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IntroductiontotheNervousSystem. Code:HMP100/UPC103/VNP100.Course:MedicalPhysiology Level1MBChB/BDS/BPharm Lecture2.FunctionalOrganisationoftheNervousSystem LectureOutline 1.1Introduction 1.2Learningoutcomes 1.3AreasoftheCerebralHemispheresinvolvedinLanguageFunction 1.4NeuronalPathwayfor 1.4.1SomaticSensory 1.4.2SomaticMotor 1.4.3PainSensation. 1.5Summary 1.6Activity 1.7FurtherReading 1.8SampleExaminationQuestions 1.1Introduction Inthelastlecture,wecoveredthebasicanatomyofthenervoussystem.Wesawthatitis dividedintomanydivisionsandparts.Nowinthislecture,wewillcoversomefunctional organisationofthenervoussystem.Youknowthatthenervoussystemcarriesoutmany differentfunctionsunlikeotherphysiologicalsystemsofourbody.Todothesemanydifferent functions,thenervoussystemhasfunctionaldivisionsinvolvingdifferentpartofthenervous system.Andthisaddstothedifficultyinstudyinghowthenervoussystemworks.Inthis lecture,wewillcoverthefunctionalorganizationof4functions:language,sensory,motor,and pain.Wewillnotcoverthemindetails;thiswillbedoneinlaterlectures.Herewewantto beingtounderstandthefunctionalorganizationofthenervoussystem. Learningwhichpartsofthenervoussystemareinvolvedincarryingoutthedifferent functionsisimportant.Thiswillhelpuspredictthetypeofdysfunctionapersonwillhave whenthereisdamagetoaparticularpartoftheirnervoussystem.Andifweknoworobserve thesymptomsintheperson,wecanpredictwhereinthenervoussystemthedamagehas 1.2LearningOutcomes Attheendofthislecture,youshouldbeableto: 1. StatethefunctionoftheNervousSystem 2. Nameandshowthelocationoftheareasofthecerebral hemisphereinvolvedinlanguagefunction. 3. Describewithadiagramthesomaticsensorysystem. 4. Describewithadiagramthesomaticmotorsystem. 5. Describewithadiagramthepain(nociceptive)system. occurred. 1 1.3AreasontheCerebralHemisphereinvolvedLanguageFunction Inrecentyears,techniquesandmethodshavebeendevelopedthatallowustostudythe activityofbrainwhenitisinvolvedinaparticulartask.Withthesetechniques,weaska persontodoaparticulartaskandseewhichareasofthecerebralhemispheresareactive whenthattaskisbeingdone.Fromthesestudies,wecanconcludethattheseareasare involvedinthatfunction.Ofcourseitdoesnottellushowthesepartsofcerebralhemispheres arecarryingoutthatfunction. Figure1.1showstheresultsofastudyonbrainactivitywhenthepersonwasaskedtodo4 differenttasks:(1)lookatwords,(2)listentowords,(3)speakwords,and(4)thinkofwords. Theimagingisofthelateralsurfaceoftheleftcerebralhemisphere.Theareasonthesurface showingredandyellowcolorareareasthatbecameactivewhenthepersonwasdoingeachof PositronEmissionTomography. Thetechniquecannottellyouhowthefunctionisbeing,e.g.,howis hearingdonebythisarea;itonlytellsyouwhichareasofthebrain areinvolved. thedifferenttasks. Figure1.1Imagingofthesurfaceoftheleftcerebralhemispherewhenapersonwasaskedto (1)lookatwords,(2)listentowords,(3)speakwords,and(4)thinkofwords.Theareasin red/yellowshowtheareaswhichbecameactiveforthedifferenttasks.Thefrontpartofthe cerebralhemisphereistowardstheleft. Whenthepersonwasaskedtolookatwords(topleftimage),wecanseethattheiroccipital lobebecamemoreactivethanotherpartsofthebrain.Inlaterlectures,youwilllearnthatthis partofthesurfaceofthecerebralcortexwithotherpartsisinvolvedinproducingimages fromthesignalsthatcomefromtheretinaofoureyes.Thisareaiscalledtheprimaryvisual cortex.Itisherethatthesignalsfromtheretinaarebeginningtobeprocessedtoproduce visualimages. 