Notes from (Plonsey and Barr) Chapter 3

hi guys,

I got a couple of emails about your Chapter 3 reading and I wanted to post my answers for you all to share. I will be posting more as I get more questions so check back often. So, here are some notes:

(1) Relative charge depletion is a way for us to guage how much (if at all) the intracellular and extracellular concentrations are modified by the movement of ions that charge the membrane. We make the assumption that concentrations don't change even after the membrane is charged and calculating relative charge depletion is a good way to test whether this is a valid assumption. There is an example in your (Plonsey and Barr) textbook on page 61 that shows how to calculate it. If you have specific questions about this example please let me know.

(2) the Nernst Potential is the voltage the cell membrane WOULD HAVE TO BE in order for a particular ion to be in equilibrium. As we discussed in class after we computed the different Nernst potentials for different ions, there is no single Nernst potential that would being all ions into equilibrium, since the Nernst potential for Calcium is different than that for Potassium etc. The parallel conductance model helps us make sense of this fact by putting variable conductances into the equation. Vm is the a weighted average of Nernst potentials depending on the conductances of the different ions.

(3) I skipped the section about contributions from Chloride since I don't think it is particularly important and confusing. Basically, the Chloride ion is not very important in determining the membrane potential, but its behavior is interesting to the authors of this book because it tracks that of potassium (which IS very important) and because its intracellular concentration is very small and is much more affected by small amounts of influx or efflux.

(4) About typos, send them onto me, maybe we can compile a lit of erratas together and send them to the publisher at the end of the course.