RESTING MEMBRANE POTENTIAL

Presents to you by ABOUT DISEASE.CO TEAM

OBJECTIVESBy 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 Goldmann-Hoghkin-Katz

equation to calculate the RMP. Explain the contribution of Sodium-Potassium Pump to the

RMP.

Points to Note:i. Opposite charges (– and +) are attracted to each other, but two

charges of the same type (– and –, or + and +) repel each other.ii. A concentration gradient can exist for molecules/

particles and ions. iii. A CHEMICAL gradient can exist in the presence of an

ELECTRICAL gradient.

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 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 Voltage Gated

Channels

MEMBRANE POTENTIAL :

MEASURING THE RESTING MEMBRANE POTENTIAL

Electrodes are created from hollow glass tubes drawn to very fine points. These micropipettes are filled with a liquid that conducts electricity and then connected to a voltmeter, which measures the electrical difference between two points in units of either volts (V) or millivolts (mV). A recording electrode is inserted through the cell membrane into the cytoplasm of the cell. A reference electrode is placed in the external bath, which represents the extracellular fluid. When the recording electrode is placed inside a living cell, the voltmeter measures the membrane potential, in other words, the electrical difference between the intracellular fluid and the extracellular fluid.

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 POTENTIALDEFINITION:

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: Concentration gradients of various ions across the cell

membrane. Membrane Permeability to those ions.

QuestionWe 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 : ICFNa+:- 150 : 15K+:- 5 : 150

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 Resting Membrane Potential is mostly due to Potassium Ions

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 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.

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 EQUATIONThe Nernst equation can be used to calculate 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:

NERNST EQUATION 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 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:

i. It keeps in mind the concentration gradients of each of the ions contributing to the RMP.

ii. It keeps in mind the membrane permeability of all the ions contributing to the RMP

iii. It can thus be used to calculate the RMP when multiple ions are involved rather than when only single ions are involved.

EquationEMF= 61.log CNa i.PNa + Cki. Pk + CcloPcl

CNao.PNa + Cko.Pk + CcliPcl

= —90 mv

What is the Physiological basis of the Resting Membrane Potential?PHYSIOLOGICAL BASIS OF THE RMP:-Calculation through the Nernst Equation and Goldman-

Hodgkin-Katz equaation (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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