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  • Resting Membrane Potential
  • Cell Membranes F5-1
    • Cell membrane distinguishes one cell from the next.
    • Cell membranes do the following:
    • a) Regulates exchange of salts, nutrients and waste with the environment.
    • b) Mediate communication between the cytosol and environment.
    • c) Maintain cell shape.
  • Fig. 03.02
  • Uneven Distribution of Solutes Amongst Body Compartments
    • Solutes are molecules which dissolve in liquid. Cell membranes prevent most solutes from diffusing amongst compartments.
    • Active transport of solutes helps create and maintain differences in solute concentrations.
    • The body is kept in a state of chemical disequilibrium .
  • Uneven Distribution of Major Ions in the Intracellular and Extracellular Compartments (mM) T5-9
    • The body is in a state of electrical disequilibrium because active transport of ions across the cell membrane creates an electrical gradient .
    • Although the body is electrically neutral, cells have excess negative ions on the inside and their matching positive ions are found on the outside.
  • Terminology Associated with Changes in Membrane Potential F8-7, F8-8
    • Depolarization - a decrease in the potential difference between the inside and outside of the cell.
    • Hyperpolarization - an increase in the potential difference between the inside and outside of the cell.
    • Repolarization - returning to the RMP from either direction.
    • Overshoot - when the inside of the cell becomes +ve due to the reversal of the membrane potential polarity.
  • Resting Membrane Potential (Difference)
    • The resting membrane potential is the electrical gradient across the cell membrane.
    • Resting : the membrane potential has reached a steady state and is not changing.
    • Potential : the electrical gradient created by the active transport of ions is a source of stored or potential energy, like chemical gradients are a form of potential energy. When oppositely charged molecules come back together again, they release energy which can be used to do work (eg. molecules moving down their concentration gradient).
    • Difference : the difference in the electrical charge inside and outside the cell (this term is usually omitted)
  • K + Ions Contribute to the Resting Membrane Potential F5-33
    • In electrical equilibrium and chemical disequilibrium.
    • Membrane is more permeable to K + ions.
    • K + leaks out of the cell down its concentration gradient.
    • Excess -ve charge buildup inside the cell as Pr - cannot cross the membrane. An electrical gradient is formed.
    • The -ve charges attract K+ ions back into the cell down the electrical gradient.
    • Net movement of K+ stops. The membrane potential at which the electrical gradient opposes the chemical gradient is known as the equilibrium potential (E). E K = -90 mV.
  • Nerst Equation
    • The equilibrium potential is calculated using the Nerst equation:
    R= gas constant (8.314 jules/ o K.mol) T= temperature ( o K) F= Faraday constant (96, 000 coulombs/mol) z= the electric charge on the ion [I] out = ion concentration outside the cell [I] in = ion concentration inside the cell
    • Derived under resting membrane conditions when the work required to move an ion across the membrane (up its concentration gradient) equals the electrical work required to move an ion against a voltage gradient.
  • RMP Dependence on [K + ] o
  • F5-34 Contribution of Na + to the Resting Membrane Potential
    • Membrane permeable to Na + only.
    • Same principles hold as in the case of K + movement across the membrane.
    • The equilibrium potential for Na + is, E Na = +60 mV.
  • Goldman Equation
    • It is used to calculate the membrane potential resulting from all the participating ions when V m is not changing:
    • P X = the relative permeability of the membrane to ion X (measured in cm/s). An ion’s contribution to the membrane potential is proportional to its ability to cross the membrane.
    • P K : P Na : P Cl = 1.0: 0.04: 0.45 at rest .
  • Electrodiffusion Model of the Cell Membrane GHK Current Eqn.: Current of ion X through the membrane
  • I-V Relationship Predicted by GHK Current Equation
  • Direction of Current Rectification is Dependent On The Ratio of Ion Concentration On Both Sides Of The Membrane As Predicted by GHK Current Equation
  • RMP Dependence on [K + ] o And 
  • Resting Membrane Potential in Real Cells
    • Most cells are 40x more permeable to K+ than Na+. As a result, the resting membrane potential is much closer to E K than E Na .
    • In actual cells, the resting membrane potential is much closer to -70 mV because a small amount of Na + leaks into the cell.
    • The Na + is pumped out and the K + pumped in by the Na + /K + -ATPase. It pumps 3 Na + ions out and 2 K + ions in.
    • Na + /K + -ATPase is also known as an electrogenic pump because it helps maintain an electrical gradient. 7-20% of the RMP is generated by the pump.
    • Not all transporters are electrogenic pumps:
      • Na + /K + /2Cl - symporter moves one +ve charge for every -ve charge.
      • HCO 3 - /Cl - antiport in red blood cells moves these ions in a one-for-one fashion.
  • Electrical Model of the Cell Membrane
  • References
    • Boron, W.F. & Boulpaep, E.L. (2005). Medical Physiology: Elsevier. Ch.3 & 6
    • Tortora, G.J. & Grabowski, S.R (2003). Principles of Anatomy & Physiology.New Jersey: John Wiley & Sons. Ch.12, pp.396-398.
    • Silverthorn, D.U (1998). Human Physiology: An Integrated Approach. New Jersey: Prentice Hall. Ch.5, pp.131-133, 136-141.
    • Johnston, D. & Wu, S. (1999). Foundations of Cellular Neurophysiology: Cambridge, Mass.:MIT Press. Ch.2