28 Sept Announcements Pick up answer sheet for Quiz 2 from front Friday absentees: pick up Quiz 1 & Andro Paper from Piano Read & bring Androstenedione paper to lab this week Test # 1 Oct 5

Today’s Topics Membrane potentials Ohm’s law The Nernst Equation The Goldman Equation Resting Membrane Potential

Fig From physics: Ohm’s Law Voltage = Current x Resistance

Fig

Fig

Fig

Fig a There is a concentration gradient favoring the diffusion of Na+ and K+ through the selectively permeable membrane which has ion channels only for potassium.

Fig b With K+ channels open, K+ diffuses down its concentraiton gradient, leaving behind CL- ions which are not permeable through the membrane. As more and more K+ move to the left, the compartment they leave becomes more and more negatively charged.

Fig c

Fig d Soon, the accumulation of negative charges seriously impeded the diffusion of K+ as the electrostatic force builds up in opposition to the concentration driving force.

Fig e Equilibrium potential = Nernst potential = diffusion potential Eventually, the electrostatic force that impedes diffusion of K+ is exactly equal to the driving force favoring diffusion based on a concentration gradient. When these two driving forces are equal and opposite, the membrane potential reaches an equilibrium at which the voltage is called So which compartment corresponds to intracellular fluid? E ion+ = 61/Z log ([conc outside]/ [conc inside]) E K+ = 61/1 log (5/150) E K+ = -90 mV

The Nernst Equation Calculate the membrane potential if only one ion species is permeable and the concentrations are known on both sides of the membrane.

Fig Now consider a situation in which only Na+ is permeable.

Fig a

Fig b

Fig c

Fig d

Fig e Equilibrium potential for Na+ E Na+ = 61/1 log (145/15) E Na + = +60 mV

Electrical and concentration gradient driving forces for Sodium and Potassium Graded potentials in dendrites and cell body, action potentials in axon because of the types of ion channels found in these membrane domains. How does the membrane potential change if 1)permeability to sodium increases 2) Permeability to potassium increases Why is resting membrane potential closer to E K than E Na ? What would happen to membrane potential if suddenly P Na became very great?

Which ion moving in which direction (into or out of cell) is responsible for depolarization and overshoot? Which ion moving in which direction (into or out of cell) is responsible for repolarization and hyperpolarization? Can the membrane potential go more negative than -90 mV?