IntroductiontotheNervousSystem.
Code:HMP100/UPC103/VNP100.Course:MedicalPhysiology
Level1MBChB/BDS/BPharm
Lecture3.CellsoftheNervousSystemandNeuralCircuits
LectureOutline
1.1Introduction
1.2Learningoutcomes
1.3NerveCells
1.4GlialCells
1.5ASimpleNeuralCircuit
1.6SignalingintheNervousSystem
1.7Summary
1.8Activity
1.9FurtherReading
1.1Introduction
Inthislecture,wewilllookatthecellsthatmakeupthenervoustissueandlookatanexample
ofaneuralcircuit.Aswewillseetherearetwoclassesofcells-neuronsandglia,bothof
whichcomeinsub-classes.Afterwehavediscussedthesecells,wewilllookataneuralcircuit
toseehowaresponsetoastimulustakesplace.Theneuralcircuitwewilllookatistheknee
jerkreflex.
1.2LearningOutcomes
Attheendofthislecture,youshouldbeableto:
1. Describeandexplainthefunctionofthenervecells
2. Describeandexplainthefunctionofthegliacells
3. Drawtheneuralcircuitdiagramofthekneejerkreflex
4. Explainhowthenervesignalistransmittedinthekneejerk
reflex
1.3NerveCells
Nervecellsorneuronsmakeuparound10-20%ofthecellpopulationofthenervoustissue;
theother80-90%ofthenervoustissueismadeofglialcells.Intermsofnumbers,wehave
around100billionnervecells.Intissuesinotherpartsofthebodywedonotfindmuch
variationbetweenthecellsthatmakeupthattissue.Soitdoesnotmatterwhichpartofthe
liverwelookat,thecelltypeswillbesimilartootherparts,andalsohowthecellsare
arranged,thecytoarchitecture,willalsobesimilarindifferentpartsoftheliver.Inthebrain,
wefindthatthereisnotthisregularityofcellsandcellulararrangement.Wefindthatnerve
cellscomeinmanydifferentformsandnervecellsarrangementsindifferentareasofthe
nervoustissuecanbeverydifferent.
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InFigure3.1twotypesofnervecellsareshown:thecorticalneuronfoundinthecerebral
cortexandthemotorneuron(alphamotorneuron)foundintheventralhornofthespinal
cordgraymatter.Bothshowafeaturethatiscommontonervecells.Thisistheprocessesthat
areproducedandextendoutfromthecellbody.Thelongestprocessistheaxonandits
endingmakesthepre-synapticpartofthesynapse.InFig3.1(lowerright),therealsoa
pictureoftheendingamotorneuronshowingthepre-synapticpartincontactwithamuscle
cell.Inotherlectures,wewilldiscusshowthesignalfromthenervecellsispassedtothe
musclecelltocausethemusclecellstocontract.
Alsoextendingfromthecellbodyareprocessesthatareshorterthantheaxonandhavelots
ofbranching.Becausethebranchingoftheseprocesseslooklikeatree,theyarecalledthe
dendritiesfromtheGreekword,dendron,meaningtree.
Figure3.1.Acorticalandmotornervecell.Notethatbothhavealongprocesscalledtheaxon
andmanyshorterprocessescalledthedendrites.
Wecandivideanervecellinto4anatomicalcompartments:(1)dendrites,(2)cellbody,(3)
axonand(4)synapse.
Likeallothercellsofthebody,thecellbodyofthenervecellscontainsallthecellular
organellesnecessaryforcellfunction.Butwemaywonderhowmaterialfromthecellbodyis
sentalongthelongaxonalprocessesastheseprocessescanbequitelong.Forexample,the
pyramidalcellinourmotorcortexsendsaxonsthatextendallthewaytothelumberregionof
ourspinalcord.Thisisadistanceofaroundameter,andifweconsiderthegiraffe,thesame
axonscanextendover2meters.Becausethespeedofdiffusionofmoleculeswouldbetoo
slowtosentmaterialrapidlyalongtheaxon,thereisamechanismmadeupofspecialized
molecularmachinerytomovematerialfromthecellbodyalongtheaxon(anterograde)and
fromthenerveendingtothecellbody(retrograde).Thisisamoleculartransportmachinery
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thatmovesmaterialatarateof400mm/day(fasttransport)andanotherthatisslower
movingthematerialatarateof200mm/day(slowtransport).
