Nervous system 2023.pdf detailing propagation through myelinated and non myelinated nerve fiber.

Nervous system, propagation of nerve impulse ppt.

1. Nervous System
Propagation of nerve impulse (myelinated
and non-myelinated nerve fibre)

2. •Sensory-detectinternalstimuli(increaseinbloodacidity)andexternalstimuli
(raindroplandingonyourarm).
•Integrative-integrates(processes)sensoryinformationbyanalyzingandstoringsome
ofitandbymakingdecisionsforappropriateresponses.
•Motor-Oncesensoryinformationisintegrated,thenervoussystemelicitanappropriate
motorresponsebyactivatingeffectors(musclesandglands)throughcranialandspinal
nerves.
FunctionsoftheNervousSystem
Consistsoftwotypesofcells:
•Neurons
•Neuroglia.

3. Neurons(nervecells)possesselectricalexcitability,theabilitytorespondtoastimulusand
convertitintoanactionpotential.
✓Stimulusisanychangeintheenvironmentthatisstrongenoughtoinitiateanaction
potential.
✓Actionpotential(nerveimpulse)isanelectricalsignalthatpropagates(travels)along
thesurfaceofthemembraneofaneuron.Itbeginsandtravelsduetothemovementof
ions(suchassodiumandpotassium)betweeninterstitialfluidandtheinsideofaneuron
throughspecificionchannelsinitsplasmamembrane.Oncebegun,anerveimpulse
travelsrapidlyandataconstantstrength.Nerveimpulsestravelatspeedsrangingfrom
0.5to130meterspersecond(1to280mi/hr).
PartsofaNeuron
(1)Cellbody
(2)Dendrites
(3)Axon
Neurons

4. Structure of a multipolar neuron

5. Nucleussurroundedbycytoplasmthatincludeslysosomes,mitochondria,Golgicomplex,
freeribosomes,neurofibrils,lipofuscinandprominentclustersofRER,termedNissl
bodies.
Cellbody/perikaryon/soma
Seriesofhighlybranchedoutgrowthsofthecellbodyandthereceivingorinput
portionsofaneuron.TheircytoplasmcontainsNisslbodies,mitochondria,andother
organelles.
Dendrites
Axons
Propagatesnerveimpulsestowardanotherneuron,amusclefiber,oraglandcell.
Containsmitochondria,microtubulesandneurofibrils.RERabsent.
Inmostneurons,nerveimpulsesariseatthejunctionoftheaxonhillockandtheinitial
segment,anareacalledthetriggerzone,fromwhichtheytravelalongtheaxonto
theirdestination.
Theaxonmayhavebranches,calledcollaterals.

6. StructuralClassificationofNeurons
Brain and spinal cord
Retina of the eye,
inner ear, olfactory
area of the brain.
Sensory receptors

7. FunctionalClassificationofNeurons

8. •Glialcellssurroundthesoma,axon,anddendritesofneuronsandprovidethemwith
physicalandmetabolicsupport.
•Gliadonotgenerateorpropagateactionpotentialsandtheycanmultiplyand
divideinthematurenervoussystem.
•Incasesofinjuryordisease,neurogliamultipliestofillinthespacesformerlyoccupied
byneurons.Braintumorsderivedfromglia,calledgliomas,tendtobehighlymalignant
andgrowrapidly.
Neuroglia
CNS
1.Astrocytes
2.Oligodendrocytes
3.Microglia
4.Ependymalcells
PNS
1.Schwann cells
2.Satellite cells

9. Star-shaped,manyprocessesandarethelargestandmostnumerousoftheneuroglia.
✓ProcessesofastrocyteswrappedaroundbloodcapillariesisolateneuronsoftheCNS
fromvariouspotentiallyharmfulsubstancesinbloodbysecretingchemicalsthat
maintaintheuniqueselectivepermeabilitycharacteristicsoftheendothelialcellsofthe
capillariescreatingblood-brainbarrier.
Astrocytes
ResponsibleforformingandmaintainingthemyelinsheatharoundCNSaxons.
Oligodendrocytes
Microglia function as phagocytes.
Microglia
Cuboidaltocolumnarcellslinetheventriclesofthebrainandcentralcanalofthespinal
cordandformtheblood-cerebrospinalfluidbarrier.
EpendymalCells

