introduction to the nervous system....organisation of the nervous system. you know that the nervous...

1

IntroductiontotheNervousSystem.Code:HMP100/UPC103/VNP100.Course:MedicalPhysiologyLevel1MBChB/BDS/BPharmLecture2.FunctionalOrganisationoftheNervousSystemLectureOutline1.1Introduction1.2Learningoutcomes1.3AreasoftheCerebralHemispheresinvolvedinLanguageFunction1.4NeuronalPathwayfor 1.4.1SomaticSensory 1.4.2SomaticMotor 1.4.3PainSensation.1.5Summary1.6Activity1.7FurtherReading1.8SampleExaminationQuestions1.1IntroductionInthelastlecture,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.

2

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.

3

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.

4

Figure1.2Theareasofthecerebralcortexthatareinvolvedinlanguagefunction.TheareainredlocatedintheparietalcortexisalsoknownastheWernicke’sarea.TheareainbluelocatedinthefrontallobeisalsocalledBroca’sarea.Theexampleofthelanguagefunctionofthenervoussystemisourfirstintroductiontotheneuronalorganisationorneuronalcircuitrythatareinvolvedincarryingoutthedifferentfunctionsofthenervoussystem.

1.4NeuronalPathwayforSomaticSensory,SomaticMotorandPainSensation.1.4.1SensorySystemHowdowe“know”thatwehavetouchedanobject,orwhensomeoneorsomethingtouchesus?Howdoweknowwhichpartofourbodytouchedtheobjectorwhereonourbodysurfaceweweretouched?Thereisaneuronalsystemcalledthesomaticsensorysystemthatfunctionstogiveusthe‘ability’ofknowingthatastimulushasbeenappliedtoourskin,andwhattypeofstimulusitis,e.g.,lighttouch,pressure,tickle,etc,andwhereonthebody.Infigure1.3,thepathwaythenervesignaltravelsfromthereceptorinthefingertothebrainisshown.Whenwetouchanobjectwithourfingertip,receptorsinthefingertipareactivated.Theyproduceanervesignal.Inlaterlectures,wewilllearnhowthenervesignalisproduced.Butfornow,letustakeitthatasignalhasbeenproduced.Thissignaltravelsalongthenervethatentersthespinalcordonitsdorsalside.(Rememberthatallsensoryinputtothespinalcordenterthroughthespinaldorsalroots).Fromherethesignaltravelsupthespinalcordinnervesthatformthedorsalcolumnsofthespinalcordwhitematter.Onreachingthetopof

Clinicalnote:Dysarthriaisaspeechdisordercausedbydisturbanceofmuscularcontrol.Thepersonhasnodifficultyinunderstandingandbuthasdifficultyinarticulatinghisorherwords.Dysphasiaisimpairmentoflanguagefunctionwhichcaneitherbeinspeakingmeaningfully(expressive)orunderstandinginwhatisbeingsaid(receptive).

5

thespinalcord,thenervescarryingthesignalmakeconnectionswithothernervecellslocatedinthedorsalcolumnnuclei.

Figure1.3Theneuronalpathwayofthesensorysystem.Inthisfigure,spinalcordsectionsareshowninthehorizontalplane,andthecerebralcortexinacoronalsection.Noticethenervefibersfromthedorsalcolumnnucleicrossthespinalmidlineandgoupintothebrainontheoppositeside.Nowsomethingunusualhappensbutwhich,asyouwilllearn,isquitecommoninthenervoussystem.ThenervecellsofthedorsalcolumnnucleiextendnervesthatcrossthemidlineoftheCNSandcontinueuptothethalamuscreatinganervepathwaycalledthemediallemniscus.Thesignalhascrossedfromonesideofthebodytotheother.Fromthethalamus,thesignalstravelalongthethalamicnervestothenervecellsinthecortexofthecerebralhemisphere,whichiscalledtheprimarysensorycortex.

Thewordnucleus(plural:nuclei)isusedtodescribedifferentstructures.ItisusedtodescribeastructureinthecellthatcontainstheDNA.Inneuroanatomy,thewordisusedtodescribeacollectionofnervecells.

