introduction to the nervous system. · functional localization of the cortex. the drawing of the...
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IntroductiontotheNervousSystem.Code:HMP100/UPC103/VNP100.Course:MedicalPhysiologyLevel1MBChB/BDS/BPharmLecture2.FunctionalOrganisationoftheNervousSystemLectureOutline1.1Introduction1.2Learningoutcomes1.3AreasoftheCerebralHemispheresinvolvedinLanguageFunction1.4NeuronalPathwayfor 1.4.1SomaticSensory 1.4.2SomaticMotor 1.4.3PainSensation1.5Summary1.6Activity1.7FurtherReading1.8SampleExaminationQuestions1.1IntroductionInthelastlecture,wecoveredthebasicanatomyofthenervoussystem.Wesawthatitisdividedintomanydivisionsandparts.Nowinthislecture,wewillcoversomefunctionalorganisationofthenervoussystem.Youknowthatthenervoussystemcarriesoutmanydifferentfunctionsunlikeotherphysiologicalsystemsofourbody.Todothesemanydifferentfunctions,thenervoussystemhasfunctionaldivisionsinvolvingdifferentpartofthenervoussystem.Andthisaddstothedifficultyinstudyinghowthenervoussystemworks.Inthislecture,wewillcoverthefunctionalorganizationof4functions:language,sensory,motor,andpain.Wewillnotcoverthemindetails;thiswillbedoneinlaterlectures.Herewewanttobeingtounderstandthefunctionalorganizationofthenervoussystem.Learningwhichpartsofthenervoussystemareinvolvedincarryingoutthedifferentfunctionsisimportant.Thiswillhelpuspredictthetypeofdysfunctionapersonwillhavewhenthereisdamagetoaparticularpartoftheirnervoussystem.Andifweknoworobservethesymptomsintheperson,wecanpredictwhereinthenervoussystemthedamagehas
occurred.
1.2LearningOutcomes Attheendofthislecture,youshouldbeableto: 1. StatethefunctionoftheNervousSystem2. Nameandshowthelocationoftheareasofthecerebralhemisphereinvolvedinlanguagefunction.
3. Describewithadiagramthesomaticsensorysystem.4. Describewithadiagramthesomaticmotorsystem.5. Describewithadiagramthepain(nociceptive)system.
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1.3AreasontheCerebralHemisphereinvolvedLanguageFunctionInrecentyears,techniquesandmethodshavebeendevelopedthatallowustostudytheactivityofbrainwhenitisinvolvedinaparticulartask.Withthesetechniques,weaskapersontodoaparticulartaskandseewhichareasofthecerebralhemispheresareactivewhenthattaskisbeingdone.Fromthesestudies,wecanconcludethattheseareasareinvolvedinthatfunction.Ofcourseitdoesnottellushowthesepartsofcerebralhemispheresarecarryingoutthatfunction.Figure1.1showstheresultsofastudyonbrainactivitywhenthepersonwasaskedtodo4differenttasks:(1)lookatwords,(2)listentowords,(3)speakwords,and(4)thinkofwords.Theimagingisofthelateralsurfaceoftheleftcerebralhemisphere.Theareasonthesurfaceshowingredandyellowcolorareareasthatbecameactivewhenthepersonwasdoingeachof
thedifferenttasks.
Figure1.1Imagingofthesurfaceoftheleftcerebralhemispherewhenapersonwasaskedto(1)lookatwords,(2)listentowords,(3)speakwords,and(4)thinkofwords.Theareasinred/yellowshowtheareaswhichbecameactiveforthedifferenttasks.Thefrontpartofthecerebralhemisphereistowardstheleft.Whenthepersonwasaskedtolookatwords(topleftimage),wecanseethattheiroccipitallobebecamemoreactivethanotherpartsofthebrain.Inlaterlectures,youwilllearnthatthispartofthesurfaceofthecerebralcortexwithotherpartsisinvolvedinproducingimagesfromthesignalsthatcomefromtheretinaofoureyes.Thisareaiscalledtheprimaryvisualcortex.Itisherethatthesignalsfromtheretinaarebeginningtobeprocessedtoproducevisualimages.
