nervous system: part iii how the neuron...
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NervousSystem:PartIIIHowtheNeuronWorks:RESTINGVS.ACTIONPOTENTIAL
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RESTINGVS.ACTIONPOTENTIAL
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• Howdoesaneuroncausethetransmissionofinforma4on?
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Cells:surroundedbychargedions• Cellsliveinaseaofchargedions
– anions(nega4ve)• moreconcentratedwithinthecell• Cl-,chargedaminoacids(aa-)
– ca4ons(posi4ve)• moreconcentratedintheextracellularfluid• Na+• K+
Na+ Na+ Na+ Na+ Na+ Na+ Na+Na+ Na+ K+ Na+ Na+
Cl-K+ Cl- Cl- Cl-
K+aa- K+ Cl- Cl-
aa-aa-aa-
aa- aa-K+
K+ +–
Measuringcellvoltage
unsFmulatedneuron=resFngpotenFalof-70mV
Voltagemeasuresthedifferenceinconcentra4onofcharges.Theposi4vesarethe“hole”youleavebehindwhenyoumoveanelectron.
Cellshavevoltage!
• Oppositechargesonoppositesidesofcellmembrane
• chargegradient• storedenergy(likeabaPery)
+ + + + + + + ++ + + + + + +
+ + + + + + + ++ + + + + + +
– – – – – – – ––– – – – –
– – – – – – – ––– – – – –
Howdoesanerveimpulsetravel?
• S4mulus:nerveiss4mulated• Na+ionsdiffuseintocell
– chargesreverseatthatpointonneuron• posi4veinside;nega4veoutside
– + + + + + + ++ + + + + + +
– + + + + + + ++ + + + + + +
+ – – – – – – –– – – – – – –
+ – – – – – – –– – – – – – –Na+
The 1st domino goes down!
Gate
+ –
+
+
channelclosed
channelopen
Howdoesanerveimpulsetravel?
• Wave:nerveimpulsetravelsdownneuron– changeinchargeopensnextNa+gatesdowntheline
– Na+ionscon4nuetodiffuseintocell– “wave”movesdownneuron
– – + + + + + +– + + + + + +
– – + + + + + +– + + + + + +
+ + – – – – – –+ – – – – – –
+ + – – – – – –+ – – – – – –Na+
wave→
The rest of the dominoes fall!
Howdoesanerveimpulsetravel?
• Re-set:2ndwavetravelsdownneuron• K+channelsopenupmoreslowlythanNa+channels
– K+ionsdiffuseoutofcell– chargesreversebackatthatpoint
• nega4veinside;posi4veoutside
+ – – + + + + +– – + + + + +
+ – – + + + + +– – + + + + +
– + + – – – – –+ + – – – – –
– + + – – – – –+ + – – – – –Na+
K+
wave→
Set dominoes back up quickly!
Howdoesanerveimpulsetravel?
• Combinedwavestraveldownneuron– waveofopeningionchannelsmovesdownneuron– signalmovesinonedirec4on→→→→→
• flowofK+outofcellstopsac4va4onofNa+channelsinwrongdirec4on
+ + – – + + + ++ – – + + + +
+ + – – + + + ++ – – + + + +
– – + + – – – –– + + – – – –
– – + + – – – –– + + – – – –Na+
wave→
K+Ready for next time!
Howdoesanerveimpulsetravel?
• Ac4onpoten4alpropagates– wave=nerveimpulse,orac4onpoten4al– brain→finger4psinmilliseconds!
+ + + + – – + ++ + + – – + +
+ + + + – – + ++ + + – – + +
– – – – + + – –– – – + + – –
– – – – + + – –– – – + + – –Na+
K+
wave→
In the blink of an eye!
Voltage-gatedchannels• Ionchannelsopen&closeinresponsetochangesin
chargeacrossmembrane• Na+channelclosedwhennerveisn’tdoinganything.
– Na+channelsopenquicklyinresponsetodepolariza4on&closeslowly
– K+channelsopenslowlyinresponsetodepolariza4on&closeslowly
+ + + + + – + ++ + + + – – +
+ + + + + – + ++ + + + – – +
– – – – – + – –– – – – + + –
– – – – – + – –– – – – + + –Na+
K+
wave→
Structure & function!