2 Now,whenthesubjectwasaskedtolistentowords(toprightimage),wecanseethatanother areaofthecerebralcortexhasbecomeactive.Thisistheprimaryauditorycortexandis locatedonthesuperiortemporalgyrusofthetemporallobe,anditisherethattheprocessing ofsignalscomingfromtheorganofCortilocatedininnerpartoftheearbeginsandwewill ‘hear’sounds. Whenthesubjectwasaskedtospeakwords,anareaabovethelateralfissureinthefrontal lobebecomesactive(lower,leftimage).ThisistheBroca’sarea,namedafterDr.PaulBroca, aneurologist,whodiscoveredthatpeoplewhohaddifficultyinspeakingwords,invariably haddamagetothisareaofthesurfaceofthecerebralcortex.However,thesamepeoplehad nodifficultyinunderstandingwhatwasbeingsaid;onlytheyhaddifficultyinspeaking. DrPierrePaulBroca Dr.CarlWernicke (1824-1880) (1848-1905) Whenthesubjectwasaskedtothinkaboutwords,severalareasofthesubject’scerebral cortexbecomeactive(lowerrightimage).Noteinparticulartheareaintheparietalcortex. ThisiscalledtheWernicke’sareaafter,offcourse,Dr.Wernicke,aneurologist,whofound thatpeoplewhohaddifficultyinunderstandinglanguagehaddamagetothisarea.However, theycouldspeakfluentlythoughnotrelatedtowhatwasbeingasked! Canyouexplainhowapersonwhohasdifficultyinunderstandingthe spokenwordcanstilltalkandwritefluentlybutcannotspecificallyprovide answerstoyourquestion? Sofromstudieslikethis,wenowknowthatdifferentareasofthebrainareinvolvedin differentfunctionsofthenervoussystem,andalsothatmanybrainfunctionsinvolveseveral differentpartsofthebrain.Soinformationisbeingsentbetweendifferentareasofthebrain producingnetworksforprocessingthesignalsandproducingabehavioraloutput.Learning theneuronalcircuitsaddstotheeffortwehavetomaketounderstandhowyourbrainworks. Infigure1.2,theareasofthecerebralhemispheresinvolvedinlanguagefunctionareshown. Thesetwoareasforlanguagefunctioninover80%ofadultpersonsarefoundonlyintheleft cerebralhemisphere,nottheright.Hence,thelefthemisphereisoftenreferredtoasthe dominanthemisphere,meaningthatitisthedominanthemisphereforlanguagefunction,not thatthelefthemisphere“dominate”therighthemisphere.In25%orsoofleft-handed persons,thelanguagehemisphereistherightone. 3 Figure1.2Theareasofthecerebralcortexthatareinvolvedinlanguagefunction.Theareain redlocatedintheparietalcortexisalsoknownastheWernicke’sarea.Theareainblue locatedinthefrontallobeisalsocalledBroca’sarea. Theexampleofthelanguagefunctionofthenervoussystemisourfirstintroductiontothe neuronalorganisationorneuronalcircuitrythatareinvolvedincarryingoutthedifferent functionsofthenervoussystem. Clinicalnote:Dysarthriaisaspeechdisordercausedbydisturbanceof muscularcontrol.Thepersonhasnodifficultyinunderstandingandbuthas difficultyinarticulatinghisorherwords.Dysphasiaisimpairmentof languagefunctionwhichcaneitherbeinspeakingmeaningfully(expressive) orunderstandinginwhatisbeingsaid(receptive). 1.4NeuronalPathwayforSomaticSensory,SomaticMotorandPainSensation. 1.4.1SensorySystem Howdowe“know”thatwehavetouchedanobject,orwhensomeoneorsomethingtouches us?Howdoweknowwhichpartofourbodytouchedtheobjectorwhereonourbodysurface weweretouched?Thereisaneuronalsystemcalledthesomaticsensorysystemthat functionstogiveusthe‘ability’ofknowingthatastimulushasbeenappliedtoourskin,and whattypeofstimulusitis,e.g.,lighttouch,pressure,tickle,etc,andwhereonthebody. Infigure1.3,thepathwaythenervesignaltravelsfromthereceptorinthefingertothebrain isshown.Whenwetouchanobjectwithourfingertip,receptorsinthefingertipareactivated. Theyproduceanervesignal.Inlaterlectures,wewilllearnhowthenervesignalisproduced. Butfornow,letustakeitthatasignalhasbeenproduced.Thissignaltravelsalongthenerve thatentersthespinalcordonitsdorsalside.