DrViral I Champaneri MD Assistant Professor Department of Physiology 1 Learning Objectives Stages of Action Potential Repolarization Positive afterpotential After hyperpolarization ID: 913944
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Slide1
Nerve Action Potential :2
Dr.Viral I. Champaneri, MDAssistant ProfessorDepartment of Physiology
1
Slide2Learning Objectives
Stages of Action Potential Repolarization
Positive
afterpotential
After
hyperpolarization
Resting membrane potential of neuronEffect of Increase / Decrease level of Na+ Effect of Increase / Decrease level of K+ Role of other Ions
2
Slide32.
Depolarization Overshoot
In Large nerve fibers,
Membrane potential
“
Overshoot”Beyond zero level Becomes Somewhat positive3
Slide42.
Depolarization Overshoot
Smaller fibers and
Many Central nervous system
(CNS)
neurons
The potential merely approaches the zero levelDoes not overshoot to the positive level4
Slide52.
Depolarization Overshoot
During overshootDirection of electrical gradient
For Na
+
is
reversed5
Slide62.
Depolarization Overshoot
Because Membrane potential is reversed
Limits Na+
influx
Voltage gated K
+ channels Open6
Slide7Rising membrane potential
Within Fraction of a millisecondCauses
7
Slide8Rising membrane potential
Beginning of Closure of Sodium channels Opening of
Potassium channelAction potential terminates
8
Slide93.
Repolarization stage of action potentialWithin few
10,000ths of a secondNa+
channels begin to close After membrane
becomes
Highly permeable to K
+ ions9
Slide103.
Repolarization stage of action potential
10
Slide113.
Repolarization stage of action potentialThe K
+ channels open
More than normallyRapid diffusion of K
+
ions to the exterior
(Higher Concentration to Lower Concentration)11
Slide123.
Repolarization stage of action potentialRe-established
The normal negative resting membrane potential (RMP: -90 mV ) called
Repolarization
12
Slide133.
Repolarization stage of action potentialOpening of the voltage-gated K
+ channelsSlower & more prolonged
Than
Opening of the Na+ channels13
Slide143.
Repolarization stage of action potentialIncrease in K
+ conductance Comes after
The increase in Na+
conductance
14
Slide15Conductance
of Na
+
ion channels
15
Slide16Conductance of the K
+ channelsWhere as the potassium channels
Only open
(Activate)And the
rate of opening
is much
Slower than for sodium channel (Prolonged)16
Slide17During the Resting stage:
The Gate of the potassium channel is Closed
Voltage-Gated Potassium Channel
17
Slide18Potassium ions
are Prevented from passing through this channel To the exterior
Voltage-Gated Potassium Channel
18
Slide19When membrane potential rises
From -90mV Towards ZeroVoltage change
Cause slow conformational opening
of the gateAllows
increased potassium diffusion outward
Voltage-Gated Potassium Channel
19
Slide20K
+ channels open Just at the same time Na
+ channels
Beginning to close Due to
I
nactivationSlowness of the K
+
Channels
20
Slide213.
Repolarization stage of action potentialThe net movement of positively charge
Out of the cell Due to
K+
efflux
Completes The process of repolarization21
Slide22Stages of Nerve Action Potential
Resting stage
Depolarization stage and Overshoot
Repolarization stage
After-
hyperpolarization
22
Slide234. “
Positive” After potentialMembrane potential becomes
more negativeThan
Original RMP (- 90 mV)
For
few milliseconds
After action potential Over23
Slide244. “
Positive” After potential“Positive” after potential
is Misnomer
Because positive
afterpotential
Is even more negative Than resting membrane potential (RMP =-90mV)24
Slide254. “
Positive” After potentialReason for calling it “Positive”
HistoricallyThe
first potential measurement Were
made on
The
outside of the nerve fiber membrane Was Positive25
Slide264. “
Positive” After potentialThan
The inside
When measured on the outsideThis
potential causes
a
positive recordRather than a negative one26
Slide274. “
Positive” After potentialCause of the positive
afterpotential M
ainlyMany potassium channels
Remain open for several milliseconds
After complete
repolarization of the membrane27
Slide28-90
28
-65
Slide294.
After-hyperpolarizationThe slow return of the K
+ channels
To the closed state explain
After-
hyperpolarization
F/b return To the resting membrane potential29
Slide305.