Howdoesthenervere-setitself?
• A\erfiringaneuronhastore-setitself– botharemovingagainstconcentra4ongradients
• needapump!!
+ + + + + – – ++ + + + + – –
+ + + + + – – ++ + + + + – –
– – – – – + + –– – – – – + +
– – – – – + + –– – – – – + +Na+Na+Na+
Na+ Na+Na+
K+K+K+K+ Na+ Na+Na+Na+Na+
Na+Na+Na+ Na+
Na+
Na+
K+K+K+K+K+K+ K+ K+
wave→
A lot of work to do here!
Howdoesthenervere-setitself?• _Dominoesneedtosetbackupagain.• Na/KpumpsareoneofthemaindrainsonATPproduc4oninyour
body.
• ac4vetransportproteininmembrane• requiresATP
– _Yourbrainisaveryexpensiveorgantorun!– re-setschargeacrossmembrane
ATP
That’s a lot of ATP ! Feed me some sugar quick!
Neuronisreadytofireagain
Na+ Na+ Na+ Na+ Na+ Na+ Na+Na+ Na+ Na+ Na+ Na+ Na+
Na+ Na+ Na+ Na+ Na+ Na+ Na+
Na+ Na+ Na+ Na+ Na+ Na+
K+K+ K+ K+ K+
K+aa- K+ K+ K+
aa-aa-aa-
aa- aa-
+ + + + + + + ++ + + + + + +
+ + + + + + + ++ + + + + + +
– – – – – – – –– – – – – – –
– – – – – – – –– – – – – – –
resFngpotenFal
Res4ngPoten4alandAc4onPoten4al
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KeyPoints
• GuidedNotesonKeyPointsbeginnow:
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KP1:Res4ngpoten4alisnegaFveinsidethecellandposiFveoutside
• Thisismaintainedby:
• Sodium-potassiumpumpsthatmove– Naoutsideand– Kinside
• Becausethereisadifferenceinchargeinside/outsidethecell,wesaythecellispolarized.
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KP1:Res4ngpoten4alisnegaFveinsidethecellandposiFveoutside
• Thecellsays“Na”toNa(andpumps3out)andsays“OK”topotassium(andpumps2in)
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Sodium/PotassiumPumps
• 3sodium(Na+)ionspumpedoutforevery2potassium(K+)ionspumpedin– RequiresATP!!!
• CreatesanegaFveresFngpotenFal(-70mVchargeinsidethecell)– Aka“polarized”
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KP2:Ac4onpoten4alscausepreviouslygatedNa+andK+channelstoopen
• Theposi4veionsfloodingthecellsignalsneighboringgatedchannelstoopen– Thiscausessignalsto“propogate”orflowdownaneuron
– Asthishappens,thecellisdepolarized(thesepara4onofchargefrominside/outsideislost)
• Thesodium-potassiumpumpsandpotassiumchannelsre-establishres4ngpoten4al(polariza4on)
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Genera4onofAc4onPoten4als:ACloserLook
• Atres4ngpoten4al:1. Mostvoltage-gatedsodium(Na+)channelsandvoltage-gatedpotassium(K+)channelsareclosedduringtheres8ngstate.
OUTSIDEOFCELL
INSIDEOFCELLInacFvaFonloop
Sodiumchannel
Potassiumchannel
Threshold
ResFngpotenFalTime
Mem
bran
epo
tenF
al
(mV)
+50
-100
-50
0
Na+K+
Key
1
1
ResFngstate
• AcFonpotenFalisgenerated:2. Voltage-gatedNa+channelsopenfirstandNa+flowsintothecell,depolarizingit.