(Rememberthatallsensoryinputtothespinal cordenterthroughthespinaldorsalroots).Fromherethesignaltravelsupthespinalcordin nervesthatformthedorsalcolumnsofthespinalcordwhitematter.Onreachingthetopof 4 thespinalcord,thenervescarryingthesignalmakeconnectionswithothernervecellslocated inthedorsalcolumnnuclei. Figure1.3Theneuronalpathwayofthesensorysystem.Inthisfigure,spinalcordsectionsare showninthehorizontalplane,andthecerebralcortexinacoronalsection.Noticethenerve fibersfromthedorsalcolumnnucleicrossthespinalmidlineandgoupintothebrainonthe oppositeside. Nowsomethingunusualhappensbutwhich,asyouwilllearn,isquitecommoninthenervous system.Thenervecellsofthedorsalcolumnnucleiextendnervesthatcrossthemidlineofthe CNSandcontinueuptothethalamuscreatinganervepathwaycalledthemediallemniscus. Thesignalhascrossedfromonesideofthebodytotheother.Fromthethalamus,thesignals travelalongthethalamicnervestothenervecellsinthecortexofthecerebralhemisphere, whichiscalledtheprimarysensorycortex. Thewordnucleus(plural:nuclei)isusedtodescribedifferentstructures.It isusedtodescribeastructureinthecellthatcontainstheDNA.In neuroanatomy,thewordisusedtodescribeacollectionofnervecells. 5 Whatthefunctionalimportanceofnervecrossingoverfromonesideofthe bodytotheotherisnotknown.Whydon’tthenervefibersgoupthesame sideofthenervoussystem? Asyoucanseeinfigure1.4,theprimarysensorycortexislocatedonthegyrusjustposterior tothecentralsulcus.Itisthisareathatinformsyouthatyouhavetouchedanobjectwithyour fingertip. Youcanalso“see”withyoursensorysystem.Saythatwehavecoinsandkeysinourpocket Closeyoureyesandhaveafriendorcolleague,placeapenonyour fingertips.Canyoutellitispen?Nowrollyourfingersaroundthepen.Can younowtellthatitisapen? andwewanttotakeoutsomecoins.Wecanputourhandinourpocketandfeeltheobjects Showonthepictureoftheleftcerebralhemisphere,thelocationofthe primarysensorycortex. andwithouthavingtolookatthemwithoureyes,wewilltakeoutthecoinsandleavethe keys.Byfeelingtheobjects,wecreateanimageofthembytouch. 6 Figure1.4Thisfigureshowstheareaofthecerebralcortexthatreceivesthesensoryinput fromthesurfaceofyourbody.Itisposteriortothecentralsulcus. Sonowwehavelearntthepartsofournervoussystemandthenervepathwaysthatcarryout thefunctionofsensation.Torecap,thepathwayinvolves3nervecells:First,thenerve connectionfromtheskintothespinalcordbynervecellslocatedinthedorsalrootganglia, secondconnectionbythenervecellsofthedorsalcolumnnucleitothethalamus,andthird,by thenervecellsofthethalamustonervescellsintheprimarysensorycortex. Byknowingthisneuralpathway,wealsolearntsomethingamazing.Thesignalsfromoneside ofourbodyaresenttotheoppositecerebralhemisphere.Sosensationsignalscomingfrom therightsideofthebodyaresenttotheleftcerebralhemisphereandviceversa. 1.4.2SomaticMotorSystem Thenextnervoussystemfunctionthatwewilllookatisourabilitytocarryoutmovement whetheritisvoluntaryornot.Therangeandvarietyofmovementthatweareabletodois verylarge.Forexample,considerthecomplexityofmovementrequiredforplayingaguitaror dribblingafootballwithourfoot.Movementiseverythingtous,withoutitwecandonothing. Therearemanypartsofthenervoussystemandpathwaysinvolvedinmovementfunction.In thissection,wewillonlydiscussonepathwayoutofthefivethatareinvolvedinmotor function.Inlaterlectures,wewilllookinmoredetailsatthepartsandpathwaysofour nervoussystemthatprovideuswiththeabilityofmovement. Unlikethesomaticsensorysystem,wewillstartfromthecortexofthecerebralhemisphere andworkourwaydowntothemuscles.Lookingbackatfigure1.4,wecanseethatthereisan areajustinfrontofthecentralsulcusofthecerebralhemisphere,whichiscalledmotor cortex.Nervecellsinthisareasendoutnervefibersthattravelthroughthecerebral hemispheresandenterthebrainstem.Thesenervesformastructureinsidethecerebral hemispherescalledtheinternalcapsule. 