1 / 13

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 )

wilona
Télécharger la présentation

Concepts

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related