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Concepts

Concepts. The intracellular and extracellular fluids have unequal concentrations of specific ions. Na + K + Cl - H + HCO 3 - The differences in concentrations are maintained by the expenditure of energy (work). Nernst Potential. V Eq = RT/ZF ln(C out /C in )

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Concepts

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  1. Concepts • The intracellular and extracellular fluids have unequal concentrations of specific ions. • Na+ K+ Cl- H+ HCO3- • The differences in concentrations are maintained • by the expenditure of energy (work).

  2. Nernst Potential • VEq = RT/ZF ln(Cout /Cin) • Naout /Nain = 12/1 VNa = + 66 mV • Kout /Kin = 1/40 VK = - 97 • Clout /Clin = 30/1 VCl = - 90 • Measured Membrane Potential Vm = - 90 mV

  3. Goldman • Theory: • Movement of ions across (through) the membrane is due to a Driving Force. • Driving Force = Diffusion Gradient + Electric Field

  4. Goldman • Assumptions: • Permeability - Velocity of ionic movement across the membrane is proportional to • ion solubility in the membrane • ion mobility in the electric field • reciprocal of the membrane thickness • Constant Electric Field - Potential varies linearly with the thickness of the membrane

  5. Goldman • Equation: • VEq = RT/ZF ln PKKout + PNaNaout + PClClin • PKKin + PNaNain + PClClout • Pi = Permeability Coefficient

  6. Goldman • If permeability of Cl- is either • very small (nerve cells) or • very large (skeletal muscle) • Cl- ions can be ignored. • Let a = PNa / PK • VEq = - RT/ZF ln Kin + aNain • Kout + aNaout

  7. Permeability • VEq = - RT/ZF ln Kin + aNain • Kout + aNaout • Presume permeability is due to macromolecules • (channels / pores) through the membrane that • are selective to specific ions (Na+ K+ Cl- H+ HCO3-). • What happens if the permeability changes? • What happens if the pores open/close completely?

  8. Ionic Currents • Electric current is the movement of charge per time. • Ions are charged particles. • How many ions per second = 1 ampere ? • Ionic Charge = 1.60 x 10-19 coulomb • Faraday’s Constant = 9.65 x 104 coulomb per mol • Avogadro’s Number = 6.02 x 1023 ions per mol

  9. Ionic Current • Net Driving Force = Vm - VEq • Vm = Membrane Potential • VEq = Nernst Potential • I » (Vm - VEq) • Let Proportionality Factor = Conductance g • Note: The transmembrane conductance is probabilistic and represents a population characteristic, NOT the value of a single pore. • If Vm - VEq < 0 implies positive ion enters the cell. • If Vm - VEq > 0 implies positive ion leaves the cell.

  10. Ionic Currents Vm = - 60 mV Na+ K+ Inject Na or K into the cell, Vm increases (less negative). Eject Na or K from the cell, Vm decreases (more negative). Therefore the flow of ions results in changes to the membrane potential.

  11. Ionic Currents • Change in Conductance • Ionic Current Flow • Change in Membrane Potential • When a single ionic species flows, the effect is to drive the membrane potential TOWARDS the equilibrium (Nernst) potential for that ion.

  12. Changes in Membrane Potential • Channel Sodium Potassium • Permeability • Conductance • Ionic/Current Flow • Vm Outward Inward

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