10. Glial cells of the central nervous system

11. SchwannCellsformthemyelinsheatharoundasingleaxons.AsingleSchwanncell
canalsoencloseasmanyas20ormoreunmyelinatedaxons.Schwanncells
participateinaxonregeneration,whichismoreeasilyaccomplishedinthePNSthanin
theCNS.
SatelliteCellsareflatcellssurroundthecellbodiesofneuronsofPNSgangliaand
providestructuralsupport.
Glial cells of the PNS

12. •Axonssurroundedbyamultilayeredlipidandproteincovering,calledthemyelin
sheath,aresaidtobemyelinated.Thesheathelectricallyinsulatestheaxonofaneuron
andincreasesthespeedofnerveimpulseconduction.
•TheouternucleatedcytoplasmiclayeroftheSchwanncell,whichenclosesthemyelin
sheath,istheneurolemma(sheathofSchwann).
•AneurolemmaisfoundonlyaroundaxonsinthePNS.Whenanaxonisinjured,the
neurolemmaaidsregenerationbyformingaregenerationtubethatguidesandstimulates
re-growthoftheaxon.
•AxonsintheCNSdisplaylittlere-growthafterinjuryduetotheabsenceofa
neurolemma,andinparttoaninhibitoryinfluenceexertedbytheoligodendrocyteson
axonre-growth.
•Theamountofmyelinincreasesfrombirthtomaturity,anditspresencegreatlyincreases
thespeedofnerveimpulseconduction.Aninfant’sresponsestostimuliareneitheras
rapidnorascoordinatedasthoseofanolderchildoranadult,inpartbecause
myelinationisstillinprogressduringinfancy.
Myelination

13. Myelinatedandunmyelinatedaxons-NoticethatonelayerofSchwanncellplasmamembrane
surroundsunmyelinatedaxons.
AxonssurroundedbyamyelinsheathproducedeitherbySchwanncellsinthePNSorby
oligodendrocytesintheCNSaresaidtobemyelinated.

14. Neuronscommunicatewithoneanotherusingtwotypesofelectricsignals:
✓Gradedpotential-usedforshort-distancecommunication
✓Actionpotential-allowcommunicationoverlongdistances
Theproductionofgradedpotentialsandactionpotentialsdependsontwobasicfeatures
oftheplasmamembraneofexcitablecells:
1.theexistenceofarestingmembranepotential
2.thepresenceofspecificionchannels
IONCHANNELS
•leakage
•ligand-gated
•mechanicallygated
•voltage-gated
Ionic Basis of Resting and Action Potentials

15. •Allcellsunderrestingconditionshaveapotentialdifferenceacrosstheirplasma
membranes,withtheinsideofthecellnegativelychargedwithrespecttotheoutside.
Thispotentialistherestingmembranepotential.Restingmembranepotentialin
neurons-rangesfrom-40to-90mV(typicalvalueis-70mV).
Resting Membrane Potential (RMP)
Factors responsible for RMP
1.UnequaldistributionofionsintheECFandcytosol–insidebeingmorenegative
(potassium,phosphatesandaminoacids)thanoutside(Na
+
,Cl
-
).PMhasmoreK+
leakagechannelsthanNa+leakagechannels.
2.Inabilityofmostanionstoleavethecell-Sinceanionsareattachedtonon-diffusible
moleculessuchasATPandlargeproteinstheyarenotinfreestatetofollowtheK+out
ofthecell.
3.ElectrogenicnatureoftheNa+/K+ATPases-Na+/K+ATPasesexpelthreeNa+for
eachtwoK+imported.Sincethesepumpsremovemorepositivechargesfromthecell
thantheybringintothecell,theycontributetothenegativityoftherestingmembrane
potential.

