RESTING MEMBRANE POTENTIALS

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RESTING MEMBRANE
POTENTIAL
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 & 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 RMP
REMEMBER:
CONCENTRATION GRADIENT:
REMEMBER:
A concentration gradient can exist for
molecules/ particles and ions. Thus, a CHEMICAL
gradient can exist in the presence of an
ELECTRICAL gradient.
LIPID
BILAYER
1. The membrane
is electrically
NEUTRAL!
2. The membrane
carries NO
charge!
3. The membrane
is SELECTIVELY
permeable.
SEMIPERMEABLE MEMBRANE
If the membrane is impermeable
or semi-permeable, THEN,
How do we make it
selectively permeable to
a specific ion?
The Role of Ion Channels
The role of Ion channels
The ion channels can be of 2 main types:
1. 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 potential
NERNST EQUILIBRIUM/
EQUILIBRIUM POTENTIAL:
ECF: Less +, more -
ICF: more +, less -
ECF:
ICF:
ECF: 3+, 5-
ICF: 5+, 5-
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.
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. Outside
PHYSIOLOGICAL BASIS OF RESTING
MEMBRANE POTENTIAL IN A NERVE
FIBRE:
MEMBRANE POTENTIAL
DEFINITION:
• The separation of charges across the
membrane.
OR
• The difference in the relative number of
cations & anions in the ICF & ECF.
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.
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??
Resting Membrane Potential in Neurons
There is a great difference in
the chemical composition of
nerve cell interior(ICF) &
exterior (ECF).
ECF
Na+:K+:-
142
4
: ICF
:
:
14
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.
Various ions try to diffuse from one side of the membrane to the
other depending upon their electrochemical gradients:
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....
So, Now:
Electrical gradient
for K+
Chemical gradient
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.
CALCULATING THE RMP:
• The RMP can be calculated using one of the 2
equations:
1. NERNST EQUATION
2. GOLDMAN’S OR GOLDMANN-HODGKIN-KATZ
EQUATION
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
Calculating the RMP by the
GOLDMAN-HODGKIN-KATZ equation:
Has 3 advantages:
1. It keeps in mind the concentration gradients of each of
the ions contributing to the RMP.
2. It keeps in mind the membrane permeability of all the
ions contributing to the RMP
3. It can thus be used to calculate the RMP when multiple
ions are involved rather than when only single ions are
involved.
4. EMF= 61.log CNa i.PNa + Cki. Pk + CcloPcl
CNao.Pna + Cko.Pk + CcliPcl
= —90 mv
PHYSIOLOGICAL BASIS OF THE RMP:
-Calculation through the Nernst Equation
(Mushtaq: chapter: 2, NEURONS & SYNAPSES,
page: 102-108, 5th edition).
- Calculation through the Goldman-Hodgkin-Katz
equation (Guyton: chapter 5, page: 59-60, 12th
edition)
RMP
• POINT TO NOTE:
Resting Membrane Potential
is DETERMINED by the
POTASSIUM IONS and has a
value of ‒90 mv.

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