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Membrane Potentials

Membrane Potentials. Overview. Introduction Diffusion potentials Nernst equation Donnan membrane equilibrium Resting membrane potential in living cells Action potentials. Introduction. An electric potential (voltage) difference Exists between inside and outside of all cells

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Membrane Potentials

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  1. Membrane Potentials Excitable tissues

  2. Overview • Introduction • Diffusion potentials • Nernst equation • Donnan membrane equilibrium • Resting membrane potential in living cells • Action potentials Excitable tissues

  3. Introduction • An electric potential (voltage) difference • Exists between inside and outside of all cells • Trans-membrane potential • Can be measured by • Intracellular recording technique Excitable tissues

  4. Intracellular recording • Microelectrode • Penetrate the membrane • Voltmeter measures • The difference in the distribution of ions • On inside versus the outside Excitable tissues

  5. Intracellular recording Excitable tissues

  6. Resting Membrane Potential • The resting membrane potential • Magnitude varies • 5mv to 100mv • Cell type • Chemical environment Excitable tissues

  7. Resting Membrane Potential • It is approximately • -80mv in excitable cells • Nerves, muscles cells • -20 to -40mv in non-excitable cells • RBC, epithelial cells Excitable tissues

  8. Excitable Cells • Capable of transmitting electrical and chemical impulses • Able to respond to changes in external environment • Leading to change in trans-membrane potential • Develop action potential • Examples: nerve and muscle cells Excitable tissues

  9. Diffusion Potential Hypothetical example • Ions will diffuse from soln 1 to soln 2 • Cl- moves faster than Na+ • Cl- moves ahead of Na+ • Charge separation • Soln 1 will have positive charge • Soln 2 will have negative charge 2 1 0.1 M NaCl 1.0 M NaCl Na+ - + Na+ Na+ Na+Cl- - + Cl- Cl- Cl- - + Excitable tissues

  10. Diffusion Potential Hypothetical example • This is diffusion potential • Depends on movement of ions from one compartment to the other • It is transient • Disappears as soon as equilibrium is attained 2 1 0.1 M NaCl 1.0 M NaCl Na+ - + Na+ Na+ Na+ Cl- - + Cl- Cl- Cl- - + Excitable tissues

  11. Diffusion Potential Hypothetical example • Solution 1 & 2 are • Separated by a semi-permeable membrane • Only K+ can diffuse • K+ will diffuse • Along concentration gradient • From solution 1 to 2 • Charge separation • Solution 1 will have – charge • Solution 2 will have + charge 2 1 0.15 M NaCl 0.15 M KCl K+ + - Na+ K+ Na+ K+ Cl- Cl- Cl- + - Cl- Cl- Na+ Cl- + - -94 mv Excitable tissues

  12. Diffusion Potential Hypothetical example • This is diffusion potential • Depends on movement of ions • From one compartment to another • Magnitude depends • On concentration of K+ in solution 1 2 1 0.15 M NaCl 0.15 M KCl K+ + - Na+ K+ Cl- Na+ K+ Cl- Cl- Cl- + - Cl- Na+ Cl- + - -94 mv Excitable tissues

  13. The Nernst Equation Hypothetical example • The concentration gradient • Which causes the net diffusion of K+ from soln 1 to 2 • Known as • Concentration force • Chemical potential 2 1 0.15 M NaCl 0.15 M KCl K+ + - Na+ K+ Cl- Na+ K+ Cl- Cl- Cl- + - Cl- Na+ Cl- + - -94 mv Excitable tissues

  14. The Nernst Equation Hypothetical example • Given by formula • RTln([K+]1/ [K+]2) • R = gas constant • T = absolute temp • Ln = natural logarithm 2 1 0.15 M NaCl 0.15 M KCl K+ + - Na+ K+ Cl- Na+ K+ Cl- Cl- Cl- + - Cl- Na+ Cl- + - -94 mv Excitable tissues

  15. The Nernst Equation Hypothetical example • The electrical force • Which opposes the diffusion of K+ from soln 1 to 2 • Known as • Electrical potential • Given by formula • EKZF • EK = equilibrium potential for K+ • Z = valence of K+ • F = Faraday’s constant 2 1 0.15 M NaCl 0.15 M KCl K+ + - Na+ K+ Cl- Na+ K+ Cl- Cl- Cl- + - Cl- Na+ Cl- + - -94 mv Excitable tissues

  16. The Nernst Equation • At equilibrium • Electrical force = concentration force • EKZF = RTln([k+]1/[k+]2) • Ek = (RT/ZF) ln([k+]1/[k+]2) • This is Nernst equation • Simplified Nernst equation • Ek = ± 61 log ([K+]1/[K+]2) • Converting to log base 10 • Evaluating (RT/ZF) at 370C • Giving the result in mV Excitable tissues

  17. Living Cells • Transport properties of cell membrane • Na+/K+ pump • Electrogenic pump • 3Na+ out • 2K+ in • Create net deficit of +ve charge inside cell • Inside –ve with respect to outside cell Excitable tissues

  18. Living Cells • Na+/K+ pump also • Cause large conc gradient of Na+ andK+ across the cell membrane • For sodium • 14 mEq/L ICF • 142 mEq/L ECF • For potassium • 140 mEq/L ICF • 4 mEq/L ECF Excitable tissues

  19. Living Cells • Leakage of K+ and Na+ through • Channel proteins • K+/ Na+ leak channels • Channels are more permeable to K+ than to Na+ Excitable tissues

  20. The Living Cells • Living cells are bound by cell membrane • Ionic composition of ICF differs from that of ECF • Cell membrane • more permeable to K+ • Na+ can permeate with difficulty • Effectively kept out by the Na+/K+ pump • Cl- kept out because • The –ve charges of impermeate anions • Thus K+ diffuse out along concentration gradient Excitable tissues