3. Duringtherisingphase,ifthethresholdiscrossed,themembranepoten4alincreasestoandpastzero
OUTSIDEOFCELL
INSIDEOFCELLInacFvaFonloop
Sodiumchannel
Potassiumchannel
Threshold
ResFngpotenFalTime
Mem
bran
epo
tenF
al
(mV)
+50
-100
-50
0
Na+
K+
Key
2
1
1
2
ResFngstate
DepolarizaFon
OUTSIDEOFCELL
INSIDEOFCELLInacFvaFonloop
Sodiumchannel
Potassiumchannel
AcFonpotenFal
Threshold
ResFngpotenFalTime
Mem
bran
epo
tenF
al
(mV)
+50
-100
-50
0
Na+
K+
Key
2
1
3
1
2
3
ResFngstate
DepolarizaFon
RisingphaseoftheacFonpotenFal
4.Duringthefallingphase,voltage-gatedNa+channelsinacFvated;voltage-gatedK+channelsopen,andK+flowsoutofthecell
OUTSIDEOFCELL
INSIDEOFCELLInacFvaFonloop
Sodiumchannel
Potassiumchannel
AcFonpotenFal
Threshold
ResFngpotenFalTime
Mem
bran
epo
tenF
al
(mV)
+50
-100
-50
0
Na+
K+
Key
2
1
3
1
2
3
ResFngstate
DepolarizaFon
RisingphaseoftheacFonpotenFal
OUTSIDEOFCELL
INSIDEOFCELLInacFvaFonloop
Sodiumchannel
Potassiumchannel
AcFonpotenFal
Threshold
ResFngpotenFalTime
Mem
bran
epo
tenF
al
(mV)
+50
-100
-50
0
Na+
K+
Key
2
1
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1
2
3
4
ResFngstate
DepolarizaFon
RisingphaseoftheacFonpotenFalFallingphaseoftheacFonpotenFal
5.Duringtheundershoot,membranepermeabilitytoK+isatfirsthigherthanatrest,thenvoltage-gatedK+channelscloseandres4ngpoten4alisrestored.
Hence,thereisanundershootorhyperpolarizaFonthat
persistsun4lthemembranepotassiumpermeabilityreturnstoitsusualvalue.
OUTSIDEOFCELL
INSIDEOFCELLInacFvaFonloop
Sodiumchannel
Potassiumchannel
AcFonpotenFal
Threshold
ResFngpotenFalTime
Mem
bran
epo
tenF
al
(mV)
+50
-100
-50
0
Na+
K+
Key
2
1
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5
1
2
3
4
5 1
ResFngstate Undershoot
DepolarizaFon
RisingphaseoftheacFonpotenFalFallingphaseoftheacFonpotenFal
Na+
AcFonpotenFal
Axon
Plasmamembrane
Cytosol1
K+
K+
Na+
Na+
AcFonpotenFal
Axon
Plasmamembrane
Cytosol
AcFonpotenFal
2
1
K+
K+
K+
K+
Na+
Na+
Na+
AcFonpotenFal
Axon
Plasmamembrane
Cytosol
AcFonpotenFal
AcFonpotenFal
2
1
3
• Duringtherefractoryperioda\eranac4onpoten4al,asecondac4onpoten4alcannotbeini4ated
• Therefractoryperiodisaresultofatemporaryinac4va4onoftheNa+channels
KP3:Inac4vatedNa+channelscausearefractoryperiodwhennoac4onpoten4alcanoccur
• Therefractoryperiodensuresthatac4onpoten4alsonlytravelinonedirec4on(towardsthesynapse)
• Thismakessurethatac4onpoten4alscannotpropagatebackwards
KP3:Inac4vatedNa+channelscausearefractoryperiodwhennoac4onpoten4alcanoccur
KP4:Myelinsheathsincreasethespeedofsignaltransmission
• Ac4onpoten4alspeedincreaseswiththeaxon’sdiameter– Invertebrates,axonsareinsulatedbyamyelinsheath
– Myelinsheathsaremadebyglia*• OligodendrocytesintheCNS• SchwanncellsinthePNS
• *Gliaarenotneurons,theyare“gluingcells”thatholdneuronsinplace,providenutri4on,andinsulatethem.