Ontheinternetlookthediffusionrateofmoleculesinaliquid.Calulatethe
timeifwouldtakeamoleculetodiffuse1meter.
Nowifwelookathowthenervecellsconnecttoeachother,wefindthatthenerveendings
fromothernervecellsmakeconnectionsonthedendritesandcellbody,notontheaxon.
Lookingcloselyatfigure3.1,weseethatdendriteshavetinyextensionsontheirbranches.
Thesearecalledthedendriticspines,anditisonthesethataxonsfromothernervecells
makeconnections.Sothedendritesandcellbodymakeuptheinputzoneofthenervecells.
Signalsfromthenervecellaresentalongtheaxontoothercells,sotheaxonistheoutput
zone.
Dendrites
Input
Cell body
Intregration
Axon
Output
Figure3.2.Diagramofanervecellshowingtheinputzone(dendrites),theintegrationzone
(cellbody),andtheoutputzone(axon).
Nowifweaskhowmanyconnectionsdoesasingenervecellreceive,theansweris
astonishing.Ontheaverageanervecellinourbrainandspinalcordreceives1,000synaptic
connections,someupto10,000.Thismeansthatthenervecellisreceiving1,000signalsand
thesesignalshavetobeintegratedto“decide”whatthenervecellshoulddo.Ournervecells
haveto“decide”whethertosendasignaltothecellsitcontactstoornot.Howdoournerve
cellsmakethis“decision”?Wewillnotdiscussthe“decision”makingprocessinthislecture
Ontheinternetlookupthemeaningofthewordintegration.
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butitwilldependonhowtheincomingsignalschangethenervecellmembraneelectrical
potential.Thistakesinthecellbodyandhencethecellbodyistheintegrationzone.
Whentheaxonmakesaconnectionwithanothernervecell,itpassesthesignaltotheother
cellbyreleasingchemicalsfromitsnerveendings.Thesearecalledneurotransmitter,andwe
willdiscusstheprocessofsignaltransmissionatthesynapse,inthelecturesonsynaptic
physiology.Weshouldalsonotethatwhentheaxonreachedthecellitwillformasynapse
withitformsseveralbranches.Wecanseethisformotorandsensorynervesinfigure3.2.
Thehumanbrainhasaround100billionnervecells.Ifontheaverageeach
nervereceives1,000synapticconnections,howmanysynapticconnections
arethereinthebrain?
Finally,weshouldknowthedifferenttypesofnervecellsthatwecanfindinthenervous
system.InFigure3.2drawingsofthedifferentshapes(morphology)ofnervecellsareshown.
Allthenervecellshavedendrites,acellbody,andanaxon.Butweseethesepartsofthenerve
cellsdifferbetweennervecellsformingnervecellswithdifferentmorphologies.Alsoinfigure
3.2,thenamesofthedifferenttypesofneurotransmittersthatarereleasedbythenerve
endingaregiven.Sonervecellsalsodifferbytheneurotransmitterthatisreleasedbytheir
nerveendingsatthesynapse.
Animportantprincipleofneuronalfunctionsisthatanervecellreleasesonly
onetypeofneurotransmitterfromallitsnerveending.Soanervecelldoes
notreleaseonetypeofneurotransmitterfromfromoneofitsnerveending
andanotherfromitsothernerveending.ItiscalledDale’sPrinciple.
Figure3.2.Diagramofthedifferenttypesofnervecellmorphologyandsomeofthe
neurotransmitterthatreleasedbynerveending.
Sointhissection,wehavelookedatthepartsofthenervecell.