PositronEmissionTomography. Thetechniquecannottellyouhowthefunctionisbeing,e.g.,howishearingdonebythisarea;itonlytellsyouwhichareasofthebrainareinvolved.
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Now,whenthesubjectwasaskedtolistentowords(toprightimage),wecanseethatanotherareaofthecerebralcortexhasbecomeactive.Thisistheprimaryauditorycortexandislocatedonthesuperiortemporalgyrusofthetemporallobe,anditisherethattheprocessingofsignalscomingfromtheorganofCortilocatedininnerpartoftheearbeginsandwewill‘hear’sounds.Whenthesubjectwasaskedtospeakwords,anareaabovethelateralfissureinthefrontallobebecomesactive(lower,leftimage).ThisistheBroca’sarea,namedafterDr.PaulBroca,aneurologist,whodiscoveredthatpeoplewhohaddifficultyinspeakingwords,invariablyhaddamagetothisareaofthesurfaceofthecerebralcortex.However,thesamepeoplehadnodifficultyinunderstandingwhatwasbeingsaid;onlytheyhaddifficultyinspeaking.
DrPierrePaulBroca Dr.CarlWernicke (1824-1880) (1848-1905)Whenthesubjectwasaskedtothinkaboutwords,severalareasofthesubject’scerebralcortexbecomeactive(lowerrightimage).Noteinparticulartheareaintheparietalcortex.ThisiscalledtheWernicke’sareaafter,offcourse,Dr.Wernicke,aneurologist,whofoundthatpeoplewhohaddifficultyinunderstandinglanguagehaddamagetothisarea.However,theycouldspeakfluentlythoughnotrelatedtowhatwasbeingasked!
Canyouexplainhowapersonwhohasdifficultyinunderstandingthespokenwordcanstilltalkandwritefluentlybutcannotspecificallyprovideanswerstoyourquestion?
Sofromstudieslikethis,wenowknowthatdifferentareasofthebrainareinvolvedindifferentfunctionsofthenervoussystem,andalsothatmanybrainfunctionsinvolveseveraldifferentpartsofthebrain.Soinformationisbeingsentbetweendifferentareasofthebrainproducingnetworksforprocessingthesignalsandproducingabehavioraloutput.Learningtheneuronalcircuitsaddstotheeffortwehavetomaketounderstandhowyourbrainworks.Infigure1.2,theareasofthecerebralhemispheresinvolvedinlanguagefunctionareshown.Thesetwoareasforlanguagefunctioninover80%ofadultpersonsarefoundonlyintheleftcerebralhemisphere,nottheright.Hence,thelefthemisphereisoftenreferredtoasthedominanthemisphere,meaningthatitisthedominanthemisphereforlanguagefunction,notthatthelefthemisphere“dominate”therighthemisphere.In25%orsoofleft-handedpersons,thelanguagehemisphereistherightone.
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Figure1.2Theareasofthecerebralcortexthatareinvolvedinlanguagefunction.TheareainredlocatedintheparietalcortexisalsoknownastheWernicke’sarea.TheareainbluelocatedinthefrontallobeisalsocalledBroca’sarea.Theexampleofthelanguagefunctionofthenervoussystemisourfirstintroductiontotheneuronalorganisationorneuronalcircuitrythatareinvolvedincarryingoutthedifferentfunctionsofthenervoussystem.
FunctionalLocalizationoftheCortex.Thedrawingofthedifferentviewsofthecerebralcortexandcerebellumshowninfigure1.3,nicelysummarizethelocationofthedifferentfunctions.
Clinicalnote:Dysarthriaisaspeechdisordercausedbydisturbanceofmuscularcontrol.Thepersonhasnodifficultyinunderstandingandbuthasdifficultyinarticulatinghisorherwords.Dysphasiaisimpairmentoflanguagefunctionwhichcaneitherbeinspeakingmeaningfully(expressive)orunderstandinginwhatisbeingsaid(receptive).
