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2. BIOPHYSICS – Membrane Potentials and Action Potential.pptx

This document provides an overview of membrane potential and action potential, explaining concepts such as resting membrane potential, the role of the sodium-potassium pump, and the Nernst and Goldman equations. It details the factors influencing resting membrane potential, including ion concentration gradients and membrane permeability, as well as the dynamics of action potential. The document emphasizes the variability of resting membrane potential across different cell types and the need for individual ion considerations in calculations.

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BIOPHYSICS – ELECTROPHYSIOLOGY Membrane Potential & Action Potential Department of Physiotherapy M.L.B Paramedical Training College Jhansi Dr Gaurav Saxena Senior Teaching Faculty ( Assoc. Prof. ) & Head of Department
Membrane Potential
Introduction
Introduction  Any biological cell consist of various ions  These ions are located either inside or outside the cell membrane  Intra cellular matrix – Inside the cell  Extra cellular Matrix- Out side the cell  This concentration of ions in ECM or ICM is not equal, this difference in concentration of ion and charge generate a electrical potential known as Membrane potential  Also known as – Transmembrane potential  Membrane potential which is resulted from equilibrium in diffusion of all ions is known as Resting membrane potential
Ion concentration in ECM and ICM
Ion concentration in ECM and ICM
 There are three main elements of Resting membrane potential Concentration difference of ions Charge difference of ions Cell wall permeability Main factor that causes membrane potential is “diffusion”
Diffusion
Osmosis
DiffusionVs Osmosis
Sodium potassium pump  NA-K pump is a protein that has been identified in many cells that maintains  the internal concentration of potassium ions [K+] higher than that in the surrounding medium (blood, body fluid, water) &  Maintains the internal concentration of sodium ions [Na+] lower than that of the surrounding medium.  It is an enzyme that uses metabolic energy to transport (pump) Na+ outward and K+ inward.  The Resting membrane potential of cells and related bioelectric phenomena such as the action potential depend on the steady state difference in concentrations of Na+ and K+ maintained by the pump.
NA- K pump uses ATP to move 3 sodium ion out and 02 potassium ions in continuously to maintain the concentration gradient for potassium and sodium
Movement of a ion Ex. Potassium  K+ has a concentration of 150mm/l inside the cell and 5 mm/l outside the cell  Due to high difference between ICM and ECM concentration of K +, a concentration gradient is created, which pushes K+ outside the cell  However NA-K pump can efflux only limited K+ hence significant amount of K + moves out of cell through potassium leak channels or inward rectifier channels  Using these channels concentration gradient pushes significant K+ out of the cell  This lead to negative charge inside the cell and positive charge outside the cell , this process continues until there is a significant difference between inside and outside of the cell polarity  This significant difference in polarity creates electrostatic gradient
Movement of Potassium  Electrostatic gradient  An electric charge generated due to difference between negative charge ( inside) and Positive charge ( outside)  it pushes K+ from outside of the cell to inside  It does not interfere the concentration gradient 
NERNST Equation
Introduction The exact point (value) when efflux of an ion ( due to concentration gradient ) is equal to influx of the same ion ( due to electrostatic gradient) is called the equilibrium potential or NERNST Potential For Potassium equilibrium potential or NERNST Potential is -92mv • In other word we need a -92mv potential to attract the quantity of potassium ion back in to the cell that is needed to balance the concentration gradient that is pushing K+ out of the cell
NERNST Potential of an ION Concentration Gradient Cell Membrane permeability for that ion
So if a cell have only one ION NERNST Potential Influx =efflux Resting Membrane Potential of a Cell
NERNST EQUATION  But in reality a cell has multiple ions with different ICM and ECM concentration  They ae permeable across the membrane  Each has its own Equilibrium potential/NERNST potential  Hence formulae for Resting Membrane potential should be based up on the NERNST potential of individual ion  This formulae is known as NERNST equation
 04 main ions which affects the Resting Membrane potential are  NA, K, Ca, Cl  NERNST Equation for these ions are
Calculation of Resting Membrane Potential  As Nernst potential of each ion is different hence , actual Resting membrane potential my be somewhere in between these individual Nernst potential or various ions  But its mainly depends up on the Nernst potential of K+ , as K + is the most moving ion of the cell ( 90% of all ion movements)  Movement of remaining all ions is less than 10 % , second most movement is of Cl - that is 08 % , Ca++ & Na + is 01 %  So calculation of actual Resting Membrane potential depends up on the proportion of individual ion movement ( out of total ion movement)  To calculate actual Resting Membrane Potential we need to multiple the individual movement proportion of each ion to its Nernst potential then add up the total  Resting Membrane potential ( RMP)= ∑Nernst potential of individual ion X ion movement proportion
Movement proportion of various ions multiplied to its NERNST potentials
 Generally in non excited state, the Resting Membrane potential of a Cell is 86mV to -90mV  Its is always closest to the Ion which has the maximum permeability across the cell membrane  Cell Resting membrane potential is highly depends up to the individual primality of an ion across the membrane  If a cell changes its permeability , that will lead to change in the movement proportion of various ions and ultimately will lead to change in the Resting membrane of the Cell.  Hence Resting membrane potential varies cell to cell ( -20 to -100V)  Neuron = -70mV  Skeletal = -90mV  Epithelial cell = -50mV  Adipose = - 40 mV  Resting Membrane Potential(RMP) indicated the potential when a cell is in non excited state
Goldman’s equation
 When the membrane is permeable to several ions the Membrane equilibrium potential that develops depends on  Polarity of each ion  Membrane permeability  Ionic concentration  This is calculated using Goldman Equation/ Goldman-Hodgkin- Katz ( GHK) Equation Vm- Membrane potential R- Ideal Gas constant T- Temp F- Ferrady constant
Important consideration  GHK does not include all the ion, it just consider the major one- Potassium, Sodium and Chloride  Rest of the ions has either very low concentration or has a very low transmembrane permeability ; hence they are not used GHK equation  Even calcium which is an important ion physiologically is left out of this equation as it has a very low intra cellular concentration  It is known that permeability of a cell is variable, and can change for an ion , hence if permeability of an ion is increased , it should be used in GHK for the calculation of membrane potential and vice versa
Action Potential
 Under resting condition a cell is polarized  Inside of a cell is Negatively charged (ICM)  Outside of a cell is positively charged (ECM) An action potential is a rapid sequence of changes in the voltage across a membrane
changes in RMP withAP
changes in RMP withAP
changes in RMP withAP
changes in RMP withAP
changes in RMP withAP
All or None Law ofAP
Physiological Basis ofAP
Restoration of membrane potential
VoltageGated ionChannels
VoltageGated ionChannels
VoltageGated ionChannels
VoltageGated ionChannels