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Membrane Potential (Em)

Membrane Potential (Em). What is it? How does the membrane potential arise? How do you estimate the Membrane Potential with the Nernst Equation? Why study membrane potential?= see next slide Page 191, 203 (ion channels), 208-209 (CF), 365-368 (Understanding mem pot, Nernst equa).

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Membrane Potential (Em)

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  1. Membrane Potential (Em) What is it? How does the membrane potential arise? How do you estimate the Membrane Potential with the Nernst Equation? Why study membrane potential?= see next slide Page 191, 203 (ion channels), 208-209 (CF), 365-368 (Understanding mem pot, Nernst equa)

  2. Why study the membrane potential Em? (Don’t memorize!!): • Our cells make ATP (possibly the most important reaction in the body) with use of the membrane potential. • Epilepsy is thought to be due to bad "voltage-gated" potassium channels (voltage-gated means that the channels are opened or closed by the membrane voltage Em). • Cystic fibrosis is due to bad chloride movement across membranes. • Heart drugs such as cardiotonic steroids (i.e., cardiac glycosides) are Na-K pump (active transport) inhibitors- this can effect Em • Neurotransmitters act by changing ion fluxes across membranes-this changes Em. • Ions move across membranes in the eye- changing Em, allowing us to see. • Ion movement across membranes and changes in Em is important for each muscle contraction and for each nerve impulse. • One of the most deadly poisons known (tetrodotoxin) is from the Puffer fish (a specialty in the Japanese diet); the poison acts by blocking sodium channels- this prevents action potentials (which are changes in Em). • Some anesthetics work by altering Em.

  3. FIG. 5-2 The cell spends a tremendous amount of energy maintaining the Membrane Potential CHEMICAL WORK CHEMICAL WORK Note positive ion moving out Makes the membrane potential With an excess of + outside, - inside

  4. The membrane potential: • VOLTAGE OF A MEMBRANE: Vm OR Em is the symbol • WHAT IS A VOLTAGE? SINCE MEMBRANES ARE HYDROPHOBIC AND PREVENT IONS FROM CROSSING, A POTENTIAL DIFFERENCE (IN UNITS OF VOLTAGE) CAN BUILD UP ACROSS THE MEMBRANE. • That is, + charges build up on one side of a Membrane, and – charges build up on the other • CELLS ARE REGULATED BY CHANGES IN THE MEMBRANE POTENTIAL (ESP. NERVE CELLS)

  5. Membrane Voltage: IS THE INSIDE OF THE CELL POSITIVE OR NEGATIVE? FIG. 13-11 -4O MILLIVOLTS CELL HAS HI [K] AND LOW [Na] + We will measure membrane potential this way - CELL

  6. If a Positive ion like K moves out of the cell, a membrane potential develops- where is the plus sign? Inside the cell or out? K+ K+ - + Voltage becomes very negative== -50 mV (or more)

  7. For most cells (cells are “rest” or resting cells), as K moves out of the cell, this makes the membrane potential

  8. Go through these web sites to review (see them on my web site for cell biology 3611) • http://carbon.cudenver.edu/~bstith/membrpotential.gif (D:\cell biol 3611\ch 7 8 9 membrane trans\membrpotential.gif) • http://carbon.cudenver.edu/~bstith/nernst.mov (D:\cell biol 3611\ch 7 8 9 membrane trans\nernst-short.mov) • http://distance.stcc.edu/AandP/AP/AP1pages/nervssys/unit10/resting.htm#what%20is%20RP • http://www.taumoda.com/web/nernstjava/ • For the computational biology students: view this paper: http://carbon.cudenver.edu/~bstith/koch.pdf

  9. Then, if a Negative Ion like Chloride (Cl-) moves out of the cell, the membrane potential decreases toward zero Cl- Cl- + - Em changes from -50 mV to a less negative number (Cl- are leaving)--Such as -20 mV

  10. How do Ions (like sodium or Na, potassium or K, Chloride or Cl) cross membranes? • IONS CROSS MEMBRANES THROUGH CHANNELS (A PROTEIN THAT CROSSES THE MEMBRANE) FIG 13-8

  11. How does the membrane potential develop? • Most cells are negative inside, about – 40 to -60 mV • This potential is typically due to potassium moving out of the cell—there are more K channels open than channels for other ions • K moves out because K concentration is very high in the cell (and low outside)-so K moves from high to low conc. (due to the NaK pump) • What would happen to the membrane potential if negative Cl moved out of the cell? We will see...

