By Dr Ayisha Qureshi Assistant Professor Physiology MBBS MPhil OBJECTIVES By the end of this lecture you should be able to Define Nernst potential Use the Nernst equation to calculate the values of Nernst potential for Na K amp Cl ID: 554414
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Slide1
RESTING MEMBRANE POTENTIAL
By
Dr.
Ayisha
Qureshi
Assistant Professor, Physiology
MBBS, MPhilSlide2
OBJECTIVES
By the end of this lecture, you should be able to:
Define Nernst potential
Use the Nernst equation to calculate the values of Nernst potential for Na, K & Cl
Define and give the
physiological basis of Resting membrane potential
Use the
Goldmann
-
Hoghkin
-Katz equation to calculate the RMP
Explain the contribution of Sodium-Potassium Pump to the RMPSlide3
REMEMBER:Slide4
CONCENTRATION GRADIENT:Slide5
REMEMBER:
A
concentration gradient can exist for molecules/ particles and ions. Thus, a
CHEMICAL gradient
can exist in the presence of an
ELECTRICAL gradient
. Slide6
LIPID BILAYER
The membrane is electrically NEUTRAL!
The membrane carries NO charge!
The membrane is SELECTIVELY permeable.Slide7
SEMIPERMEABLE MEMBRANESlide8
If the membrane is impermeable or semi-permeable, THEN,
How do we make it selectively permeable to a specific ion?Slide9
The Role of Ion ChannelsSlide10
The role of Ion channels
The ion channels can be of 2 main types:
Leak channels:
Include ion channels specific for Na
+
, K
+
,
Cl
-
etc. As long as the size of the ion is appropriate, the ion will go through them.
2.
Gated channels:
The gates are part of the protein channel and can open or close in response to certain stimuli.
Ligand Gated Channels
– Channels which are opened through ligand binding (the ligand can be a hormone or a neurotransmitters or some other chemical.)
Voltage Gated Channels
– Channels which are opened by changes in the membrane potentialSlide11
Nernst equilibrium/ Equilibrium potential: Slide12
ECF:
Less +, more -
ICF
:
more +, less
-Slide13
ECF:
ICF:
Slide14
ECF:
3+, 5-
ICF:
5
+, 5-Slide15
NERNST EQUILIBRIUM/ EUILIBRIUM POTENTIAL
“
The membrane potential at which the
electrical gradient
exactly opposes the concentration or
chemical gradient
is called the Equilibrium potential.”
It is calculated by the Nernst equation.
At this potential, the net movement of that particular ion STOPS. Slide16
NERNST EQUATION
The Nernst equation can be used to calculate Nernst potential for any
univalent ion
at normal body temperature:
EMF= ±61 log
Conc
. Inside
Conc
.
OutsideSlide17
Physiological basis of resting membrane potential in a nerve fibre:Slide18Slide19
MEMBRANE POTENTIAL
DEFINITION:
The separation of charges across the membrane.
OR
The difference in the relative number of
cations
& anions in the ICF & ECF.Slide20
RESTING MEMBRANE POTENTIAL
DEFINITION:
The constant membrane potential present in the cells of excitable & non-excitable tissues when they are at rest (i.e. when they are not producing any electrical signals) is called their Resting membrane potential. Slide21
We know that the Resting Membrane Potential of human nerve cell membrane is —90
mv
.
What is the Physiological Basis of this RMP & how is it calculated??Slide22
Resting Membrane Potential in Neurons
There is a great difference in the chemical composition of nerve cell interior(ICF) & exterior (ECF).
ECF
:
ICF
Na
+
:- 142 : 14
K
+
:- 4 : 140
The nerve cell
interior (ICF)
is rich in
potassium ions (K)
and
negatively charged proteins
while the
ECF
is rich in
Sodium
&
Chloride
ions.Slide23
Various ions try to diffuse from one side of the membrane to the other depending upon their electrochemical
gradients:Slide24
The neuron plasma membrane at rest is
100 times
more permeable to K ions than to the Na ions!!!!
This is through the help of the
Potassium
leak channels....Slide25Slide26
So, Now:
Electrical gradient Chemical gradient
for K
+
for K
+
This is the
membrane potential at which the
electrical gradient
exactly opposes the concentration or
chemical gradient
and it is
called the Equilibrium
potential or the Nernst Potential for Potassium.
Using the Nernst equation, when the
Nernst potential for Potassium
is calculated, it is -94 mv. Slide27
CALCULATING THE RMP:
The
R
MP can be calculated using one of the 2 equations:
NERNST EQUATION
GOLDMAN’S OR GOLDMANN-HODGKIN-KATZ EQUATIONSlide28
Calculating the RMP by the Nernst Potential:
Potassium ions:
Nernst Potential for
K+
= —94mv
Sodium ions:
A very small number of Sodium ions move to the inside of the nerve cell despite a low permeability of the membrane to the
Sodium
ions. This is because of the small no. of Sodium leak channels present. They make a contribution of a small amount of electro positivity to the cell interior.
Its value is=
+8mv
Sodium-Potassium Pump:
expels 3 Na
+
in exchange for 2 K
+
.
It contributes=
—4 mv
So the total Resting Membrane Potential of a nerve cell is:
RMP= —94 +8 —4 (mv)
= —90 mv Slide29
Calculating the RMP by the
GOLDMAN-HODGKIN-KATZ equation:
Has
3
advantages:
It keeps in mind the concentration gradients of each of the ions contributing to the RMP.
It keeps in mind the membrane permeability of all the ions contributing to the RMP
It can thus be used to calculate the RMP when multiple ions are involved rather than when only single ions are involved.
EMF= 61.log
C
Na
i
.
P
Na
+
C
k
i
.
P
k
+
C
cl
o
P
cl
C
Na
o
.
Pna
+ Cko.Pk +
CcliPcl
= —90 mvSlide30
PHYSIOLOGICAL BASIS OF THE RMP:
-Calculation through the Nernst Equation (
Mushtaq
: chapter: 2, NEURONS & SYNAPSES, page: 102-108, 5
th
edition).
- Calculation through the Goldman-Hodgkin-Katz equation (Guyton: chapter 5, page: 59-60, 12
th
edition)Slide31
RMP
POINT TO NOTE:
Resting Membrane Potential is
DETERMINED
by the
POTASSIUM IONS
and has a value of
‒
90 mv.