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 Resting Membrane Potential.
•Give the physiological basis of Resting membrane potential.
•Use the Nernst equation and Goldman-Hodgkin-Katz equation to calculate the RMP.
•Explain the contribution of Sodium-Potassium Pump to the RMP.
Points to Note:
1.Opposite charges(–and +) are attracted to each other, but two charges of the same type
(–and –, or + and +) repel each other.
- A concentration gradient can exist for molecules/ particlesand ions.
- A CHEMICAL gradientcan exist in the presence of an ELECTRICAL gradient.
SEMIPERMEABLE MEMBRANE
If the membrane is impermeable or semi-permeable, how do we make it selectively permeable to a specific ion?
By introducing channels or carrier proteins.
THE ROLE OF 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 pass through.
- Gated channels:
The gates are part of the protein channel and can open or close in response to certain stimuli. E.g.
-Mechanically Gated Channels
-Ligand Gated Channels
-Voltage Gated Channels
MEMBRANE POTENTIAL
Measuring the Resting Membrane Potential
Separation of Charges is called Membrane Potential
MEMBRANE POTENTIAL
DEFINITION:
•It is the separation of charges across the membrane.
OR
•It is the difference in the relative number of cations & anions in the ICF & ECF.
PHYSIOLOGICAL BASIS OF RESTING MEMBRANE POTENTIAL IN A NERVE FIBRE:
RESTING MEMBRANE POTENTIAL
DEFINITION:
The constant membrane potential present in all living cells when they are at rest (i.e. when they are not producing any electrical signals) is called their Resting membrane potential.
All living cells show resting membrane potential which results from the uneven distribution of ions across the cell membranes.
2 Factors influence the Membrane Potential:
1.Concentration gradients of various ions across the cell membrane.
2.Membrane Permeability to those ions.
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
These potential differences between the different ions across the ECF and ICF is maintained by the Na-K PUMP.
Various ions try to diffuse from one side of the membrane to the other depending upon their electrochemical gradients:
The Resting Membrane Potential is mostly due to Potassium Ions
The neuron plasma membrane at rest is 25-30 timesmore permeable to K ions than to the Na ions!!!!
This is through the help of the Potassium leak channels…
So, Now:
Electrical gradient Chemical gradient
for K+for K+
ThisisthemembranepotentialatwhichtheelectricalgradientexactlyopposestheconcentrationorchemicalgradientanditiscalledtheEquilibriumpotentialortheNernstPotentialforPotassium.
UsingtheNernstequation,whentheNernstpotentialforPotassiumiscalculated,itis-94mv.
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 the Nernst potential for any univalent ion at normal body temperature:
EMF= ±61 log Conc. Inside
Conc. Outside
Nernst Potential for Different Ions
Using the Nernst equation, the Nernst Potential for different ions can be calculated.
•Na: + 66mv
•K: —94mv
•Cl: —90mv
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
Point to Remember:
The greater the permeability of the plasma membrane for a given ion, the greater is the tendency for that ion to drive the membrane potential toward the ion’s own equilibrium potential.
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 Sodiumions. 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 3advantages:
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.
What is the Physiological basis of the Resting Membrane Potential?
PHYSIOLOGICAL BASIS OF THE RMP:
-Calculation through the Nernst Equation and Goldman-Hodgkin-Katz equation (Mushtaq: chapter 2, NEURONS & SYNAPSES, page: 102-108, 5thedition).
-Calculation through the Goldman-Hodgkin-Katz equation (Guyton: chapter 5, page: 62, 13thedition)
RMP
•POINT TO NOTE:
Resting Membrane Potential is DETERMINED by the POTASSIUM IONS and has a value of ‒90 mv.