網膜到大腦皮質之間傳導纖維的健康狀況以及視皮質功能活動狀況。當視力喪失患者的EOG和ERG檢查都正常時，則病變在神經節細胞以上到大腦皮質之間。在此段落的病變除視野檢查外，VEP是唯一有效的檢查方法。7475爲76VEP77VEP爲癔784.8THEELECTROENCEPHALOGRAM(EEG,)79EEG8081EEG82EEG83EEG84EEG8586EEG87DifferenttypesofnormalEEGwaves.88AbnormalEEG89BuriedwithintheEEGisasignalwhichismorerevealingaboutinformationprocessinginthebrain.Thissignalcanbeobtainedbytime-lockingtherecordingoftheEEGtotheonsetofeventssuchasapersonreadingawordonacomputerscreen,listeningtoamusicalnoteplayedonaninstrument,orviewingapictureinamagazine.Theresultingactivityiscalledanevent-relatedpotential(ERP),90---頻率爲8～