1.4GlialCells
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Inthissection,wewilldiscusstheglialcells,whichwereconsideredtogluethenervecells
together,hencethenameglia.Butnowweknow,thattheglialcellscarryoutmanydifferent
functionsinthenervoussystem.Unfortunately,wetendnottotakethemtooseriously,being
morefascinatedbythenervecells.
Figure3.3nicelysummariesthemanyfunctionsthatourglialcellscarryout.Theyaredivided
intothosefoundinthePNSandCNS.InthePNS,theSchwanncellswraparoundtheaxons,a
processcalledmyelination,whileintheCNSitistheoligodendrocytesthatmakethemyelin
sheath.Themyelinsheathincreasesthespeedatwhichnervesignalscantravelalongthe
axon.Wewilllearnmoreaboutthisprocessduringthelecturesonelectricalsignalingin
nervecells.IntheCNS,therearealsoaclassofglialcellscalledastrocytes,socalledbecause
theylook
likestars.
Figure3.3.Diagramshowingthedifferentclassesofglialcellsandtheirfunctions.
Infigure3.4stainedingreenaretheastrocytesandstainedinredaretheoligodendrocytes.
Thebluecirclesarethenucleiofthecells.Astrocytesperformmanyfunctionsinthebrain.
Onefunctionistosecreteneurotrophicfactors.Theseareproteinsthatarenecessaryforthe
growthandsurvivalofdevelopingnervecellsandmaintainthehealthofthematurenerve
cells.Themicroglialcellsfunctionsasimmunecellsandareactivatedwhenthereisinfection
ortraumatoourbrain.Theyremovedeadcellularmaterialinthebrain.
Figure3.4.Glialcellswithastrocytesstainedgreenandoligodendrocytesinred.Bluecircles
arecellnuclei.
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Unlikeotherpartsofthebody,theCNShasabarrierthatcontrolswhichsubstancecanenter
andleavetheCNS.Thisiscalledtheblood-brainbarrierandinvolvesboththeependymal
cellsandtheastrocytes.
Nowthatwehavediscussedthecellsofthenervoussystem,wewilllookatasimpleneuronal
circuitandseehowaresponseisproducedtoastimulus.
1.5ASimpleNeuronalCircuit
Theneuralcircuitthatwewilllookatisthekneejerkreflex.Thisaoneofthetendonjerk
reflexes.Figure3.5,showsthatwhenthepatellartendonofthekneeisstruckwithareflex
hammer,itcausesanextensionofthelowerleg.Thisextensionofthelowerlegiswithoutthe
personneedingto“command”anextension;theirlowerlegextendsinvoluntarilywhenthe
patellartendonisstruck.Itisareflexaction.Reflexiveactivitytakesplacewithoutushaving
toconsciouslyproducearesponsetoastimulus.Therearemanyreflexesinthebodyandeach
hasaphysiologicalfunction.Usuallythecircuitofthereflexisarrangedsoitactsasanegative
feedbackcontrolsystem.
Thestimulus,inthiscasetheextensionofthepatellartendoncausedbythereflexhammer,
causesnervesignalstobegeneratedinthereceptor.Inthiscasethemusclespindlelocated
inthequadricepsmuscleofthethigh.Thenervesignalstravelalongthesensorynervefiber
tothespinalcordandwherethenervefibermakesasynapticcontactonmotorneuron.This
anexcitatorysynapseandstimulatesmotorneurontoproducenervesignals.Thesesignals
travelalongthenervefiberofthemotornervecelltoquadricepsmusclecausingittocontract.
Andthisresultsintheextensionofthelowerleg.Sothisissimpleneuronalcircuitconsistinga
receptor,twonervecells,andeffector–thequadriceps.Thiscircuitisshowninredinfigure
3.5.
Figure3.5.Circuitdiagramofthekneejerkreflex.
