Transmission of Nerve impulses ( physiology)pdf

Animal physiology topic of nerve impulse transmission

1. TRANSMISSION OF NERVE IMPULSES
PRESENTED BY
KUMARI SHIKHA
M.Sc. Zoology (2
nd
Sem.)
Department Of Zoology

2. CONTENTS
S.NO. CONTENTS
1 NEURON
2 ELECTRICAL SIGNALS IN NEURON
3 RESTINGMEMBRANE POTENTIAL
4 ACTIONPOTENTIAL
5 PROPAGATION OF ACTION POTENTIAL
6 SIGNAL TRANSMISSION AT SYNAPSES
7 EXCITATORY AND INHIBITATORYPOST SYNAPTIC POTENTIALS
8 REFERENCES

3. NEURON
Neuronsarenervecellsspecializedforintercellularcommunication.
Neuronspossesselectricalexcitability,theabilitytorespondtoastimulusandconvertitintoanactionpotential.
Mostneuronshavethreeparts-1.CellBody2.Dendrites3.Axon

4. FUNCTIONAL CLASSIFICATION OF NEURONS

5. ELECTRICAL SIGNALS IN NEURONS
Theelectricalsignalsproducedbyneuronsrelyonfourkindsofionchannels:leakchannels,ligand-
gatedchannels,mechanicallygatedchannels,andvoltage-gatedchannels.
ION CHANNELS IN NEURONS

6. RESTING MEMBRANE POTENTIAL
Definition–Restingmembranepotential
ofacellisdefinedastheelectrical
potentialdifferenceacrosstheplasma
membranewhenthecellisunderresting
condition.
Inneurons,therestingmembrane
potentialrangesfrom−40to−90mV.A
typicalvalueis−70mV.
Method to measure the resting membrane potential of a neuron

7. Therestingmembranepotentialarises
fromthreemajorfactors:
1.UnequaldistributionofvariousionsintheECF
andcytosol:ExtracellularfluidisrichinNa+
andchlorideions(Cl−).Incytosol,however,
themaincationisK+,andthetwodominant
anionsarephosphatesattachedtomolecules,
suchasthethreephosphatesinATP,andamino
acidsinproteins
2.TrappedanionscannotfollowK+outofthe
cellbecausetheyareattachedtonondiffusible
moleculessuchasATPandlargeproteins.
3.TheelectrogenicNa+–K+ATPaseexpels3Na+
ionsforevery2K+ionsimported.

8. ACTION POTENTIAL
Nervesignalsaretransmittedbyactionpotentials,whicharerapidchangesinthemembranepotential
thatspreadrapidlyalongthenervefibermembrane.
Toconductanervesignal,theactionpotentialmovesalongthenervefiberuntilitcomestothefiber’s
end.
All-or-nonePrinciple:anerveimpulseeitheroccurscompletelyoritdoesnotoccuratall.This
characteristicofanerveimpulseisknownastheall-or-noneprinciple.
Theshold:themembranepotentialatwhichanactionpotentialbeginsiscalledthethreshold,threshold
foranaxonistypicallybetween-60mVand-55mV.
Thesuccessivestagesoftheactionpotentialareasfollows.
•RestingStage.
•DepolarizationStage
•RepolarizationStage.

9. 1.RESTINGSTATE 2.DEPOLARIZING PHASE

10. 3.REPOLARIZING PHASEBEGINS 4.REPOLARIZATION PHASECONTINUES

11. •HYPERPOLARIZATION
Eventuallythecellgetssonegative,itactuallyovershootstheoriginalrestingpotential.Thisiscalled
hyperpolarization
Duringthisphase,themembranepotentialismorenegativethanitwouldnormallybe.Thismakesitharderfora
neurontoreachthethresholdtosendasignalthannormal,limitingthenumberofsignalsthatcanbesentbacktoback
,thisiscalledrefractoryperiod,whereitismoredifficultforacelltostartanactionpotential.
Changes in the membrane potential at one location during the generation of an action potential

12. Theeventsthatgenerateanaction
potentialtakeplaceinasmallportion
ofthetotalplasmamembranesurface,
butactionpotentialspreadalongthe
entireexcitablemembrane,itiscalled
propagationorconduction.
Therearetwotypesofpropagation:
PROPAGATION OF ACTION POTENTIAL
ACTION
POTENTIAL
PROPAGATION
CONTINUOUS
PROPAGATION
SALTATORY
PROPAGATION

