-
8/7/2019 Physiology 2 Membrane potential
1/18
mediclassonline.comSeries of courses
Physiology 2
Genesis ofmembrane potential
&
Action potential
-
8/7/2019 Physiology 2 Membrane potential
2/18
Genesis of membrane potential
-The membrane potential is alwaysmeasured inside the cell as
outsideenvironment is non static.
-Assume that the flux of ions during an
action potential do not significantly altersthe inside
environment.
-Calculate the Electrochemical gradientacross the
membrane(resting membranepotential)
-
8/7/2019 Physiology 2 Membrane potential
3/18
Genesis of membrane potential
Calculating the resting membranepotential:
The potential is actually the sum of allionic 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 theyalways are open
-
8/7/2019 Physiology 2 Membrane potential
4/18
Genesis of membrane potential
Forces on individual ions:
10mM
1mM
-
8/7/2019 Physiology 2 Membrane potential
5/18
Genesis of membrane potential
Total forces of Na:
- Concentration forces-Electrical forces
Concentration force
Na 10mM Na 1mMElectrical force
Na 60mv Na
-
8/7/2019 Physiology 2 Membrane potential
6/18
Genesis of membranepotential
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 to60)
FZ
Log ten of 1/10 is 1
Therefore we will multiply 60x1 whichis =60
Hence in this case the force
-
8/7/2019 Physiology 2 Membrane potential
7/18
-
8/7/2019 Physiology 2 Membrane potential
8/18
Genesis of membrane potential
Equilibrium potential
10mM
1mM60
60
Net force= 0
+
-
8/7/2019 Physiology 2 Membrane potential
9/18
Genesis of membrane potentialNa equilibrium potential is in + as
you have to puta +ve potential
Inside also
Na+
60mV
60mV
+ +
-
8/7/2019 Physiology 2 Membrane potential
10/18
Genesis of membrane potential
For K+ we have to have a ve membrane potentialin equilibrium as
it is going out .
Equilibirium potential for K = -90mv
K
K+ 134mNK+ 4.2mM
+
-
-
8/7/2019 Physiology 2 Membrane potential
11/18
Genesis of membrane potential
Concentration of ions inside and outside
mammalian spinal motor neurons
Ion
Concentration(mmol/L of H20)
Inside CellOutside cell
EquilibriumPotential
(mV)
Na + 15 150 +60
K +150
5.5 -90
-
8/7/2019 Physiology 2 Membrane potential
12/18
Genesis of membrane potential
Resting membrane potential:
RMP can be calculated by Goldmanconstant-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 andoutside in this
equation and you will get theresting membrane potential.
-
8/7/2019 Physiology 2 Membrane potential
13/18
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 toopening of their
respective gates
It occurs in Neurons, skeletal muscles,smooth muscles and
Cardiac muscles
It is all-or-non in character
-
8/7/2019 Physiology 2 Membrane potential
14/18
Action potential in sequence
A i i l
-
8/7/2019 Physiology 2 Membrane potential
15/18
Action potential
Action potential occurs due to incoming of
Na from outside to inside which brings the+ve ions, the
negativity inside the cellchanges to positivity
Na gates are voltage gated. Open onlywhen membrane potential
reachesthreshhold which is about 15mV above theresting mem.
potential.
This is depolarisation phase
Than K gates open and K starts going out,+ve charge goses out
and again negativity
-
8/7/2019 Physiology 2 Membrane potential
16/18
Action potential
-70
-55
0
+55
Depolariz
ation----
Repolariz
atio
n-----
-
8/7/2019 Physiology 2 Membrane potential
17/18
Action potential
-70
-55
0
+55
Absolute refractory period Relative refractory period
-
8/7/2019 Physiology 2 Membrane potential
18/18
Action potential
