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Electrochemical Potentials

Electrochemical Potentials. A. Factors responsible. 1. ion concentration gradients on either side of the membrane. - maintained by active transport. Electrochemical Potentials. A. Factors responsible. 2. selectively permeable ion channels. B. Gradients not just chemical, but electrical too.

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Electrochemical Potentials

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  1. Electrochemical Potentials A. Factors responsible 1. ion concentration gradients on either side of the membrane - maintained by active transport

  2. Electrochemical Potentials A. Factors responsible 2. selectively permeable ion channels

  3. B. Gradients not just chemical, but electrical too 1. electromotive force can counterbalance diffusion gradient 2. electrochemical equilibrium

  4. C. Establishes an equilibrium potential for a particular ion based on Donnan equilibrium

  5. RT [Na+]out ___ ___ ENa = ln [Na+]in zF R = Gas constant T = Temp K z = valence of X F = Faraday’s constant Nernst equation 1. What membrane potential would exist at the true equilibrium for a particular ion? - What is the voltage that would balance diffusion gradients with the force that would prevent net ion movement? 2. This theoretical equilibrium potential can be calculated (for a particular ion). For K+ around -90mV For Na+ around +60mV

  6. Resting Membrane Potential A. Vrest 1. represents potential difference at non-excited state -normally around -70mV in neurons 2. not all ion species may have an ion channel 3. there is an unequal distribution of ions due to active pumping mechanisms - contributes to Donnan equilibrium - creates chemical diffusion gradient that contributes to the equilibrium potential

  7. Resting Membrane Potential B. Ion channels necessary for carrying charge across the membrane 1. the  the concentration gradient, the greater its contribution to the membrane potential 2. K+ is the key to Vrest (due to increased permeability)

  8. Resting Membrane Potential C. Role of active transport ENa is +55 mV in human muscle Vm is -65-70 mV in human muscle

  9. Action Potentials large, transient change in Vm depolarization followed by repolarization propagated without decrement consistent in individual axons “all or none”

  10. Action Potentials A. Depends on 1. ion chemical gradients established by active transport through channels 2. these electrochemical gradients represent potential energy 3. flow of ion currents through “gated” channels - down electrochemical gradient 4. voltage-gated Na+ and K+ channels

  11. Action Potentials B. Properties 1. only in excitable cells - muscle cells, neurons, some receptors, some secretory cells

  12. Action Potentials B. Properties 2. a cell will normally produce identical action potentials (amplitude)

  13. Action Potentials B. Properties 3. depolarization to threshold - or just local response (potential) if it does not reach threshold - rapid depolarization - results in reverse of polarity

  14. Action Potentials B. Properties a. threshold current (around -55 mV) b. AP regenerative after threshold (self-perpetuating)

  15. Action Potentials B. Properties 4. overshoot: period of positivity in ICF 5. repolarization a. return to Vrest b. after-hyperpolarization

  16. Action Potentials C. Refractory period 1. absolute 2. relative a. strong enough stimulus can elicit another AP b. threshold is increased

  17. Action Potentials D. ∆ Ion conductance - responsible for current flowing across the membrane

  18. Action Potentials D. ∆ Ion conductance 1. rising phase:  in gNa overshoot approaches ENa (ENa is about +60 mV) 2. falling phase:  in gNa and  in gK 3. after-hyperpolarization continued  in gK approaches EK (EK is about -90 mV)

  19. Gated Ion Channels A. Voltage-gated Na+ channels 1. localization a. voltage-gated

  20. Gated Ion Channels A. Voltage-gated Na+ channels 2. current flow a. Na+ ions flow through channel at 6000/sec at emf of -100mV b. number of open channels depends on time and Vm

  21. Gated Ion Channels A. Voltage-gated Na+ channels 3. opening of channel a. gating molecule with a net charge

  22. Gated Ion Channels A. Voltage-gated Na+ channels 3. opening of channel b. change in voltage causes gating molecule to undergo conformational change

  23. Gated Ion Channels A. Voltage-gated Na+ channels 4. generation of AP dependent only on Na+ repolarization is required before another AP can occur K+ efflux

  24. Gated Ion Channels A. Voltage-gated Na+ channels 5. positive feedback in upslope a. countered by reduced emf for Na+ as Vm approaches ENa b. Na+ channels close very quickly after opening (independent of Vm)

  25. Gated Ion Channels B. Voltage-gated K+ channels 1. slower response to voltage changes than Na+ channels 2. gK increases at peak of AP

  26. Gated Ion Channels B. Voltage-gated K+ channels 3. high gK during falling phase decreases as Vm returns to normal channels close as repolarization progresses

  27. Gated Ion Channels B. Voltage-gated K+ channels 4. hastens repolarization for generation of more action potentials

  28. Does [Ion] Change During AP? A. Relatively few ions needed to alter Vm B. Large axons show negligible change in Na+ and K+ concentrations after an AP.

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