Download 1 Introduction to the Nervous System. Code: HMP 100/ UPC 103

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.
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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.
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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.
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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
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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.
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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.
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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.
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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.
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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
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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.
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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
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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?
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