7 Figure1.5.Showsoneofthefivemotorpathway.Itstartsfromanareaofthecortexofthe cerebralhemispherejustinfrontofthecentralsulcusandgoesallthewaytothemuscles (corticospinaltract). Whenthenervesenterthebrainstem,thenervepathwayformediscalledthebasispedunli. Itisontheventralpartofthebrainstem.Whenthenervesreachesthemedulla,theyforma structurecalledthepyramidsandstarttocrossthemidlineoftheCNS.Thiscrossingiscalled thepyramidaldecussation.Inthespinalcord,thesenervestraveldownthewhitematterof thespinalcordmakingthelateralcorticospinaltract.Allalongthespinalcord,thenerves leavethelateralcorticospinaltractandenterthespinalcordgraymatter.Inthespinalcord Nomenclature:Thereisamethodfornamingnervefibertracts.Thefirst partofthewordgivesthelocationofthenervecellsandthesecondpartthe terminationpointofthenervefibers.Sothecorticospinaltracthasnerve cellsinthecerebralcortex(cortico-)andthenervefibersthatendinthe spinalcord(-spinal).Ifthenervecellbodieswereinthespinalcordandthe nervefibresendedinthecortexthenthenervefibertractwouldbecalled thespinocortical. 8 Clinicalnote:Thecorticospinaltractisclinicallyimportant,asitistheonly pathwaythroughwhichyoucancarryoutvoluntarymovement.Damageto thisfibertractresultsinparalysis,whichislossofvoluntarycontrolofthe skeletalmuscles.Whenapersonhasastrokeinonecerebralhemisphere, thekeyclinicalfeatureislossofvoluntarymotorcontrolontheopposite sideofthebody. graymatter,thenervesmakeaconnectionwithnervecells,calledthealphamotorneurons, whichsendnervesfibersoutfromtheventralrootofthethespinalcord.Thesemotornerve fiberstravelthroughoutthebodyandmakeconnectionwithstriatedmusclecells. Theinnervationofthesmoothandcardiacmusclesandtheirstateof contractionorrelaxationiscontrolledbytheautonomicnervoussystem. Sowhenwewanttomakeamovement,weproducesignalsinthenervecellsoftheprimary motorcortexandthesearesentalongnervepathwaystothemusclestomakethemcontract. Andtoremindourselves,thisisnottheonlypathwayinvolvedinourabilitytocarryout movement.Thereare4otherpathwaysandwewilldiscusstheseinthelecturesonlectures onthemotorsystem. 1.4.3PainPathways Finally,anoverviewofaspecialsensorysystemthatservestoprotectourbodyfromstimulus thatcancauseustissueinjury.Thisiscalledthepainornociceptivepathway.Notethatpainis subjectivefeelingproducedbyourbrain.Nociception,fromtheLatin,nicer,‘toharmorhurt’, istheprocessingofharmfulstimuli.Nociceptionmaynotresultinpain.Forexample,when weneedtohaveasurgicalprocedurewearegivenananestheticchemicalthatblockspain nervesignalsreachingourbrain.Sothoughourpainreceptorsareproducingsignals,thesedo notreachourbrainsowedonotfeelthesensationofpain.Sothoughtthereisnociceptive activity,thereisnopain. Wehavereceptorsonourskinthatrespondonlytostrongstimulusorwhentheskinhas beendamaged.Thesereceptorsaredifferentfromthereceptorsforthesomaticsensory system;theydonotreacttolighttouchbutif,forexample,astrongpressureorhighorvery lowtemperatureisapplied,thisissensedasapotentialnociceptivestimulusandthe appropriateprotectiveactiontakentopreventinjury.Wearefamiliarwiththereactionwe producewhenweatouchhotobject;wequicklyremoveourhand.Thisiscalledthe withdrawalreflexandservestomoveourhandrapidlyawayfromthenociceptivestimulusto preventtheskinonourhandbeingburnt. Infigure1.6,wecanseethatthenervefibersfromthereceptorsenterthroughthedorsalside ofthespinalcord.