Inthecircuitdiagramshowninfigure3.5,wecanseethatthesensorynervebringingthe
signaltothespinalcordhasabranchthatmakescontactwithanothernervecellshownin
blackcolour,whichturnmakesaconnectionwithanothermotornervecellshowninlight
browncolour.Thenervecellshowninblackiscalledaspinalinterneuron.Itsaxondoesnot
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leavethespinalcordbutassistinmakingconnectionsbetweennervecellswithinthespinal
cord.Thesignalsitproducesstopsthemotornervecelltowhichitisconnectedfrom
producinganysignals.Thusitiscalledaninhibitoryinterneuron;itinhibitsthenervecells
itisconnectedtofromproducinganysignals.
Themotornervecellshowninlightbrowncoloursendsanervefiberthatmakessynaptic
connectionswiththemusclefibersofthehamstringmuscle.Bypreventingproductionof
nervesignalsinthismotorneuron,thehamstringmusclesarenotstimulatedtocontractand
sodonotopposetheextensionlowerleg.
Testingoftendonreflexesisroutinelyusedbydoctorsduringaclinical
examination.
Sointhissection,wehavelookedatasimpleneuralcircuitinwhichaparticularresponse
(extensionofthelowerleg)isproducedwhenparticularstimulusisapplied.Thisissimilarto
theelectricalwiringinahouse.Differentswitchescontroldifferentlights.Soturningonthe
switchisthestimulussendinganelectriccurrentthroughthecircuitandlightcomingonis
theresponse.
1.6SignalingintheNervousSystem
Inlastsection,wediscussedtheneuronalcircuitforthekneejerkreflex.Wesawthatthe
circuitinvolvesreceptors,nervecellsandmusclecells.Welookedatthepaththesignaltook
throughthecircuittoproducetheresponse.Inthissection,wearegoingtodiscusswhatthese
signalsare?
Signalinginthenervoussysteminvolveschemicalsandelectricity.Wewilllearnabout
chemicalsignalingduringthelecturesonsynapticphysiology,andaboutelectricalsignalingin
lecturesaboutelectrophysiology.Herewewillbrieflygooverthedifferenttypesofsignaling
used.
Inthenervoussystem,chemicalareusedtopasssignalsfromanervecelltoothercells.We
callthechemicalsusedforthispurposeinthenervoussystem,neurotransmitters.The
neurotransmittersarereleasedbythenerveendingsandinteractwithreceptorsonother
cells.Theregionwherethistransferofsignalfromanervecelltotheothercelltakesiscalled
asynapse.
Electricalsignalingisusedbynervecellsandmusclecells.Tounderstandhowthiselectrical
signalingcantakeplace,weneedtoknowthatbetweentheoutsideandinsideofnerveor
musclecellthereisanelectricalvoltagedifference.Thisiscalledthemembranepotential.It
isquitesmallandmeasuredinmillivolts.Bychangingthevalueofthemembranepotential,
eitherincreasingitordecreasingit,electricalsignalsareproduced.Andthesechangesinthe
membranepotentialcanmovedalongthecellmembraneofthenervecellanditsfibers.
Infigure3.6,thewholeneuralcircuitforthekneejerkreflexislaidoutastraightlinetomake
iteasiertoseethesignalingprocess.Startingfromthemusclespindle,wecanseethatwhenit
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isstretched(thestimulus),thiscausesthemembranepotentialinthenerveendingthat
makescontactwiththemusclespindletochange.Thisisshownintheboxundertheheading
sensorysignals.Itislabelledgradedreceptorpotential.Thanthereisanotherchangeinthe
membranepotentialproduced,whichcalledtheactionpotential.Whilethegradedreceptor
potentialisfoundonlyinthenerveending,theactionpotentialtravelsalongthenervefiber
tillitreachestheendofthesensorynerveinthespinalcord.Hereitcausesthereleaseof
neurotransmitters.Inthecaseofthekneejerkreflex,theneurotransmitterreleasedis
glutamate.
Glutamatecausesthemembranepotentialinthecellbodyofthemotorneurontochange.