End of action potentialVoltage-gated K+
channels Bring the action potential
To the end
Cause closer of their gates through
Negative feedback process30
Slide31Negative feedback loop
during Repolarization
31
Slide32Resting Membrane Potential in
NeuronsAbout -70mV
Close to the equilibrium potential for K
+Because there are
more open K
+
channelsThan Na+ channels at restMembrane permeability to K+ is greater at rest32
Slide33Resting Membrane Potential in
neuronsIntracellular and extracellularConcentration of K
+
Prime determinant of the RMP (Nernst potential)Therefore
RMP is close to equilibrium potential of K
+
33
Slide34Decrease ECF level of Na
+AP
Decrease ECF [Na+]
Hyponatraemia
The
external level of Na
+ concentrationReduce the size of action potential34
Slide35Depolarization stage of action potential
35
Slide36Decrease ECF level of Na
+ RMP
Hyponatraemia
Little effect on the RMP
Because
Permeability of the membrane to
Na+ at rest is relatively low36
Slide37Decrease
/ Increase ECF level K+
Resting membrane potential Is close to equilibrium potential for K
+Change in external concentration of K
+
ions
Major effects on the RMP37
Slide38Increase ECF level K
+ Hyperkalemia
ECF level of K
+ is increased
Hyperkalemia
The
RMP ( of Neuron : -70 mV) moves closer To the threshold for eliciting an action potentialNeuron becomes More excitable
38
Slide39Decrease ECF level K
+ Hypokalemia
ECF level of K
+ is Decreased
H
ypokalemia
RMP (-70mV) ReducedNeuron Hyperpolarized39
Slide40Role of other Ions During the Action Potential
Impermeant Negatively Charged Ions (Anions) inside the Axon
Calcium Ions
40
Slide41Impermeant
Anions inside the axonInside the axonMany negatively charged ions (Anions
)That can not go through the membrane channels
41
Slide42Impermeant
Anions inside the axonIncludes
Anions of the Protein molecules
Anions of many Organic phosphate
compounds
Anions of
Sulfate compounds42
Slide43Impermeant
Anions inside the axonBecause these ions Cannot leave the
interior of the axon43
Slide44Impermeant
Anions inside the axonExcess of these impermeant anions
Deficit of positive ions inside the membrane44
Slide45Impermeant
Anions inside the axonResponsible For the negative charge inside the fiber
When there is deficit
of positive charged K
+
And other positive ions45
Slide46Calcium Ions
Membranes of almost all cells of the bodyHave Ca
2+ pump Similar to Na
+ pump
46
Slide47Calcium ions serves
Along with or Instead of Na
+ In some cells
To cause most of action potential
47
Slide48Calcium pump
Like Sodium (Na+
) pump Pumps Ca2+
ions
48
Slide49Calcium pump
Ca2+
From the interior
To the exterior
of the cell membrane
Or
To endoplasmic reticulum (ER)49
Slide50Calcium ions gradient
Of 10,000 folds due to it
Internal cell concentration of calcium ions of
10-7 molar
External concentration
of 10
-3 molar50
Slide51Voltage gated Ca
2+ ChannelsSlightly permeable to Na+
ions alsoWhen channels open
Both Ca
2+
and Na
+ ions Flow exterior of the fiber51
Slide52Ca
2+-Na+ Channels Slow channels
Slow to activation
Require 10-20 times a long for activation
As the
Sodium channels Fast channels52
Slide53Ca
2+ channels Numerous
Cardiac muscle Smooth muscle
53
Slide54Some types of Smooth Muscle
Fast sodium channels Hardly present
54
Slide55Some types of Smooth Muscle
Action potential caused Entirely by
Activation of the slow calcium channels
55
Slide56Mechanism
: Ca2+ affect the Na+ channel
Ca2+ ions bind
To the exterior surfaces
of
The Na+ channel protein molecules56
Slide57Mechanism
: Ca2+ affect the Na+ channel
Positive charges of Ca2+ ions
In turn
Alter the electrical state of the channel protein
itself
57
Slide58Mechanism
: Ca2+ affect the Na+ channel
Altering the voltage level required To open
The sodium gate
58
Slide59Voltage-Gated Sodium Channel
Inside
Outside
59
Slide60Deficit of Calcium Ions
(Hypocalcaemia)Na+
channels become activated (Opened)By very little increase
Of the membrane potential
From normal very negative level
60
Slide61Calcium Ions
falls 50% below normalSpontaneous discharge in peripheral nerves
61
Slide62Calcium Ions
falls 50% below normalOften causing muscle “Tetany
”
Lethal
Death
Tetanic contraction of the respiratory muscles62
Slide63Attend Your Roll Call
63