physiology, cell membrane
TRANSCRIPT
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mediclassonline.com
Series of courses
Physiology 2
Genesis of membrane
potential
&
Action potential
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Genesis of membrane potential
-The membrane potential is always measured inside
the cell as outside environment is non static.
-Assume that the flux of ions during an action potential
do not significantly alters the inside environment.
-Calculate the Electrochemical gradient across the
membrane(resting membrane potential)
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Genesis of membrane potential
Calculating the resting membrane potential:
The potential is actually the sum of all ionic forces
acting across the membrane.
The major ions involved are Na, Cl, and K
Na have voltage regulated gates on their channels
Cl and K have ungated channels so they always are
open
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Genesis of membrane potential
Forces on individual ions:
10mM
1 mM
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Genesis of membrane potential
Total forces of Na:
- Concentration forces-Electrical forces
Concentration force
Na 10mM Na 1mMElectrical force
Na 60mv Na
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Genesis of membrane potential
Nernst Equation
Ex = RT log [Xo]FZ [Xi]
Where R: Gas constant
T:Absolute temperatureF: Faraday ( Coulombs/mol)
Z: Valence (- or+)
The RT IS constant is calculated as 61.5 ( rounded to 60)FZ
Log ten of 1/10 is 1
Therefore we will multiply 60x1 which is =60Hence in this case the force
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Genesis of membrane potential
Net force on Na
10mM
1mM60
60
Net force= 60+60
=120
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Genesis of membrane potential
Equilibrium potential
10mM
1mM60
60
Net force= 0
+
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Genesis of membrane potentialNa equilibrium potential is in + as you have to put a +ve
potential
Inside also
Na+
60mV
60mV
+ +
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Genesis of membrane potential
For K+ we have to have ave membrane potential in equilibrium
as it is going out .
Equilibirium potential for K = -90mv
K
K+ 134mNK+ 4.2mM
+ -
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Genesis of membrane potential
Concentration of ions inside and outside mammalian
spinal motor neurons
Ion
Concentration
(mmol/L of H20)
Inside Cell Outside cell
Equilibrium
Potential
(mV)
Na + 15 150 +60
K + 150 5.5 -90
Cl- 9 125 -70
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Genesis of membrane potential
Resting membrane potential:
RMP can be calculated by Goldman constant-field
equation:
V= RT in Pk[Ko+] + P Na [Nao+] + PCl [Cli-]
F Pk [Ki+] + P Na[Nai
+] + PCl [Clo
-]
Put all relevant ionic concentration inside and outside in
this equation and you will get the resting membranepotential.
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Action potential
It is the rapid depolarization followed by
repolarization of a cell. Occurs in excitable cells (Nerve, muscle etc)
It is due to rapid movements of ions due to opening
of their respective gates It occurs in Neurons, skeletal muscles, smooth
muscles and Cardiac muscles
It is all-or-non in character
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Action potential in sequence
A t t t
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Act on potent a
Action potential occurs due to incoming of Na from
outside to inside which brings the +ve ions, the
negativity inside the cell changes to positivity
Na gates are voltage gated. Open only when
membrane potential reaches threshhold which isabout 15mV above the resting mem. potential.
This is depolarisation phase
Than K gates open and K starts going out, +ve charge
goses out and again negativity is restored.
This is repolarizatioin.
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Action potential
-70
-55
0
+55
Threshold
overshoot
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Action potential
-70
-55
0
+55
Absolute refractory period Relative refractory period
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Action potential