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Figure1.3.Theareasofthecerebralcortexthatareinvolvedindifferentfunctions.Source:www.dana.orgFinally,beforeweleavetheissueofthefunctionallocalizationofbrainfunction,wemustnotforgetthattherearesubcorticalstructures,thatis,neuronalstructureswithineachcerebralhemispheres.Therearealsoneuronalstructureswithinthecerebellum,whicharecalledthedeepnuclei.Betweenthecortexandsubcorticalstructures,thereareneuronalconnectionsformingneuralnetworksthatcarrymanyofthewhatwecallthehigherbrainfunctions.Infigure1.4,asagittalMRIimageofthebrainisshown.Markedindifferentcoloursarethesubcorticalstructureandthecorticalareathattogethermakeuptherewardcircuitofthebrain.Thiscircuitistheonethatmakesusseekoutthingsthatmakeus‘feel’good,thatistheactivitywedohasarewardingfeeling.Thisisalsothecircuitthatiscurrentlyconsideredtheonethatischangedwhenapersontakesdrugsofabuse,e.g.,cocaine,amphetaminederivatives(MDMA,ecstasy),khat,alcohol,syntheticcathinones,amongothers.
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Figure1.4.Thecorticalandsubcorticalrewardnetwork.(Sara B Taylor, Candace R Lewis and M Foster Olive. The neurocircuitry of illicit psychostimulant addiction: acute and chronic effects in humans. Substance Abuse and Rehabilitation 2013:4 29–43) 1.4NeuronalPathwayforSomaticSensory,SomaticMotorandPainSensation.1.4.1SensorySystemHowdowe“know”thatwehavetouchedanobject,orwhensomeoneorsomethingtouchesus?Howdoweknowwhichpartofourbodytouchedtheobjectorwhereonourbodysurfaceweweretouched?Thereisaneuronalsystemcalledthesomaticsensorysystemthatfunctionstogiveusthe‘ability’ofknowingthatastimulushasbeenappliedtoourskin,andwhattypeofstimulusitis,e.g.,lighttouch,pressure,tickle,etc,andwhereonthebody.Infigure1.5,thepathwaythenervesignaltravelsfromthereceptorinthefingertothebrainisshown.Whenwetouchanobjectwithourfingertip,receptorsinthefingertipareactivated.Theyproduceanervesignal.Inlaterlectures,wewilllearnhowthenervesignalisproduced.Butfornow,letustakeitthatasignalhasbeenproduced.Thissignaltravelsalongthenervethatentersthespinalcordonitsdorsalside.(Rememberthatallsensoryinputtothespinalcordenterthroughthespinaldorsalroots).Fromherethesignaltravelsupthespinalcordinnervesthatformthedorsalcolumnsofthespinalcordwhitematter.Onreachingthetopof
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thespinalcord,thenervescarryingthesignalmakeconnectionswithothernervecellslocatedinthedorsalcolumnnuclei.
Figure1.5.Theneuronalpathwayofthesensorysystem.Inthisfigure,spinalcordsectionsareshowninthehorizontalplane,andthecerebralcortexinacoronalsection.Noticethenervefibersfromthedorsalcolumnnucleicrossthespinalmidlineandgoupintothebrainontheoppositeside.Nowsomethingunusualhappensbutwhich,asyouwilllearn,isquitecommoninthenervoussystem.ThenervecellsofthedorsalcolumnnucleiextendnervesthatcrossthemidlineoftheCNSandcontinueuptothethalamuscreatinganervepathwaycalledthemediallemniscus.Thesignalhascrossedfromonesideofthebodytotheother.Fromthethalamus,thesignalstravelalongthethalamicnervestothenervecellsinthecortexofthecerebralhemisphere,whichiscalledtheprimarysensorycortex.
Thewordnucleus(plural:nuclei)isusedtodescribedifferentstructures.ItisusedtodescribeastructureinthecellthatcontainstheDNA.Inneuroanatomy,thewordisusedtodescribeacollectionofnervecells.