(Rememberallsignalscomingintothespinalcordcomethroughthespinal dorsalside.)Thesenervefibersmakeconnectionswithnervecellsinthedorsalhornofspinal cordgraymatter.Fromherethenervefibersfromthesenervecellsofthedorsalhornspinal 9 graymattermoveupthespinalcordforafewsegmentsbeforecrossingtheCNSmidlineto theotherside. Figure1.6.Cartoonshowingthepathwayforthepainsignalsfromthefoot.Notetheinthe spinalcord,thesecondordernervecellscrosstheCNSmidlineandtravelupthespinalcord intheoppositeside. Thenervefibersnowclimbupthespinalcordallthewaytothecerebralhemispheresending onthenervecellsofthethalamus.Thepathwayiscalledthespinothalamicpathway.The nervecellsofthethalamussendnervefiberstotheprimarysensorycortex(providing locationofthestimuli),thecingulategyrus(providingemotionalcontent)andinsular cortex(producingbehaviorassociatedwithpain). Bothinthespinalcordandcerebralcortexthepainfulinformationissenttothemotorpartof theCNSsowecanproducequickmotorresponseandavoidbeinginjured. 1.5Summary Sointhislecturewehavelookedatthepartsofthenervoussystemthatareinvolvedinsome ofthedifferentfunctionsofthenervoussystem.Itisimportantthatweknowthesepathways soifweseeapersonshowsdifficultyinsensing,movementorpainresponse,wecanwork wherethedamagehastakenplaceinthenervoussystem. Inotherlecturesonthenervoussystem,wewilllearnabouttheneuronalcircuits(pathways) thatareinvolvedinthefunctionofvision,audition,olfaction,gustation,memoryandlearning, andemotionandmotivationaswellasoursleep/wakecycle. 10 1.6Activities 1.7FurtherReading 1.8SampleExaminationQuestions MultipleChoiceQuestions(MCQs).Selectthebestonecorrectanswer. 1) Theareaofthecerebralhemispherethatisinvolvedinunderstandingspeechislocatedon the a) Frontallobe b) Parietallobe c) Temporallobe d) Occipitallobe e) Insularlobe 2) Theareaofthecerebralhemispherethatisinvolvedinproducingspeechislocatedonthe a) Frontallobe b) Parietallobe c) Temporallobe d) Occipitallobe e) Insularlobe 3) Whichareaofthecerebralcortexislikelytobemostactivewhenyouarelookingat words? a) Primarymotorcortex b) Primarysensorycortex c) Primaryauditoryarea d) Primaryvisualcortex e) OrganofCorti 4) Ifapersonisright-handed,whatistheprobabilitythatishisorherlefthemisphere controlsthelanguagefunction? a) >80% b) 60-80% c) 40-60% d) 20-40% e) <20% 5) Theprimarysensorycortexislocated a) Anteriortothecentralsulcus b) Posteriortothecentralsulcus c) Dorsaltothecentralsulcus d) Ventraltothecentralsulcus e) Noneoftheabove 6) Atwhichlevelofthenervoussystemdothenervesignalinthesomaticsensorysystem comingfromtherightsideofthebodycrossovertotheleftsideofthenervoussystem? a) Spinalcordlevel b) Dorsalcolumnslevel c) Dorsalcolumnnucleilevel 11 d) Thalamiclevel e) Thereisnocrossingover 7) Thenervesignalscomingfromtheprimarymotorcortexwillcrossthenervoussystem midlineatwhichlevel? a) Cerebralcortexlevel b) Diencephalonlevel c) Ponslevel d) Medullalevel e) Spinallevel ShortAnswerQuestions(SAQs).Theanswertothequestionrequires5keypoints. 1) Forthefollowingstatements,fillintheblanks: a) Thenervecellsonwhichthenerveendingsofthelateralcorticospinaltractmake synapticconnectionsarecalled_____________________________. b) Thecrossingofthelateralcorticospinaltractasthenervefibersdescendfromthe primarymotorcortexiscalled_____________________________. c) Thenervepathwaysthatcarrynociceptiveinformationiscallthe______________________. d) Damagetothe_______________________areaofsurfacecerebralhemispherewouldmakeit difficultytounderstandwhatisbeingsaid. e) Damagetothe_______________________areaofsurfacecerebralhemispherewouldmakeit difficultytoproducemeaningfulspeech. 2) Givesomeexamplesoftheusefulnessofknowingthefunctionalorganizationofthe nervoussystem. 3) Thebrainimagingmethodofpositronemissiontomographyisusefulinlearningwhat aboutthebrain. 4) Drawalabelleddiagramofthesomaticsensorysystem. 5) Drawalabelleddiagramofthesomaticmotorsystem. 6) Drawalabelleddiagramofthenociceptivepathway. 7) Whatisthedifferencebetweenthetermsnociceptionandpain? 12