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.
