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Membrane potential
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There is an unequal distribution of ions
The membrane is permeable to these
ions
Potential dierence across themembrane
Membrane potent
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Membrane potential atwhich there is no net
movement of given ionacross the membrane
(i.e. inux=eux)
Equilibrium potential
At equilibrium, forces balance
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oncentration of some ions insi!e "outsi!e mammalian spinal motor
neurons#on $#ntracellular%$&xtracellular%
(mmol')(mmol')
a* + +-
* +- .
l/ 0 +1
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For Cl-,
oncentration gra!ient inwar!&lectrical gra!ient outwar!
For K+
,oncentration gra!ient outwar!&lectrical gra!ient inwar!
For Na+,2oth gra!ients inwar!
3orces acting on ions4
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&quilibrium potential for given ion4(Eion)5escribe! b6 ernst equation,
where,o4 concentration outside the
celli4 concentration inside the
cell74 the valence of the ionEl = /8- m9
E = /0- m9
Ea = *:- m9
t t t
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est ng em rane otent a(RMP)
;he potential !i
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Genesis & Ionic basis of RMP:
K+eu passivel6 through lea6 channels
K+permeabilit! is more than that of a*
(a*inux !oes not compensate for the *eux )
ontribution of "a#K $%Pase pump
b6 maintaining the ionic gra!ients
b6 electrogenic nature
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E'
I'
>
1
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erve cells respon! to electrical,
chemical, or mechanical stimuli
?h6siochemical !isturbances4
ocal, non/propagate! potentials
(gra!e!, s6naptic, generator potentials)
?ropagate! potentials
(Action potentials or nerve impulses)
erve excitablit6
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?@A#B&5 B;A;&A net positive charge outsi!e " a netnegative charge insi!e the membrane
5&?@A#7A;#@Membrane potential becomes less Cvean! move towar!s *ve
DE?&?@A#7A;#@Membrane potential becomes more Cve
than M?
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$ction potential
Rapid change in the membrane potential in
response to a threshold stimulus, leads to
propagation of impulse in an excitable cell
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5evelopment an! ionic
basis of nerve actionpotential
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Membranepotential(mV)
+30
0
-55
-70
Time (ms)
Rap
id
depo
lariz
ati
on
Initial depolarization
Threshold potential
Rep
olariz
ation
Hyperpolarization
Depolarizin stim!l!s
"#ershoot
RM$
Threshold potential (firing level)
?hases of the Action ?otential
RM$
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#onic basis of !i
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on c as s o erenphases of A?
Rapi* *epolariation4 api! so!ium
inux through voltage gate! a* channels
Pea- .alue4 #nactivation of voltage/gate!
a* channels an! a* inux stops
Repolariation p,ase4 voltage gate! *channels open an! * eux starts
/!perpolariation4 continuous eux of* !ue to prolonge! opening of *
channels
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?roperties of actionpotential
?ropagation
efractor6 perio!
All or none law
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?ropagation of A?
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Continuousconductioninunm6elinate!neurons
Saltatoryconductionin
m6elinate! neurons
e n an sa tator
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6e n an sa tator6con!uction
Bchwann cells in the peripheralnervous s6stem " @ligo!en!roc6tesin the B
a* channels are concentrate! inthe no!es of anvier
saltator6 = Fto leapF
B l ! i
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Baltator6 on!uction
M6elin is an e
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@rtho!romic " Anti!romicon!uction
#mpulses pass from s6naptic Hunctions or receptors along
axons to their termination. Buch con!uction is calle!ort,o*romic0
on!uction in the opposite !irection is calle! anti*romic0
2ecause s6napses permit con!uction in one !irection onl6
An anti!romic impulse will fail to pass the Irst s6napse
the6 encounter an! !ie out at that point.
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efractor6 perio!
;he perio! !uring action potential, atwhich a secon! stimulus will notpro!uce secon! response
;he neuron is refractor6 to the secon!stimulus
;wo t6pes4
Absolute refractor6 perio!
elative refractor6 perio!
f i !
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Membrane
potential(mV)
+30
0
-55
-
70
Time (ms)
efractor6 perio!
%tart o& a&ter'depolarization
3 o& Repolarization
Threshold
Absolute refractor6perio! elative refractor6 perio!
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Absolute efractor6 ?erio!
3rom Iring level to +'> ofrepolariGation.
secon! stimulus will not excite thenerve, no matter how strong the
stimulus is.
9oltage gate! a* channels either
alrea!6 open or inactivate!
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elative efractor6 ?erio!
3rom latter 1'> of repolariGation to startof after !epolariGation
Btronger secon! stimulus can causeexcitation
Bome of the 9oltage gate! a*channels but not all have re/opene!
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All/or/one ?rinciple
A sub/threshol! stimulus !oes not elicitaction potential
A threshol! stimulus elicit full/e!ge! A?
A stimulus more than threshol! elicit A?
with same amplitu!e as that cause! b6the threshol! stimulus.
J ! ! i l
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Appl6ing sub/threshol! stimuli of Ixe! !uration lea!sto a localiGe! !epolariGing potential calle! gra!e!potential
Jra!e! potential rises an! !eca6s exponentiall6 with
time.
As the strength of the current is increase!, theresponse is greater !ue to the increasing a!!ition of
a local responseof the membrane
+ m9 of !epolariGation (potential of C m9), the1ring le.el is reache! an! an action potential
occurs.
Jra!e! potential
Jra!e! potential
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Jra!e! potential
(5epolariGing
gra!e! potential)
(h6perpolariGing gra!e! potential))
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GR$2E2P3%E"%I$45
$'%I3"P3%E"%I$45
on propagate! ?ropagate!Das no threshol! Das threshol!
5epolariGation orh6perpolariGation
@nl6 !epolariGation
5oes not obe6 all ornone law
@be6 all or none law
o refractor6 perio! Das refractor6 perio!
an summate annot summate
5epen! on ph6sio/chemical changes
5epen!s on voltagegate! channels