17. •ChangesinmembranepotentialthatareconfinedtoarelativelysmallregionofthePM.
•Electricalsignalsvaryinamplitude(size),dependingonthestrengthofthestimulus.
•Gradedpotentialmakesthemembraneeithermorepolarized(insidemorenegative)
termedhyperpolarizinggradedpotentialorlesspolarized(insidelessnegative)termed
depolarizinggradedpotential.
•Occurswhenastimuluscausesmechanicallygatedorligand-gatedchannelstoopenor
closeinanexcitablecell’sPM.
•Strengthofdependsonhowmanyionchannelshaveopenedorclosed.
GradedPotentials

18. DecrementalConduction
Summation
•Twodepolarizinggradedpotentialssummate=largerdepolarizinggradedpotential.
•2hyperpolarizinggradedpotentialssummate=largerhyperpolarizinggradedpotential.
•onedepolarizing+onehyperpolarizing=canceleachotherout,theoverallgraded
potentialdisappears.
Propertiesofgradedpotential
The graded nature of graded potentials.
Summationoftwodepolarizinggraded
potentialshappensinresponsetotwo
stimuliofthesamestrengththatoccur
veryclosetogetherintime.

19. Anactionpotential(AP)orimpulseisanelectricalsignalthatpropagates(travels)along
thesurfaceofthemembraneofaneuron.Itbeginsandtravelsduetothemovementofions
betweeninterstitialfluidandtheinsideofaneuronthroughspecificionchannelsinits
PM.Oncebegun,anerveimpulsetravelsrapidlyandataconstantstrength.
•AnAPhasthreemainphases:
✓Depolarizingphase-membranepotentialbecomeslessnegative,reacheszero,and
thenbecomespositive.
Voltage-gatedNa+channels,allowNa+torushintothecell.
✓Repolarizingphase-potentialisrestoredtotherestingstateof-70mV.Voltage-
gatedK+channelsopen,allowingK+toflowout,whichproducestherepolarizing
phase.
✓After-hyperpolarizingphase-membranepotentialtemporarilybecomesmore
negativethantherestinglevel.
Theafter-hyperpolarizingphaseoccurswhenthevoltage-gatedK+channelsremain
openaftertherepolarizingphaseends.
ActionPotential

21. •Whenadepolarizinggradedpotentialorsomeotherstimuluscausesthemembraneof
theaxontodepolarizetothreshold,voltage-gatedNa+channelsopenrapidly.
•InflowofNa+changesthemembranepotentialfrom-55mVto+30mV
•Eachvoltage-gatedNa+channelhastwoseparategates,anactivationgateandan
inactivationgate.
➢Restingstate-theinactivationgateisopenandactivationgateisclosed.
➢Atthreshold,boththeactivationandinactivationgatesinthechannelareopen
andNa+inflowbegins.
➢Inactivation–activationgateisopenandinactivationgateisclosed
DepolarizingPhase

22. •Asthemembranepotentialapproachesitspeakvalue,thesodiumpermeabilityabruptly
declinesasinactivationgatesbreakthecycleofpositivefeedbackbyblockingtheopen
sodiumchannels.
•Meanwhile,thedepolarizedstateofthemembranehasbeguntoopentherelatively
sluggishvoltage-gatedk+channels,andtheresultingelevatedK+fluxoutofthecell
rapidlyrepolarizesthemembranetowarditsrestingvalueproducingrepolarizingphase
oftheactionpotential.
•RepolarizationalsoallowsinactivatedNa+channelstoreverttotherestingstate.
RepolarizingPhase

23. •ImmediatelyafteranAPthereisaperiodwhenK+permiabilityremainsaboveresting
levelsbecausevoltage-gatedK+channelscloserelativelyslowly.
•Therefore,membraneistransientlyhyperpolarizedtowardthepotassiumequilibrium
potential.Thisportionoftheactionpotentialisknownastheafter-hyperpolarization.
After-hyperpolarizingPhase

24. •TheperiodoftimeafteranAPbeginsduringwhichanexcitablecellcannotgenerate
anotherAPinresponsetoanormalthresholdstimulusiscalledtherefractoryperiod.
•AbsoluteRefractoryPeriod-Evenaverystrongstimuluscannotinitiateasecond
AP.ThisperiodcoincideswiththeperiodofNa+channelactivationand
inactivation.
•RelativeRefractoryPeriod-Followingtheabsoluterefractoryperiod,thereisan
intervalduringwhichasecondAPcanbeproduced,butonlyifthestimulusstrengthis
considerablygreaterthanusual.Coincidesroughlywiththeperiodofafter-
hyperpolarization.Duringtherelativerefractoryperiod,somebutnotallofthe
voltage-gatedNa+channelshavereturnedtoarestingstate,andsomeoftheK+
channelsarestillopen.
•Therefractoryperiodslimitthenumberofactionpotentialsanexcitablemembrane
canproduceinagivenperiodoftime.
•Undernormalbodyconditions,themaximumfrequencyofnerveimpulsesin
differentaxonsrangesbetween10-1000persecond.
RefractoryPeriod

