Resting (membrane) Potential

1. Resting (membrane) Potential DENT/OBHS 131Neuroscience 2009

2. Electrical signaling in neurons • dendritic synaptic inputs • transfer to the soma • generate APs • axonal propagation • ionic basis of RMP • AP initiation & propagation

3. Learning Objectives • Explain how the concentration gradient of potassium ions across the membrane gives rise to the resting membrane potential • Compute the equilibrium potential of an ion using the Nernst equation • Predict the effect of changing the concentration of an ion (or it’s relative permeability) on the membrane potential

4. 0 How familiar are you with resting and active properties of membranes? • Not at all • Somewhat • Very • Intimately

5. 10 The major ion involved in setting the resting membrane potential is... • Sodium • Calcium • Chloride • Potassium • Bicarbonate • Hydrogen

6. Learning Objective #1 • Explain how the concentration gradient of potassium ions across the membrane gives rise to the resting membrane potential

7. The RMP

8. The membrane acts to…… • separate and maintain (pumps) gradients of solutions with different concentrations of charged ions • selectively allow certain ionic species (K+) to cross the membrane…

9. initial conditions: different distribution of a K-salt membrane is only permeable to K there is no potential difference across the membrane • at equilibrium: • K ions diffuse down concentration gradient • anions are left behind: net negativity develops inside the cell • further movement of ions is opposed by the potential difference

10. Electrical difference….. IN vs. OUT + - 0 mV DS Weiss

11. Electrical difference….. IN vs. OUT + - 0 mV -70 mV DS Weiss

12. [x]outside RT Ex = ln [x]inside zF Can we calculate the potential? • The Nernst equation determines the voltage at which the electrical and chemical forces for an ion (X) are balanced; there is NO net movement of ions.

13. Learning Objective #2 • Compute the equilibrium potential of an ion using the Nernst equation

14. 10 [K]OUT RT 60 -60 mV Ex = ln = log = z [K]IN 100 zF The Nernst potential for K+ • if K is 10-fold higher on the inside • in excitable cells the RMP is primarily determined by K ions

15. 10 If we lowered the [K+]OUT 10-fold to 1 mM, the RMP would….. • Not change • Hyperpolarize • Depolarize

16. 1 [K]OUT RT 60 -120 mV Ex = ln = log = z [K]IN 100 zF The Nernst potential for K+ • What about hyperkalemia?

17. Learning Objective #3 • Predict the effect of changing the concentration of an ion (or it’s relative permeability) on the membrane potential

18. Other ions affect RMP • different ions have different distributions • cell membrane is not uniformly permeable (“leaky”) to all ions • relative permeability of an ion determines its contribution to the RMP • a small permeability to Na and Cl offsets some of the potential set up by K • in reality the cell membrane is < negative than EK

19. Concentrations of other ions…..

20. ENa +67 membrane potential (mV) RMP ECl -90 EK -98 General rule(s) • relationship between: • membrane potential • ion equilibrium potentials • if the membrane becomes more permeable to one ion over other ions then the membrane potential will move towards the equilibrium potential for that ion (basis of AP) - DRIVING FORCE • artificial manipulation of MP - reverse direction of current flow (hence reversal or equilibrium potential)

21. ion flux explanation • driving force on an ion X will vary with MP • = (Em - Ex) • Ohm’s law • V = IR = I/g, or transformed I = gV • Ix = gx (Em - Ex) • there will be no current if: • no channels for ion X are open (no conductance, g) • no driving force (MP is at Ex)