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Asyoucanseeinfigure1.4,theprimarysensorycortexislocatedonthegyrusjustposteriortothecentralsulcus.Itisthisareathatinformsyouthatyouhavetouchedanobjectwithyourfingertip.Youcanalso“see”withyoursensorysystem.Saythatwehavecoinsandkeysinourpocket
andwewanttotakeoutsomecoins.Wecanputourhandinourpocketandfeeltheobjects
andwithouthavingtolookatthemwithoureyes,wewilltakeoutthecoinsandleavethekeys.Byfeelingtheobjects,wecreateanimageofthembytouch.
Closeyoureyesandhaveafriendorcolleague,placeapenonyourfingertips.Canyoutellitispen?Nowrollyourfingersaroundthepen.Canyounowtellthatitisapen?
Showonthepictureoftheleftcerebralhemisphere,thelocationoftheprimarysensorycortex.
Whatthefunctionalimportanceofnervecrossingoverfromonesideofthebodytotheotherisnotknown.Whydon’tthenervefibersgoupthesamesideofthenervoussystem?
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Figure1.6.Thisfigureshowstheareaofthecerebralcortexthatreceivesthesensoryinputfromthesurfaceofyourbody.Itisposteriortothecentralsulcus.Sonowwehavelearntthepartsofournervoussystemandthenervepathwaysthatcarryoutthefunctionofsensation.Torecap,thepathwayinvolves3nervecells:First,thenerveconnectionfromtheskintothespinalcordbynervecellslocatedinthedorsalrootganglia,secondconnectionbythenervecellsofthedorsalcolumnnucleitothethalamus,andthird,bythenervecellsofthethalamustonervescellsintheprimarysensorycortex.Byknowingthisneuralpathway,wealsolearntsomethingamazing.Thesignalsfromonesideofourbodyaresenttotheoppositecerebralhemisphere.Sosensationsignalscomingfromtherightsideofthebodyaresenttotheleftcerebralhemisphereandviceversa.1.4.2SomaticMotorSystemThenextnervoussystemfunctionthatwewilllookatisourabilitytocarryoutmovementwhetheritisvoluntaryornot.Therangeandvarietyofmovementthatweareabletodoisverylarge.Forexample,considerthecomplexityofmovementrequiredforplayingaguitarordribblingafootballwithourfoot.Movementiseverythingtous,withoutitwecandonothing.Therearemanypartsofthenervoussystemandpathwaysinvolvedinmovementfunction.Inthissection,wewillonlydiscussonepathwayoutofthefivethatareinvolvedinmotorfunction.Inlaterlectures,wewilllookinmoredetailsatthepartsandpathwaysofournervoussystemthatprovideuswiththeabilityofmovement.Unlikethesomaticsensorysystem,wewillstartfromthecortexofthecerebralhemisphereandworkourwaydowntothemuscles.Lookingbackatfigure1.6,wecanseethatthereisanareajustinfrontofthecentralsulcusofthecerebralhemisphere,whichiscalledmotorcortex.Nervecellsinthisareasendoutnervefibersthattravelthroughthecerebralhemispheresandenterthebrainstem.Thesenervesformastructureinsidethecerebralhemispherescalledtheinternalcapsule.
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Figure1.7.Showsoneofthefivemotorpathway.Itstartsfromanareaofthecortexofthecerebralhemispherejustinfrontofthecentralsulcusandgoesallthewaytothemuscles(corticospinaltract).Whenthenervesenterthebrainstem,thenervepathwayformediscalledthebasispedunli.Itisontheventralpartofthebrainstem.Whenthenervesarriveatthemedulla,theyformastructurecalledthepyramidsandstarttocrossthemidlineoftheCNS.Thiscrossingiscalledthepyramidaldecussation.Inthespinalcord,thesenervestraveldownthewhitematterofthespinalcordmakingthelateralcorticospinaltract.Allalongthespinalcord,thenervesleavethelateralcorticospinaltractandenterthespinalcordgraymatter.Inthespinalcord
Nomenclature:Thereisamethodfornamingnervefibertracts.Thefirstpartofthewordgivesthelocationofthenervecellsandthesecondparttheterminationpointofthenervefibers.Sothecorticospinaltracthasnervecellsinthecerebralcortex(cortico-)andthenervefibersthatendinthespinalcord(-spinal).Ifthenervecellbodieswereinthespinalcordandthenervefibresendedinthecortexthenthenervefibertractwouldbecalledthespinocortical.