Axon Myelinsheath
Schwanncell
NodesofRanvier
NodeofRanvier
Layersofmyelin
Axon
SchwanncellNucleusofSchwanncell
0.1µm
The sheaths help signals move more quickly by allowing the action potential to “jump” from each node of Ranvier to the next.
GUIDEDPRACTICE
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Takeamomenttoputtheseinthecorrectorder
• Depolariza4on• Res4ngstate• Repolariza4on• Hyperpolariza4on• Res4ngstate
Sequencedinorderofoccurrence
• Res4ngstate• Depolariza4on• Hyperpolariza4on• Repolariza4on• Res4ngstate
• Res4ngstate• Depolariza4on• Hyperpolariza4on• Repolariza4on• Res4ngstate
?
?
?Time
Mem
bran
epo
tenF
al
(mV)
+50
-100
-50
0
1
2
3
4
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Whatisrepresentedbyeachques4onmark?
• ?:Res4ngPoten4al• ?:Threshold• ?:Ac4onPoten4al
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• Describewhatisoccurringat1,2,3,4,5usingthewords:Res4ngstate,depolariza4on,repolariza4on,andhyperpolariza4on
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• AtResFngPotenFal:• 1.Mostvoltage-gatedsodium(Na+)channelsandvoltage-gated
potassium(K+)channelsareclosedduringtheres8ngstate• AcFonpotenFalisgenerated:• 2.Voltage-gatedNa+channelsopenfirstandNa+flowsintothecell,
depolarizingit.• 3.Duringtherisingphase,ifthethresholdiscrossed,themembrane
poten4alincreasestoandpastzero• 4.Duringthefallingphase,voltage-gatedNa+channelsinacFvated;
voltage-gatedK+channelsopen,andK+flowsoutofthecell• 5.Duringtheundershoot,membranepermeabilitytoK+isatfirsthigher
thanatrest,thenvoltage-gatedK+channelscloseresul4nginrepolariza8onandres8ngpoten8alisrestored
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A(n)___inNa+permeabilityand/ora(n)___inK+permeabilityacrossaneuron’splasmamembranecouldshi\membranepoten4alfrom−70mVto−80mV.
a.increase;increaseb.increase;decreasec.decrease;increased.decrease;decrease
A(n)___inNa+permeabilityand/ora(n)___inK+permeabilityacrossaneuron’splasmamembranecouldshi\membranepoten4alfrom−70mVto−80mV.
a.increase;increaseb.increase;decreasec.decrease;increased.decrease;decrease
a.theres4ngmembranepoten4altodropto0mV.b.theinsideoftheneurontobecomemorenega4verela4vetotheoutside.
c.theinsideoftheneurontobecomeposi4velychargedrela4vetotheoutside.
d.sodiumtodiffuseoutofthecellandpotassiumtodiffuseintothecell.
AddingapoisonthatspecificallydisablestheNa+/K+pumpstoacultureofneuronswillcause
a.theresFngmembranepotenFaltodropto0mV.b.theinsideoftheneurontobecomemorenega4verela4vetotheoutside.
c.theinsideoftheneurontobecomeposi4velychargedrela4vetotheoutside.
d.sodiumtodiffuseoutofthecellandpotassiumtodiffuseintothecell.
AddingapoisonthatspecificallydisablestheNa+/K+pumpstoacultureofneuronswillcause
Namethreespecificadap4ons(transport
proteins)oftheneuronmembranethatallowittospecializeinconduc4on
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(transportproteins)oftheneuronmembrane
• voltage-gatedK+channels,• sodium/potassiumpumps,• voltage-gatedNa+channels.
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IndependentPrac4ce:Labelthegraphoftheac4onpoten4al
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Figure 48.11a
AcFonpotenFal
Threshold
ResFngpotenFal
Time
Mem
bran
epo
tenF
al
(mV)
+50
-100
-50
0
1
2
3
4
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IndependentPrac4ce
CLOSING
• hPp://bcs.whfreeman.com/webpub/Ektron/Hillis%20Principles%20of%20Life2e/Animated%20Tutorials/pol2e_at_3402_The_Ac4on_Poten4al/pol2e_at_3402_The_Ac4on_Poten4al.html
•
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