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2. Today Review membrane potential What establishes the ion distributions? What confers selective permeability? Ionic basis of membrane potential Next Lectures Action Potentials

3. Membrane Potential –Inside of cell is negative compared to outside –Depends on: –High concentration K+ inside –Selective permeability of membrane

4. What causes the different ion distributions in cells? 1.Passive distribution – Donnan equilibrium 2.Active Transport

5. Passive distribution – Donnan equilibrium The ratio of positively charged permeable ions equals the ratio of negatively charged permeable ions II I K+K+ Cl - II I [K + ] = [K + ] [Cl - ] = [Cl - ] Start Equilibrium

6. Donnan Equilibrium Mathematically expressed: Another way of saying the number of positive charges must equal the number of negative charges on each side of the membrane

7. Passive Distribution BUT, in real cells there are a large number of negatively charged, impermeable molecules (proteins, nucleic acids, other ions) call them A - II I K+K+ Cl - Start A- II I [K + ] > [K + ] [Cl - ] < [Cl - ] Equilibrium A-

8. Passive Distribution II I [K + ] > [K + ] [Cl - ] < [Cl - ] Equilibrium A- [K + ] I = [A - ] I + [Cl - ] I [K + ] II = [Cl - ] II If [A-] I is large, [K+] I must also be large +’ve = -’ve space-charge neutrality

9. The presence of impermeable negatively charged molecules requires more positively charged molecules inside the cell.

10. Donnan Equilibrium Example A - = 100 K + = 150 Cl - = 50 A - = 0 K + = 150 Cl - = 150 III Initial Concentrations Are these ions in electrochemical equilibrium? No, E K + = 0 mV E Cl - = -27 mV

11. Solve for X, 7500 + 200X + X 2 = 22500 - 300X + X 2 X=30 Let X be the amount of K+ and Cl- that moves

12. A - = 100 K + = 180 Cl - = 80 A - = 0 K + = 120 Cl - = 120 III Final Concentrations Are these ions in electrochemical equilibrium? Yes, E K + = -10 mV E Cl - = -10 mV space-charge neutrality

13. What causes the different ion distributions in cells? 1.Passive distribution – Donnan equilibrium 2.Active Transport

14. Active Transport ATP-powered pumps –Proteins that are capable of pumping ions from one side of the cell membrane to the other –Use energy

15. Active Transport Na + - K + pump outside inside 3 Na + 2 K + ATPADP + Pi

16. Active Transport Na + - K + pump –3 Na + move out –2 K + move in –Hydrolyzes ATP Maintains the concentration gradient

17. Active Transport Na + - K + pump outside inside 3 Na + 2 K +  Electrogenic net loss of 1 positive charge from inside Inside becomes more negative contributes a few mV to resting potential

18. Na+/K+ pump is required –Due to low permeability for Na+ to leak into the cell –Without pump,  Gradual accumulation of +’ve charge inside  Eventually lose the membrane potential Active ingredient in rodent poison, ouabain, poisons the Na/K pump

19. What causes the different ion distributions in cells? 1.Passive distribution – Donnan equilibrium 2.Active Transport

20. What confers selective permeability? Ion channels –Non-gated –Leakage channels –Open at rest – allow K+ to flow out along its concentration gradient K+K+

21. Membrane Potential Summary 1.Selective permeability Ion channel – K + leak channel 2.Unequal distribution of ions Passive distribution (Donnan) Active transport – Na+/K+ pump 3.The equilibrium potential of each ion is described by the Nernst equation 4.The total membrane potential is described by the Goldman equation

22. Ionic basis of membrane potential

23. K+1405-84 mV Na+10120+63 mV Cl-4110-83 mV Ca 2+ 0.0015+107 mV Outside (mM) Equilibrium (Nernst) Potential Inside (mM) Mammalian Cell

24. Is the resting membrane potential controlled by one ion, or several? Do an experiment –Measure membrane potential (Vm) of a cell –Change extracellular concentration of an ion in the bathing solution –If Vm really depends on E ion than Vm should change as E ion changes

25. Measuring Membrane Potential cell microelectrode amplifier 0 mV -80 mV time Resting potential Reference electrode

26. Expt #1 vary extracellular Na Assume [Na] in = 10 mM 1-58 mV 5-17 100 2017 10058 20075 [Na] out prediction

27. 152 10 20 50 100 200 External Na+ concentration (mM) Membrane Potential (mV) Prediction of E Na From Nernst equation -120 -100 -80 -60 -40 20 0 40 -20 Measured Vm

28. Therefore, –Conclude that Vm does not follow E Na

29. Expt #2 vary extracellular K Assume intracellular [K] = 140 mM 1-124mV 5-84 10-66 20-49 100-8 2009 Extracell [K] prediction

30. 152102050100200 External K+ concentration (mM) Membrane Potential (mV) Prediction of E K From Nernst equation Deviation at low [K+] due to slight permeability of Na+ -120 -100 -80 -60 -40 -20 0 Measured Vm

31. Therefore, the resting membrane potential (Vm) is very close to the equilibrium potential for K+ (E K )

32. Summary 1.At rest P K >>P Na, P Cl, P Ca 2.Therefore, at rest, the membrane potential is close to E K 3.In general, the membrane potential will be dominated by the equilibrium (Nernst) potential of the most permeable ion

33. Sample Question K = 140 Na = 10 Cl = 30 K = 5 Na = 145 Cl = 110 At rest Vm of this typical cell is -75 mV. What would Vm be if PNa >> Pk,PCl? Answer: Calculate E Na using Nernst equation. Assume Vm  E Na = +67 mV

34. The resting membrane potential is the basis for all electrical signaling Next Lecture: Action Potentials