Concepts The intracellular and extracellular fluids have unequal concentrations of specific ions. Na+ K+ Cl- H+ HCO3- The differences in concentrations.

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Presentation transcript:

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).

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

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

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

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

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

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?

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

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.

Ionic Currents Na+ K+ Vm = - 60 mV 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.

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.

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