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Membrane potentials

Membrane potentials. Huan Ma （马欢）， PhD Department of Physiology Room 515, Block C, Research Building School of Medicine, Zijingang Campus Email: mah@zju.edu.cn Tel: 88208068. OUTLINE. Resting potential Graded potential Action potential Refractory period. Electrocardiogram ECG.

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Membrane potentials

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  1. Membrane potentials Huan Ma（马欢），PhD Department of Physiology Room 515, Block C, Research Building School of Medicine, Zijingang Campus Email: mah@zju.edu.cn Tel: 88208068

  2. OUTLINE Resting potential Graded potential Action potential Refractory period

  3. Electrocardiogram ECG

  4. Electroencephalogram EEG

  5. Electromyogram EMG

  6. Agenda • Membrane potentials – what they are • Formation of membrane potentials • Types and uses of membrane potentials • The significance of membrane potentials

  7. Membrane Potentials What They Are Membrane Potential Any animal cell’s phospholipid bi-layer membrane and associated structures A difference in electrical charge across (ECF – ICF) this membrane, representing potential energy. Can also be called a transmembrane potential.

  8. Formation of Membrane Potentials • Requires a selectively permeable membrane • Due to membrane components • Force involved is electrochemical • “Electro” due to the charges of the ions on either side of the membrane • “chemical” due to the number and types of ions on either side of the membrane • Main components? • Na+ & K+ • Cl- • A- negatively charged anions • H+ (proton gradient) – specialized use

  9. Formation of Membrane Potential • Ion Concentrations (millimoles/liter)

  10. Electrical gradient for K+ Concentration gradient for Na+ K+ Na+ A- EK = -90mV ENa = +60mV Formation of Membrane Potential So, if we take those numbers and look graphically at what happens between Na+, K+ and A-… [Na+]=150 mmole/L [K+]=5 mmole/L ECF + + + + + + + + + + + + + + + + + + + Electrical gradient for Na+ Concentration gradient for K+ - - - - - - - - - - - - - - - - - - ICF [K+]=150 mmole/L [Na+]=15 mmole/L

  11. Maintenance of Membrane Potential • Without energy, the membrane potential would eventually be destroyed as • K+ leaks out the cell due to membrane leakage channels • There are just more of the K+ leakage channels than Na+, giving us the difference in membrane permeability • Na+ leaks in due to membrane leakage channels • Na+/K+ ATPase (Sodium-Potassium Pump) restores the balance pumping Na+ out and K+ back in.

  12. Types and Uses of Membrane Potentials • Resting membrane potential • Just described at -70mV • Threshold membrane potential • The electrical change that causes specialized channels to cycle through open/close confirmations • This occurs in mot excitable tissues at -55mV • Action potentials • This is a change in the membrane potential due to rapid influxes and effluxes of ions (Na+ and K+) • Causes adjacent cell membrane to undergo same rapid change • Continues on to end of the membrane • Used for communication • Graded potentials • Change in membrane potential that is variable based on the rate of and location of stimuli on the membrane • Used for integration

  13. The Significance of Membrane Potentials • Why do we care? • What would happen if membrane potentials didn’t exist? • What would happen if the membrane potentials were different? (higher or lower)

  14. Extracellular Recording

  15. Intracellular Recording

  16. Opposite charges attract each other and will move toward each other if not separated by some barrier.

  17. Only a very thin shell of charge difference is needed to establish a membrane potential.

  18. Resting membrane potential A potential difference across the membranes of inactive cells, with the inside of the cell negative relative to the outside of the cell Ranging from –10 to –100 mV

  19. Overshoot refers to the development of a charge reversal. A cell is “polarized” because its interior is more negative than its exterior. Repolarization is movement back toward the resting potential. Depolarization occurs when ion movement reduces the charge imbalance. Hyperpolarization is the development of even more negative charge inside the cell.

  20. unequal ion distribution (chemical gradient) across the membrane • selective membrane permeability (cell membrane is more permeable to K+) • Na+-K+ pump

  21. electrochemical balance - - - - - - - - - - - - - - - - - ++++++++++++++++ chemical driving force electrical driving force

  22. The Nernst Equation: K+ equilibrium potential (EK) (37oC) R=Gas constant T=Temperature Z=Valence F=Faraday’s constant

  23. PK+[K+]o + PNa+[Na+]o Vm = 61 log PK+[K+]i + PNa+[Na+]i Formation of Membrane Potential • The cell membrane is about 40 times less permeable to Na+ than K+, putting the resting potential closer to EK+ (which is -90mV) • The equilibrium potentials of K+, Na+, Cl- and A- result in a membrane potential of -70mV • This determined by the Goldman-Hodgkin-Katz equation This equation boils down to – the resting membrane potential is calculated by the combined effects of concentration gradients times membrane permeability for each ion, and really just concerning Na and K.