6

Asyoucanseeinfigure1.4,theprimarysensorycortexislocatedonthegyrusjustposteriortothecentralsulcus.Itisthisareathatinformsyouthatyouhavetouchedanobjectwithyourfingertip.Youcanalso“see”withyoursensorysystem.Saythatwehavecoinsandkeysinourpocket

andwewanttotakeoutsomecoins.Wecanputourhandinourpocketandfeeltheobjects

andwithouthavingtolookatthemwithoureyes,wewilltakeoutthecoinsandleavethekeys.Byfeelingtheobjects,wecreateanimageofthembytouch.

Closeyoureyesandhaveafriendorcolleague,placeapenonyourfingertips.Canyoutellitispen?Nowrollyourfingersaroundthepen.Canyounowtellthatitisapen?

Showonthepictureoftheleftcerebralhemisphere,thelocationoftheprimarysensorycortex.

Whatthefunctionalimportanceofnervecrossingoverfromonesideofthebodytotheotherisnotknown.Whydon’tthenervefibersgoupthesamesideofthenervoussystem?

7

Figure1.4Thisfigureshowstheareaofthecerebralcortexthatreceivesthesensoryinputfromthesurfaceofyourbody.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.Lookingbackatfigure1.4,wecanseethatthereisanareajustinfrontofthecentralsulcusofthecerebralhemisphere,whichiscalledmotorcortex.Nervecellsinthisareasendoutnervefibersthattravelthroughthecerebralhemispheresandenterthebrainstem.Thesenervesformastructureinsidethecerebralhemispherescalledtheinternalcapsule.

8

Figure1.5.Showsoneofthefivemotorpathway.Itstartsfromanareaofthecortexofthecerebralhemispherejustinfrontofthecentralsulcusandgoesallthewaytothemuscles(corticospinaltract).Whenthenervesenterthebrainstem,thenervepathwayformediscalledthebasispedunli.Itisontheventralpartofthebrainstem.Whenthenervesreachesthemedulla,theyformastructurecalledthepyramidsandstarttocrossthemidlineoftheCNS.Thiscrossingiscalledthepyramidaldecussation.Inthespinalcord,thesenervestraveldownthewhitematterofthespinalcordmakingthelateralcorticospinaltract.Allalongthespinalcord,thenervesleavethelateralcorticospinaltractandenterthespinalcordgraymatter.Inthespinalcord

Nomenclature:Thereisamethodfornamingnervefibertracts.Thefirstpartofthewordgivesthelocationofthenervecellsandthesecondparttheterminationpointofthenervefibers.Sothecorticospinaltracthasnervecellsinthecerebralcortex(cortico-)andthenervefibersthatendinthespinalcord(-spinal).Ifthenervecellbodieswereinthespinalcordandthenervefibresendedinthecortexthenthenervefibertractwouldbecalledthespinocortical.

9

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.Infigure1.6,wecanseethatthenervefibersfromthereceptorsenterthroughthedorsalsideofthespinalcord.(Rememberallsignalscomingintothespinalcordcomethroughthespinaldorsalside.)Thesenervefibersmakeconnectionswithnervecellsinthedorsalhornofspinalcordgraymatter.Fromherethenervefibersfromthesenervecellsofthedorsalhornspinal

Clinicalnote:Thecorticospinaltractisclinicallyimportant,asitistheonlypathwaythroughwhichyoucancarryoutvoluntarymovement.Damagetothisfibertractresultsinparalysis,whichislossofvoluntarycontroloftheskeletalmuscles.Whenapersonhasastrokeinonecerebralhemisphere,thekeyclinicalfeatureislossofvoluntarymotorcontrolontheoppositesideofthebody.

Theinnervationofthesmoothandcardiacmusclesandtheirstateofcontractionorrelaxationiscontrolledbytheautonomicnervoussystem.

10

graymattermoveupthespinalcordforafewsegmentsbeforecrossingtheCNSmidlinetotheotherside.

Figure1.6.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.

11

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

12

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?