13. Inanunmyelinatedaxon,anactionpotentialmovesalongby
continuouspropagation.Theactionpotentialspreadsby
depolarizingtheadjacentregionoftheaxonmembrane.
STEP1–Asanactionpotentialdevelopsat
theinitialsegment(1),themembrane
potentialatthissitedepolarizesto+30mV.
STEP2-Alocalcurrentthendevelopsasthe
sodiumionsenteringat(1)spreadawayfrom
theopenvoltagegatedchannels.Agraded
depolarizationquicklybringstheaxon
membraneinsegment(2)tothreshold.
CONTINUOUS PROPAGATION ALONG AN UNMYLELINATED AXON

14. STEP 3-An action potential
now occurs in segment (1)
while segment (2) begins
repolarization.
STEP 4-As the sodium ions
entering at segment (2) spread
laterally, a graded depolarization
quickly brings the membrane in
segment (3) to threshold, and the
cycle is repeated

15. SALTATORY PROPAGATION ALONG A MYELINATED AXON
Because myelin limits the movement of ions across the axon membrane ,
the action potential must jump from node to node during propagation .
This results in much faster propagation along the axon.
STEP 1–An Action
potential has occurred at the
initial segment (1)
.
STEP 2-A local current produces a
graded depolarization that brings the
axon membrane at the next node to
threshold .

16. STEP 3–An action potential develops at
node (2)
STEP 4-A local current produces
a graded depolarization that
brings the axolemmaat node (3) to
threshold

17. Thespeedofpropagationofanerveimpulseisaffectedbythreemajorfactors:amountofmyelination,
axondiameter,andtemperature.
1.Amountofmyelination:Asyouhavejustlearned,nerveimpulsespropagatemorerapidlyalong
myelinatedaxonsthanalongunmyelinatedaxons.
2.Axondiameter:Largerdiameteraxonspropagatenerveimpulsesfasterthansmalleronesdueto
theirlargersurfaceareas.
3.Temperature:Axonspropagatenerveimpulsesatlowerspeedswhencooled
FACTORSTHATAFFECTTHESPEEDOFPROPAGATION

18. SIGNAL TRANSMISSION AT THE SYNAPSES
Synapseisaregionwherecommunication
occursbetweentwoneuronsorbetweena
neuronandaneffectorcell(musclecellor
glandularcell)
Presynapticneuron(pre=before)referstoa
nervecellthatcarriesanerveimpulsetoward
asynapse.Itisthecellthatsendsasignal.A
postsynapticcellisthecellthatreceivesa
signal.

19. TYPES OF SYNAPSES
1. ELECTRICAL SYNAPSES
Atelectricalsynapses,thereisdirectphysicalcontact
betweenthecells.Thepresynapticandpostsynaptic
membranesofthetwocommunicatingcellsarelocked
togetheratgapjunctions.
ElectricalsynapsesareextremelyrareinboththeadultCNS
andPNS.
2. CHEMICAL SYNAPSES
Achemicalsynapseiswhereoneneuronsendschemical
signalstoanothercell,oftenasecondneuron.
Everychemicalsynapseinvolvestwocells(1)axonterminalof
thepresynapticcell(2)thepostsynapticcell,Anarrowspace
calledthesynapticcleftseparatesthetwocells.Itisaspaceof
20–50nmthatisfilledwithinterstitialfluid.

20. ATypicalChemicalSynapsetransmitsasignalasfollows

21. Aneurotransmittercauseseitheranexcitatoryoraninhibitorygradedpotential.
Aneurotransmitterthatcausesdepolarizationofthepostsynapticmembraneisexcitatorybecauseit
bringsthemembraneclosertothresholdAdepolarizingpostsynapticpotentialiscalledanExcitatory
postsynapticpotential(EPSP).
Aneurotransmitterthatcauseshyperpolarizationofthepostsynapticmembraneisinhibitory.During
hyperpolarization,generationofanactionpotentialismoredifficultthanusualbecausethemembrane
potentialbecomesinsidemorenegativeandthusevenfartherfromthresholdthaninitsrestingstate.A
hyperpolarizingpostsynapticpotentialistermedanInhibitorypostsynapticpotential(IPSP).
Excitatory and Inhibitory Postsynaptic Potentials

22. REFERENCES
1.Tortora,G.J.,&Derrickson,B.(2017).Principlesofanatomy&physiology.Fifteenthedition;
WileyLoose-LeafPrintCompanion.JohnWiley&Sons,Inc.
2.Hall,J.E.(2015).Guytonandhalltextbookofmedicalphysiology(13thed.).WBSaunders.
1.Chrysafides,S.M.,Bordes,S.J.,&Sharma,S.(2023).Physiology,RestingPotential.InStatPearls.
StatPearlsPublishing.