25. AnAPoccursinthemembraneoftheaxonofaneuronwhendepolarization
reachesacertainleveltermedthethreshold(~-55mVgenerally),constantfora
particularneuron.
✓Subthresholdstimulus-astimulusthatisweaktodepolarizethemembrane
andcannotbringthemembranepotentialtothreshold.APwillnotoccur.
✓Thresholdstimulus-astimulusthatisjuststrongenoughtodepolarizethe
membranetothreshold.APwilloccurinresponsetoathresholdstimulus,
✓Suprathresholdstimulus-SeveralAPwillforminresponsetoa
suprathresholdstimulus,astimulusthatisstrongenoughtodepolarizethe
membraneabovethreshold.Eachoftheactionpotentialscausedbya
suprathresholdstimulushasthesameamplitude(size)asanAPcausedbya
thresholdstimulus.
ThresholdStimulus

27. ALL-OR-NONE PRINCIPLE
AnAPisgeneratedinresponsetoathresholdstimulusbutdoes
notformwhenthereisasub-thresholdstimulus.Inotherwords,an
actionpotentialeitheroccurscompletelyoritdoesnotoccuratall.

28. Certainshellfishandotherorganismscontainneurotoxins,thatproducetheirpoisonous
effectsbyactingonthenervoussystem.Oneparticularlylethalneurotoxinis
tetrodotoxin(TTX),presentinthevisceraofJapanesepufferfish.TTXeffectively
blocksAPbyinsertingitselfintovoltage-gatedNa+channelssotheycannotopen.
Localanestheticsaredrugsthatblockpainandothersomaticsensations.Eg.procaine
(Novocaine®)andLidocaine,usedtoproduceanesthesiaintheskinduringsuturing,in
themouthduringdentalwork,orinthelowerbodyduringchildbirth.thesedrugsactby
blockingtheopeningofvoltage-gatedNa+channels.
Localizedcoolingofanervecanalsoproduceananestheticeffectbecauseaxons
propagateactionpotentialsatlowerspeedswhencooled.Theapplicationoficeto
injuredtissuecanreducepainbecausepropagationofthepainsensationsalongaxonsis
partiallyblocked.
NeurotoxinsandLocalAnesthetics

29. Continuousconduction
Involvesstep-by-stepdepolarization
andrepolarizationofeach
adjacentsegmentofthePMin
unmyelinatedaxons.
Continuous Conduction

30. •APpropagationalongmyelinatedaxons.
•DuringAPpropagationalongamyelinatedaxon,anelectriccurrentflowsthrough
theextracellularfluidsurroundingthemyelinsheathandthroughthecytosolfrom
onenodetothenext.
•TheAPatthefirstnodegeneratesioniccurrentsinthecytosolandextracellular
fluidthatdepolarizethemembranetothreshold,openingvoltage-gatedNa+
channelsatthesecondnode.
Saltatory Conduction

31. ✓Myelination-APpropagatemorerapidlyalongmyelinatedaxonsthan
alongunmyelinatedaxons.
✓Axondiameter-Larger-diameteraxonspropagateAPfasterthan
smalleronesduetotheirlargersurfaceareas.
✓Temperature-AxonspropagateAPatlowerspeedswhencooled.
Factors that Affect the Speed of Propagation

32. https://jhs.jisd.org/apps/video/watch.jsp?v=95237

33. REFERENCES
•Tortora, G.J. and Derrickson, B.H. (2009). Principles of Anatomy and
Physiology, XII Edition, John Wiley and Sons, Inc.
•WidmaierE, Raff H and Strang K. (2013) Vander’s Human Physiology: The
Mechanism of Body Functions. McGraw-hillEducation 13
th
Edition.