Chapter11-SensorySystemsTHEEARMarcel-Andr茅BoillatAnatomyTheearisthesensoryorganresponsibleforhearingandthemaintenanceofequilibrium,viathedetectionofbodypositionandofheadmovement.Itiscomposedofthreeparts:theouter,middle,andinnerear;theouterearliesoutsidetheskull,whiletheothertwopartsareembeddedinthetemporalbone(figure11.1).Figure11.1DiagramoftheearTheouterearconsistsoftheauricle,acartilaginousskin-coveredstructure,andtheexternalauditorycanal,anirregularly-shapedcylinderapproximately25mmlongwhichislinedbyglandssecretingwax.Themiddleearconsistsofthetympaniccavity,anair-filledcavitywhoseouterwallsformthetympanicmembrane(eardrum),andcommunicatesproximallywiththenasopharynxbytheEustachiantubes,whichmaintainpressureequilibriumoneithersideofthetympanicmembrane.Forinstance,thiscommunicationexplainshowswallowingallowsequalizationofpressureandrestorationoflosthearingacuitycausedbyrapidchangeinbarometricpressure(e.g.,landingairplanes,fastelevators).Thetympaniccavityalsocontainstheossicles-themalleus,incusandstapes-whicharecontrolledbythestapediusandtensortympanimuscles.Thetympanicmembraneislinkedtotheinnerearbytheossicles,specificallybythemobilefootofthestapes,whichliesagainsttheovalwindow.Theinnerearcontainsthesensoryapparatusperse.Itconsistsofabonyshell(thebonylabyrinth)withinwhichisfoundthemembranouslabyrinth-aseriesofcavitiesformingaclosedsystemfilledwithendolymph,apotassium-richliquid.Themembranouslabyrinthisseparatedfromthebonylabyrinthbytheperilymph,asodium-richliquid.聽Thebonylabyrinthitselfiscomposedoftwoparts.Theanteriorportionisknownasthecochleaandistheactualorganofhearing.Ithasaspiralshapereminiscentofasnailshell,andispointedintheanteriordirection.Theposteriorportionofthebonylabyrinthcontainsthevestibuleandthesemicircularcanals,andisresponsibleforequilibrium.Theneurosensorystructuresinvolvedinhearingandequilibriumarelocatedinthemembranouslabyrinth:theorganofCortiislocatedinthecochlearcanal,whilethemaculaeoftheutricleandthesacculeandtheampullaeofthesemicircularcanalsarelocatedintheposteriorsection.HearingorgansThecochlearcanalisaspiraltriangulartube,comprisingtwoandone-halfturns,whichseparatesthescalavestibulifromthescalatympani.Oneendterminatesinthespiralligament,aprocessofthecochlea'scentralcolumn,whiletheotherisconnectedtothebonywallofthecochlea.Thescalavestibuliandtympaniendintheovalwindow(thefootofthestapes)androundwindow,respectively.Thetwochamberscommunicatethroughthehelicotrema,thetipofthecochlea.Thebasilarmembraneformstheinferiorsurfaceofthecochlearcanal,andsupportstheorganofCorti,responsibleforthetransductionofacousticstimuli.Allauditoryinformationistransducedbyonly15,000haircells(organofCorti),ofwhichtheso-calledinnerhaircells,numbering3,500,arecriticallyimportant,sincetheyformsynapseswithapproximately90%ofthe30,000primaryauditoryneurons(figure11.2).Theinnerandouterhaircellsareseparatedfromeachotherbyanabundantlayerofsupportcells.Traversinganextraordinarilythinmembrane,theciliaofthehaircellsareembeddedinthetectorialmembrane,whosefreeendislocatedabovethecells.ThesuperiorsurfaceofthecochlearcanalisformedbyReissner'smembrane.Figure11.2Cross-sectionofoneloopofthecochlea.Diameter:approximately1.5mmThebodiesofthecochlearsensorycellsrestingonthebasilarmembranearesurroundedbynerveterminals,andtheirapproximately30,000axonsformthecochlearnerve.Thecochlearnervecrossesth...
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