Re: Do we really understand the resting membrane potential?

Martin - helluva question! Makes a change from 'Why is the sky blue?' 
although probably far fewer people will want to read this answer than that 
one:

Note:

I referred mostly to Chang (1981) ‘Physical Chemistry with Applications to 
Biological Systems’ (2nd ed), Macmillan. I’m not sure I was able to dust 
of my knowledge of this topic enough to make sense of the question, never 
mind to compose an adequate answer. Please get back to MadSci if you need 
more detail.

Answer:

“Macroscopic electroneutrality” I would presume (not having met this 
phrase before) to mean that the cells and their immediate surroundings 
remain neutral. There are species of electric eel and rays where this is 
obviously not the case, but these are exceptional. Evidently, the interior 
of the cell is not neutral with respect to its immediate surroundings or 
there would be no potentials to speak of.

The membrane cannot be a simple capacitor because it is permeable to 
charge. The potential is generated by charge separation (anions from 
cations) across the membrane. With a capacitor the charges derive from 
elsewhere in a circuit.

I’m not sure what you mean by ‘infinitesimal differences’: A voltage is a 
voltage – the potential difference between two points. It doesn’t make 
sense in this case to add them up as if you have a battery of cells.

Chang gives the potassium concentrations as 400 mM inside an axon and 15 
mM outside. This would imply a membrane potential of -84mV i.e. if the 
inside is -84mV relative to the outside, then that would account for the 
lack of diffusion of potassium ions from the inside.
Because the potential is only -75mV potassium will diffuse out of the 
cell, and pumps are needed to maintain the 400/15 concentration ratio. 
The -75mV potential can be calculated (more-or-less) using the Goldman-
Hodgkin-Katz equation taking sodium ion concentrations as: 20mM inside and 
440 mM outside, and a 100-fold greater permeability of the membrane to 
potassium than to sodium. The equation works in practice – I’m not sure 
why you have a problem with its derivation.

The ‘arbitrary’ cancellation of ion fluxes is anything but an arbitrary 
assumption. If there was any net ion flux we would observe a change in 
resting potential, wouldn’t we? Of course, the ions are flowing into and 
out of the cell in dynamic equilibrium, but the influx and outflow of each 
type of ion is equal.

Ewen McLaughlin