  12. ION CHANNELS allow ions to move across the membrane • ION CHANNELS ALLOW ONLY IONS TO CROSS, CHANGING MEMBRANE POTENTIAL OF CELL • CHANGE Em, TURN ON/OFF NEURON. • MANY MEDICINES AFFECT ION CHANNELS TO AFFECT NEURON (ANTIDEPRESSENT). • Typically, there are more K channels open, so K moves out of the cell and sets the membrane potential to negative inside. • In an action potential, sodium channels open up and sodium movement sets the membrane potential

  13. In the Xenopus Oocyte • Potassium channels are more open, so K efflux sets the membrane potential to about -50 mV • However, if chloride channels open, chloride moves out of the cell and this reduces the membrane potential to about -20 mV.

  14. HOW DO YOU ESTIMATE THE MEMBRANE POTENTIAL?Nernst Equation: Vm or Em=(RT/ZF)ln([C]out/[C]in) (EQUATION 13.1 (old 9.1) IN TEXT) R= 1.987 cal/deg mole; Z is charge of the ion, F = 23,062 cal/volt equiv; T =temp in Kelvin (C +273), Ln is natural log base e= 2.718. • [C] = concentration of the ion that is most permeable (its channels are more open)- concentration outside the cell or inside the cell

  15. outside the cell membrane: [Na]o =140 mM [K]o = 5 mM [Cl]o = 100 mM inside the cell [Na]in = 14 mM [K]in = 124 mM [Cl]in = 40 mM Example: Estimate Plasma Membrane Potential (Em)-write this down

  16. Estimate the Membrane Potential for human cell (37C) Em = (RT/ZF) ln ([C]out/[C]in) Plug in what ion concentrations since it is the most permeable ion (more these ion channels are open so the ion’s concentrations determine the membrane potential) R= 1.987 cal/deg mole; Z is charge of the ion, F = 23,062 cal/volt equiv; T =temp in Kelvin (deg C +273), Ln is natural log base e= 2.718. • [C] = concentration of the ion that is most permeable (its channels are more open)- concentration outside the cell or inside the cell

  17. Do the calculation

  18. K is most permeable: Em = (RT/ZF) ln ([C]out/[C]in) = (1.987)(273+37)/ (+1)(23062) ln (5 mM/124 mM) = 0.0267 x -3.21 = -0.0857 Volts (note the concentrations must be in same units) Usually, scientists report answer in milliVolts: = - 85.7 mV is the estimate for the membrane potential of the liver cell For practice: questions 13-2, -4, and -5 in book

  19. ACTION POTENTIAL in nerve cell IS DUE TO OPENING OF SODIUM ION CHANNELS, THEN CLOSING OF THESE ION CHANNELSRESTING CELL before ActPot: K IS MOST PERMEABLE (SOME K CHANNELS OPEN)PEAK OF ACTION POTENTIAL: Na IS MOST PERMEABLE

  20. What if Na channels open? Em now set by Na Concentrations • Em = [(1.987x310)/(+1)23062]ln (140/14) = +61 mV (make sure that you can perform this calculation-on exam)

  21. Before,K channels are more open and K determines the Em, then the Na channel open to determine Em (and closes) to make the action potential (turns on nerve cell) Na determines membrane potential S Na CH. open Na CH. closes K CH. open K CH. open Fig. 13-12

  22. WHAT IF Chloride becomes the MOST PERMEABLE ION? • DOES THIS HAPPEN WITH ANIMAL CELLS? Yes. • Chloride channels open up to allow Cl- movement • The membrane potential changes from about -40 to -20 mV.

  23. Estimate the Membrane Potential with Chloride channels open • [Cl]in = 40 mM [Cl]out = 100 mM • Cl has a negative charge (z=-1) Em = (RT/ZF) ln ([C]out/[C]in) = HORMONES RELEASE CALCIUM INTO THE CYTOPLASM AND CALCIUM OPENS CHLORIDE CHANNELS TO CHANGE THE Em • WE WILL MEASURE THIS CHANGE IN Em…

  24. Chloride Channels in human disease- cystic fibrosis • Chloride channels need to be present and functional for cells to function • In cystic fibrosis, chloride channels do not make it to the plasma membrane AND CHOLORIDE DOES NOT MOVE ACROSS THE MEMBRANE • So, the symptoms of cystic fibrosis develop (high salt in sweat, destruction of organs, thick mucus in lungs that causes infections). • Pgs. 208-209 in 6th edition of our text

  25. Chloride Channel Fig. 13-8a

  26. We will study the Chloride Channel in Xenopus frog oocytes • Acetylcholine binds to a membrane receptor • To increase the concentration of Calcium in the cytoplasm • Calcium binds to and opens the Chloride channel • Chloride moves across the membrane and out of the cell • This Cl movement causes the membrane potential to change from -50 mV to a lower value: -25 mV

  27. PA (new hormone?) Acetylcholine Calcium Ach receptor Cl- Note that chloride Efflux reduces the Membrane potential (less negative inside, Less positive outside) - - - + + + +

  28. Today, we will use the NeuroLab program (later, maybe the Neuroscience Program from HHMI)

  29. end

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