  24. 5 + 6 = 61 log = 61 log 150 + .6 1(5) + .04(150) 11 Vm = 61 log 1(150) + .04(15) 150.6 PK+[K+]o + PNa+[Na+]o Vm = 61 log PK+[K+]i + PNa+[Na+]i Formation of Membrane Potential Here’s How it Works… Vm = 61(log of .073) = 61 (-1.37) = -69mV +1mV (for the Na+/K+ pump effect) = -70mV PK+ = permeability for Potassium = 1PNa+ = permeability for Sodium = .04 [K+]o = concentration of Potassium outside the cell = 5[K+]i = concentraiton of Potassium inside the cell = 150 [Na+]o = concentration of Sodium outside the cell = 150[Na+]i = concentration of Sodium inside the cell = 15

  25. Begin: K+ in Compartment 2, Na+ in Compartment 1; BUT only K+ can move. Ion movement: K+ crosses into Compartment 1; Na+ stays in Compartment 1. At the potassium equilibrium potential: buildup of positive charge in Compartment 1 produces an electrical potential that exactly offsets the K+ chemical concentration gradient.

  26. Begin: K+ in Compartment 2, Na+ in Compartment 1; BUT only Na+ can move. Ion movement: Na+ crosses into Compartment 2; but K+ stays in Compartment 2. At the sodium equilibrium potential: buildup of positive charge in Compartment 2 produces an electrical potential that exactly offsets the Na+ chemical concentration gradient.

  27. Difference between EK and directly measured resting potential Mammalian skeletal muscle cell -95 mV -90 mV Frog skeletal muscle cell -105 mV -90 mV Squid giant axon -96 mV -70 mV Ek Observed RP

  28. Goldman-Hodgkin-Katz equation

  29. Role of Na+-K+ pump: • Electrogenic • Hyperpolarizing Establishment of resting membrane potential: Na+/K+ pump establishes concentration gradient generating a small negative potential; pump uses up to 40% of the ATP produced by that cell!

  30. Origin of the normal resting membrane potential • K+ diffusion potential • Na+ diffusion • Na+-K+ pump

  31. Electrotonic Potential

  32. Graded potential Graded potentials are changes in membrane potential that are confined to a relative small region of the plasma membrane

  33. The size of a graded potential (here, graded depolarizations) is proportionate to the intensity of the stimulus.

  34. Graded potentials can be: EXCITATORY or INHIBITORY (action potential (action potential is more likely) is less likely) The size of a graded potential is proportional to the size of the stimulus. Graded potentials decay as they move over distance.

  35. Graded potentials decay as they move over distance.

  36. Graded potentials (Local response, local excitation, local potential) • Not “all-or-none” • Electrotonic propagation: spreading with decrement • Summation: spatial & temporal

  37. Threshold Potential: level of depolarization needed to trigger an action potential (most neurons have a threshold at -50 mV)

  38. Excitable cells: a cell in which the membrane response to depolarisations is nonlinear, causing amplification and propagation of the depolarisation (an action potential). Action potential Some of the cells (excitable cells) are capable to rapidly reverse their resting membrane potential from negative resting values to slightly positive values. This transient and rapid change in membrane potential is called an action potential

  39. A typical neuron action potential Positive after-potential Negative after-potential Spike potential After-potential

  40. Ionic basis of action potential

  41. (1) Depolarization: Activation of voltage-gated Na+ channel Blocker: Tetrodotoxin (TTX)

  42. (2) Repolarization: Inactivation of Na+ channel Activation of K+ channel Blocker: Tetraethylammonium (TEA)

  43. The rapid opening of voltage-gated Na+ channels explains the rapid-depolarization phase at the beginning of the action potential. The slower opening of voltage-gated K+ channels explains the repolarization and after hyperpolarization phases that complete the action potential.

  44. An action potential is an “all-or-none” sequence of changes in membrane potential. The rapid opening of voltage-gated Na+ channels allows rapid entry of Na+, moving membrane potential closer to the sodium equilibrium potential (+60 mv) Action potentials result from an all-or-none sequence of changes in ion permeability due to the operation of voltage-gated Na+ and K + channels. The slower opening of voltage-gated K+ channels allows K+ exit, moving membrane potential closer to the potassium equilibrium potential (-90 mv)

  45. Click here to play the Voltage Gated Channels and Action Potential Flash Animation

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