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graymatter,thenervesmakeaconnectionwithnervecells,calledthealphamotorneurons,whichsendnervesfibersoutfromtheventralrootofthethespinalcord.Thesemotornervefiberstravelthroughoutthebodyandmakeconnectionwithstriatedmusclecells.
Sowhenwewanttomakeamovement,weproducesignalsinthenervecellsoftheprimarymotorcortexandthesearesentalongnervepathwaystothemusclestomakethemcontract.Andtoremindourselves,thisisnottheonlypathwayinvolvedinourabilitytocarryoutmovement.Thereare4otherpathwaysandwewilldiscusstheseinthelecturesonlecturesonthemotorsystem.1.4.3PainPathwaysFinally,anoverviewofaspecialsensorysystemthatservestoprotectourbodyfromstimulusthatcancauseustissueinjury.Thisiscalledthepainornociceptivepathway.Notethatpainissubjectivefeelingproducedbyourbrain.Nociception,fromtheLatin,nicer,‘toharmorhurt’,istheprocessingofharmfulstimuli.Nociceptionmaynotresultinpain.Forexample,whenweneedtohaveasurgicalprocedurewearegivenananestheticchemicalthatblockspainnervesignalsreachingourbrain.Sothoughourpainreceptorsareproducingsignals,thesedonotreachourbrainsowedonotfeelthesensationofpain.Sothoughtthereisnociceptiveactivity,thereisnopain.Wehavereceptorsonourskinthatrespondonlytostrongstimulusorwhentheskinhasbeendamaged.Thesereceptorsaredifferentfromthereceptorsforthesomaticsensorysystem;theydonotreacttolighttouchbutif,forexample,astrongpressureorhighorverylowtemperatureisapplied,thisissensedasapotentialnociceptivestimulusandtheappropriateprotectiveactiontakentopreventinjury.Wearefamiliarwiththereactionweproducewhenweatouchhotobject;wequicklyremoveourhand.Thisiscalledthewithdrawalreflexandservestomoveourhandrapidlyawayfromthenociceptivestimulustopreventtheskinonourhandbeingburnt.Infigure1.8,wecanseethatthenervefibersfromthereceptorsenterthroughthedorsalsideofthespinalcord.(Rememberallsignalscomingintothespinalcordcomethroughthespinaldorsalside.)Thesenervefibersmakeconnectionswithnervecellsinthedorsalhornofspinalcordgraymatter.Fromherethenervefibersfromthesenervecellsofthedorsalhornspinal
Clinicalnote:Thecorticospinaltractisclinicallyimportant,asitistheonlypathwaythroughwhichyoucancarryoutvoluntarymovement.Damagetothisfibertractresultsinparalysis,whichislossofvoluntarycontroloftheskeletalmuscles.Whenapersonhasastrokeinonecerebralhemisphere,thekeyclinicalfeatureislossofvoluntarymotorcontrolontheoppositesideofthebody.
Theinnervationofthesmoothandcardiacmusclesandtheirstateofcontractionorrelaxationiscontrolledbytheautonomicnervoussystem.
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graymattermoveupthespinalcordforafewsegmentsbeforecrossingtheCNSmidlinetotheotherside.
