Nernst equations - Can anyone help me with this?

GREGORY C.O'KELLY gokelly at delphi.com
Sun May 22 00:54:29 EST 1994


	Questions about Nernst equations and the offical view

	The Nernst equation was derived by Walter Nernst in 1888 
from thermodynamic principles.  He was attempting to find a way to 
estimate potential difference due to ion gradients.  He expressed 
this potential difference in volts.  Neuroscience has assumed that 
these were the volts of electricity.  Electricity is the movement of 
electrons across or along a conductor involving the valence shells of 
the atoms of the conductor.
	Nernst involved valence in his formula.  If, across the 
membrane wall, there is a greater concentration of K+ outside the 
cell, the potential is positive.  If the concentration is greater inside, 
the potential is negative.  For sodium, this is what we see.  For Cl-, 
if the concentration is greater outside, unlike for K+, the potential 
is negative, the opposite of what we find with greater 
concentrations of sodium or potassium outside the wall.
	The Nernst equation does not directly translate into the 
potential difference of electricity.  In the case of the squid giant 
axon we find that the Nernst equation results in for Na+, K+, and Cl- 
simultaneously +55mV, -75mV, and -60mV.  If these values were 
actually electrical values, then we would have -80mV for the 
resting membrane potential, Vmr.
	When techniques for intracellular recording were developed in 
the 1940's it was found that the electrical potential across the 
squid giant axon was -60mV.  This was a lower negative number than 
that predicted for Ek where Ek was the Nernst potential for 
potassium based on intracellular and extracellular recordings of ion 
concentration.  It was then hypothesized that observed values 
differed from theoretical values because of the permeability of the 
membrane to other ions.  It should be pointed out, however, that this 
approach assumed that theoretical membrane potential was not only 
a result exclusively of ion gradients of potassium, but that it 
couldn't also simultaneously exist, as it did in the squid axon, with 
an Ena of +55.  This approach equated E with V, Nernst membrane 
potentials with electrical potentials, and insisted that Ek or Ena 
must prevail, but that the two could not be simultaneous as they 
were in the squid giant axon.  In other words, Vm would go from Ek 
to Ena as the action potential passed and Na+ flowed across the 
membrane.
	Furthermore, because, with the passing of an action potential, 
the Vm went from negative to positive, this was taken that Na+ 
rushed in to the membrane, and K+ rushed out.  According to the 
Nernst equations, if the concentration of Na+ intracellularly is 
increased to more nearly what it is outside, then Ena is smaller than 
+55mV.  Still it was thought that because Vm went from -60mV to 
+45 or +50mV, and because, unlike in the giant axon of the squid, 
these Ek and Ena could not exist simultaneously, and because Vm 
was equated with Ex,  sodium was replacing potassium 
intracellularly (in which case, according to the Nernst equations, 
Ena should have been far smaller than +55mV).
	I suspect that the conflation between electrical potential 
differences and Nernst potential differences, even though they are 
expressed in the same terms, falsely equates ion gradients with 
voltage.  I am told I don't know what I am talking about, and that this 
all makes sense, that sodium pumps are legitimate ad hoc 
stratagems to allow for ion currents which are purportedly 
electrical.  I am not denying membrane permeability, and ion 
channels.  What I am questioning is the equating of Ex and Vm and 
the insistence that Nernst equations tell us the latter too;  that Ek 
and Ena cannot exist simultaneously across the same membrane wall 
as they do in the squid, i.e., that Na+ displaces K+;  and that Em must 
be one or the other.
	Can anyone shed some light on this matter for my own 
enlightenment without becoming ad hominem?  I will admit to being 
a beginner in this area, so maybe there is something the textbook did 
not cover.  If not, there could be some problems.



More information about the Neur-sci mailing list