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Chapter8HearingTheAuditorySystemsReferenceP142-151P442-454P651-662ContentPropertiesofSoundRoleofMiddleEarinSoundTransmissionFunctionofOrganofCortiHomeostaticImbalancesofhearing.Part1.PropertiesofSoundSoundtravelsinwavesasdoeslight1.Pitch:determinedby“frequency,”thenumberofcyclespersecondofasoundwave,measuredinhertz(Hz)2.Loudness:determinedby“amplitude”(height)ofthesoundwave,measuredindecibels

(dB)

3.Timbre:determinedby“complexityandshape”ofthesoundwave,giveseachsounditsuniquequalityLoudnessofSound0dB=hearingthreshold50dB=normalconversation90dB=dangerzone120dB=Rockconcert130dB=PainthresholdPart2RoleofMiddleEarinSoundTransmissionMechanismsInvolvedinTransformerProcessSizedifferencebetweenTympanicMembraneandStapesFootplateLeveractionFirstComponentofMiddleEarTransformerActionSizeDifferenceTympanicmembrane.59cm2Stapesfootplate.032cm2PressureformulaPressure=force/areaImpactonsoundtransmissionPressuregain:0.59/0.032=18.4(times)TransformerActionofMiddleEar

LeverActionFulcrumEffectpressuregain:1.3timesTRANSFORMERACTION

AMOUNTOF

AMPLIFICATIONPressureGain Contributionfrom:

18.4 TM(TympanicMembrane)

tostapesfootplate 1.3 Leveraction 23.9 Totalpressuregain

(18.6x1.3)

Part3FunctionofOrganofCortiastructurerestsatopthebasilarmembranealongitslengthcontainsapprox.16,000cochlearhaircells1.Howtodiscriminatethefrequencyofthesound?---TravelingWaveTheoryVibrationofBasilarMembraneandtheTravelingWaveTheorySoundwaveenteringattheovalwindowistocausethebasilarmembraneatthebaseofthecochleatovibratedifferentfrequenciescausevibrationsatdifferentlocations(places)alongbasilarmembranehigherfrequenciesatbase,lowerfrequenciesattop2.ElectricalPotentialsDCvs.ACDirectCurrent(DC)=stimulusdoesn’tchangewithtime,constant;i.e.batteryAlternatingCurrent(AC)=alwayschangingovertime,lookslikeasinewaveCochleaPerilymph-similarincompositiontoextracellularfluid.HighinNa+andlowinK+.Endolymph-foundinthescalamedia.Similartointracellularfluid.HighinK+andlowinNa+TwoDCPotentials(EP)EndocochlearPotential(EP)+80mVpotentialwithrespecttoaneutralpointonthebodyduetotheStriaVascularis-80mVReticularLamina+80mVIntracellularPotential(IP)ororganofcortipotential(restingpotential)Recorded-80mVinsidecellsoforganofcortiTwoDCPotentials(IP)HairCellintheOrganofCortiWhenthebasilarmembranemoves,ashearingactionbetweenthetectorialmembraneandtheorganofCorticauseshaircellstobendK+comesintothehaircellanddepolarizesthehaircell.

TheconcentrationofK+intheendolymphisveryhighsowhenitcomesintothehairthepositiveionscometothecellcausingadepolarization.Therearelittlemechanicalgatesoneachhaircellthatopenwhentheyarebent.TwoACPotentialsCochlearMicrophonicPotentialReproducesfrequencyandwaveformofasinusoidperfectlyGeneratedfromhaircellActionPotential(AP)ElectricalactivityfromtheVIIINerveCanbemeasuredfromanywhereinthecochleaorintheauditorynervePart4HomeostaticImbalancesofhearing.Deafness.Conductiondeafness-

possiblecausesinclude:perforatedeardrum,inflammation,otosclerosisSensi

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