Figure1.8.Cartoonshowingthepathwayforthepainsignalsfromthefoot.Notetheinthespinalcord,thesecondordernervecellscrosstheCNSmidlineandtravelupthespinalcordintheoppositeside.Thenervefibersnowclimbupthespinalcordallthewaytothecerebralhemispheresendingonthenervecellsofthethalamus.Thepathwayiscalledthespinothalamicpathway.Thenervecellsofthethalamussendnervefiberstotheprimarysensorycortex(providinglocationofthestimuli),thecingulategyrus(providingemotionalcontent)andinsularcortex(producingbehaviorassociatedwithpain).BothinthespinalcordandcerebralcortexthepainfulinformationissenttothemotorpartoftheCNSsowecanproducequickmotorresponseandavoidbeinginjured.1.5SummarySointhislecturewehavelookedatthepartsofthenervoussystemthatareinvolvedinsomeofthedifferentfunctionsofthenervoussystem.Itisimportantthatweknowthesepathwayssoifweseeapersonshowsdifficultyinsensing,movementorpainresponse,wecanworkwherethedamagehastakenplaceinthenervoussystem.Inotherlecturesonthenervoussystem,wewilllearnabouttheneuronalcircuits(pathways)thatareinvolvedinthefunctionofvision,audition,olfaction,gustation,memoryandlearning,andemotionandmotivationaswellasoursleep/wakecycle.
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1.6Activities1.7FurtherReading1.8SampleExaminationQuestionsMultipleChoiceQuestions(MCQs).Selectthebestonecorrectanswer.1) Theareaofthecerebralhemispherethatisinvolvedinunderstandingspeechislocatedon
thea) Frontallobeb) Parietallobec) Temporallobed) Occipitallobee) Insularlobe
2) Theareaofthecerebralhemispherethatisinvolvedinproducingspeechislocatedonthe
a) Frontallobeb) Parietallobec) Temporallobed) Occipitallobee) Insularlobe
3) Whichareaofthecerebralcortexislikelytobemostactivewhenyouarelookingat
words?a) Primarymotorcortexb) Primarysensorycortexc) Primaryauditoryaread) Primaryvisualcortexe) OrganofCorti
4) Ifapersonisright-handed,whatistheprobabilitythatishisorherlefthemisphere
controlsthelanguagefunction?a) >80%b) 60-80%c) 40-60%d) 20-40%e) <20%
5) Theprimarysensorycortexislocated
a) Anteriortothecentralsulcusb) Posteriortothecentralsulcusc) Dorsaltothecentralsulcusd) Ventraltothecentralsulcuse) Noneoftheabove
6) Atwhichlevelofthenervoussystemdothenervesignalinthesomaticsensorysystem
comingfromtherightsideofthebodycrossovertotheleftsideofthenervoussystem?a) Spinalcordlevelb) Dorsalcolumnslevelc) Dorsalcolumnnucleilevel
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d) Thalamiclevele) Thereisnocrossingover
7) Thenervesignalscomingfromtheprimarymotorcortexwillcrossthenervoussystem
midlineatwhichlevel?a) Cerebralcortexlevelb) Diencephalonlevelc) Ponsleveld) Medullalevele) Spinallevel
ShortAnswerQuestions(SAQs).Theanswertothequestionrequires5keypoints.1) Forthefollowingstatements,fillintheblanks:
a) Thenervecellsonwhichthenerveendingsofthelateralcorticospinaltractmakesynapticconnectionsarecalled_____________________________.
b) Thecrossingofthelateralcorticospinaltractasthenervefibersdescendfromthe
primarymotorcortexiscalled_____________________________.
c) Thenervepathwaysthatcarrynociceptiveinformationiscallthe______________________.
d) Damagetothe_______________________areaofsurfacecerebralhemispherewouldmakeitdifficultytounderstandwhatisbeingsaid.
e) Damagetothe_______________________areaofsurfacecerebralhemispherewouldmakeit
difficultytoproducemeaningfulspeech.2) Givesomeexamplesoftheusefulnessofknowingthefunctionalorganizationofthe
nervoussystem.3) Thebrainimagingmethodofpositronemissiontomographyisusefulinlearningwhat
aboutthebrain.4) Drawalabelleddiagramofthesomaticsensorysystem.5) Drawalabelleddiagramofthesomaticmotorsystem.6) Drawalabelleddiagramofthenociceptivepathway.7) Whatisthedifferencebetweenthetermsnociceptionandpain?