Thesechangesinthemembranepotentialcausedbyneurotransmittersarecalledgraded
synapticpotentials,andcanleadtoproductionofactionpotentials,whichtravelalongthe
axonuntiltheyreachthenerveending.Inthiscasethenerveendingismakingcontactwitha
musclecellanditreleasestheneurotransmitter,acetylcholine,ontothesurfaceofthe
muscle.Thiscausesthemembranepotentialofthemusclecelltochange(gradedsynaptic
potential),whichcauseactionpotentialsinthemusclecellandthisinturnleadstomuscle
contraction.Howdoesproducinganactionpotentialinthemusclecellscausesthemuscleto
contract?Wewilllearnaboutinthelecturesonmusclephysiology.
Figure3.6.Theelectricalandchemicalsignalsthatareproducedintheneuralcircuitofthe
kneejerkreflextocauseextensionofthelowerlegwhenthepatellartendonisstruck.
Sointhissection,wehavecoveredthenatureofthesignalsfoundinthenervoussystem.
Thesearechemicals,calledneurotransmitters,andtheremanydifferenttypesof
neurotransmitters.Theothertypeofsignalsproducedareelectricalandtheyalsocomein
differenttypes.
1.7Summary
Inthislecture,wehavelookedatthetwodifferentcelltypesthatarefoundinthenervous
system-thenervecellsandtheglialcells.Thenwelookedatasimpleneuralcircuittosee
howaresponseisproducedtoastimulus.Thiswasthekneejerkreflex.Finally,wediscussed
thesignalsusedbyournervoussystem,whicharechemicalandelectrical.Inthefollowing
lectures,wewillcoverinmoredetailtheprocessofproducingelectricalandchemicalsignals,
andthen,differentneuralcircuitsthatcarryoutthedifferentfunctionsofournervoussystem.
1.8Activities
1.9FurtherReading
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1.10SampleExaminationQuestions
SampleExamQuestions
MultipleChoiceQuestions(MCQs)
Selectforeachquestion,thebestcorrectanswer.
1) Thesensoryreceptorforthekneejerkreflexisthe
a) Skinreceptors
b) Painreceptors
c) Musclespindle
d) Touchreceptors
e) Golgitendonorgan
2) Sensoryinputtothespinalcordcomesfromthe
a) Dorsalside
b) Ventralside
c) Lateralside
d) Sagittalside
e) Coronalside
3) Theinputzoneofthenervecellisthe
a) Dendritesandcellbody
b) Cellbodyandtheaxons
c) Axons
d) Projectionnervefiber
e) Noneoftheabove
4) Thegliathatprovidethemyelinsheathintheperipheralnervoussystemarethe
a) Schwanncells
b) Oligodendrocytes
c) Astrocytes
d) Microglia
e) Epyendemalcells
5) Thegliathatprovidethemyelinsheathintheperipheralnervoussystemarethe
a) Schwanncells
b) Oligodendrocytes
c) Astrocytes
d) Microglia
e) Ependymalcells
6) Theneurotransmitterreleasedbythesensorynerveofthekneejerkreflexis
a) Glutamate
b) Acetycholine
c) Dopamine
d) Serotonin
e) Gammaaminobutyricacid(GABA)
7) Thechangeinthemembranepotentialproducedbythereleaseoftheneurotransmitteris
calledthe
a) Generatorpotential
b) Actionpotential
c) Synapticpotential
d) Receptorpotential
e) Membranepotential
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ShortAnswerQuestions(SAQs)
Note:SAQshavemaximumof5marks.Theanswertothequestionhastobeprecisegiving
allthekeypointsrequired.IftheSAQrequiresadiagramorillustration,youmust
providethedrawingotherwiseyouwillimmediatelylosehalfthemarks.Iwould
recommendthatyoupracticedrawingoutkeyillustrationordiagramsgiveninthe
lectureandinthetextbooks.
1. Drawtheneuralcircuitdiagramofthekneejerkreflex.
2. Namethedifferentgliacellsofthenervoussystemandtheirfunction.
3. Drawadiagramofanervecellandlabelthe4anatomicalcompartmentandtherole
thesecompartmentsplayinnervecellfunction.
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