  21. The Living Cells • K+ diffuse out of the cell • Create a diffusion potential • Magnitude of potential given by the Nernst Equation • Ek = (RT/ZF) ln([K+]1/[K+]2) • = - 61log (140/4) • = - 61 log 35 • = -61 x 1.54 mv • = - 94 mv Excitable tissues

  22. The Living Cells • Equilibrium potential for Na+ would be • ENa = - 61log (14/142) • = - 61 log (1/10) • = + 61 mv Excitable tissues

  23. The Living Cells • The measured resting membrane potential is about • -85 mv in muscles and -70mv for nerves • The reason for this deviation • Membrane is not completely impermeable to Na+ • Na+ tend to move into cell along conc gradient • Reduces the magnitude of negative charge inside Excitable tissues

  24. Goldman Equation • The magnitude of the resting membrane potential at any given time is dependent on • Distribution of K+, Na+ and Cl- • Permeability of the membrane to these ions Excitable tissues

  25. Goldman Equation • Given by the following equation • V = (RT/F) {(pK[k+]o/ pK[k+]i) + (pNa[Na+]o/ pNa[Na+]i) + (pCl[Cl-]i/ pCl[Cl-]o)} • pK+ = permeability of potassium ion • pNa+ = permeability of sodium ion • pCl- = permeability of chloride ions Excitable tissues

  26. The action Potential • The RMP is • -70 mv in nerve cells • -85 in muscle cells • If the cells are disturbed (stimulated) • Rapid change occur in membrane pot from • -70 mv to + 30 mv then • Back to -70mv Excitable tissues

  27. The action Potential • This rapid changes in membrane potential • Known as Action Potential (AP) • Duration of AP • 1 msec in nerve • 10 msec in muscle • 200 to 300 msec in cardiac Excitable tissues

  28. Ionic Bases of Action Potential • Resting membrane is 50 to 100 times more permeable to K+ than it is to Na+ • RMP is closer to K+ equilibrium potential than it is to Na+ equilibrium potential Excitable tissues

  29. Equilibrium Potentials Na+ Equilibrium Potential + 61 mV Zero Potential 0 mV Resting Membrane Potential - 84 mV - 94 mV K+ Equilibrium Potential Excitable tissues

  30. Ionic Bases of Action Potential • On stimulation • Permeability of membrane to K+ and Na+ changes • Both K+ and Na+ permeability increases • Na+ permeability increases much more than that for K+ Excitable tissues

  31. Depolarization • Stimulation causes • Voltage gated Na+ channels to open • Na+ diffuse into cell • Entry of +ve charges • Membrane pot to become +ve Excitable tissues

  32. Depolarization • Inside become +ve relative to outside • Depolarization phase • Na+ gates open • Brief moment • Then they close Excitable tissues

  33. Re-polarization • The voltage changes occurring during depolarization cause • Voltage gated K+ channels to open • K+ diffuse out Excitable tissues

  34. Re-polarization • Exit of K+ causes • Membrane pot to become –ve • The inside once again • Become –ve relative to outside • Repolarization Excitable tissues

  35. Action Potential • Depolarization • Na+ channels open • Na+ diffuse IN • Repolarization • Na+ channels close • K+ open • K+ diffuse out Excitable tissues

  36. Action Potential • Point A • Resting phase • Na+ and K+ channels closed • Point B • Depolarization • N+ channels open • K+ channels still closed Excitable tissues

  37. Action Potential • Point C • Repolarization • Na+ channels closed • K+ channels open • Point D • Hyper-polarization • K+ channels remain open longer time Excitable tissues

  38. Silverthorn: Textbook of Physiology Excitable tissues

  39. Threshold Stimulus • Action potential occur only when the membrane is stimulated enough • To cause Na+ channels to open • The minimum stimulus needed • To achieve an action potential is called • The threshold stimulus Excitable tissues

  40. Threshold Stimulus • Sub-threshold stimulus causes • Membrane potential to become less negative • No matter how small the stimulus is • Causes few Na+ channels to open • Na+ move inside (positive feedback effect) Excitable tissues

  41. Threshold Stimulus • If the membrane potential reaches the threshold potential (generally 5 – 15 mV less negative than the RMP) • Enough voltage gated Na+ channels open causing • Influx of Na+ into the cell • Depolarization occurs Excitable tissues

  42. Absolute Refractory Period • During action potential (AP) • Second stimulus will not produce another AP no matter how strong the stimulus is • Corresponds to the time when the Na+ channels are open ( few msec) Absolute refractory period Relative refractory period Excitable tissues

  43. Relative Refractory Period • During this period • Another AP can be produced • Using stimulus greater than the threshold stimulus • Corresponds to the period when the K+ channels are open Absolute refractory period Relative refractory period Excitable tissues

  44. Spread of Action Potential • In un-myelinated nerve • AP elicited on one portion • Usually excite adjacent portion • Local current flow • Active region stimulate adjacent inactive region Excitable tissues

  45. Spread of Action Potential • AP is conducted • Away from stimulus point • From one region to another Excitable tissues

  46. Saltatory Conduction • Myelin sheath wrap around the nerve to form an insulator • At nodes of Ranvier there is no myelin • RMP and AP are generated at the nodes Excitable tissues

  47. Saltatory Conduction • When the fibre is depolarized • Only the nodes become active • Local current flow from active node through the ICF to the inactive node • And through the ECF from inactive to active Excitable tissues

  48. Saltatory Conduction • The outward flow of current at the inactive node depolarizes the membrane • Excitation jumps from one node to another • This is known as saltatory conduction Excitable tissues

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