Nernst equations - Can anyone help me with this?
root at qnm33.NoSubdomain.NoDomain
root at qnm33.NoSubdomain.NoDomain
Wed Jun 1 11:31:23 EST 1994
SALUT A TOUSIn article <Cq87HM.Ks2 at cunews.carleton.ca>, ataylor at superior.carleton.ca (alex taylor) writes:
|> In article <Rq2Ndh9.gokelly at delphi.com>,
|> GREGORY C.O'KELLY <gokelly at delphi.com> wrote:
|> > 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.
|>
|> This is an incorrect definition. An electrical current
|> results whenever any charged particle moves. Weather this is an
|> electron or not is totally irrelevant. Voltage is simply a measure of
|> a difference in potential energy-and by the way the Nernst potential
|> is equivalent to voltage in an electrical circuit.
|>
|> > 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.
|>
|> Chloride equilibrates accross the membrane. It contributes
|> very little if anything the resting potential. P.S. electrochemical
|> gradients are a little more complicated than this. You must include
|> concentration of ion on both sides of the membrane as well as the
|> species of ion. This is usually covered in first year physics or
|> chemistry-try the relevant text.
|>
|> >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.
|>
|> Actually, the Goldmann equation makes no such assumption,
|> nor are E and V treated as strictly equivalent. These equations are
|> derived in "Ionic Channels of Excitable Membranes" and in may other
|> sources. The real problem with the H and H model of the membrane has
|> to do with the time course of activation and inactivation of the
|> sodium and potassium currents. With the advent of patch-clamping it
|> was discovered that the timecourses of the currents were different in
|> the ensemble average (as modelled by H and H) than they were at the
|> single-channel level.
|>
|> > 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).
|>
|> You have an incorrect concept, actually very little
|> charge moves. The membrane exists in a steady-state, not in
|> equilibrium. This is why H and H had to use the Goldmann equation to explain
|> what was going on. The voltage changes in a neuron because of the
|> capacitive discharge of the membrane, not because the intracellular
|> space is being filled up with sodium ion.
|>
|> > 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,
|>
|> You don't know what you are talking about. Voltage is
|> voltage. An intracellular recording rig is basically a glorified
|> voltmeter. The Nernst equation was simply one attempt to model a
|> phenomenon that was already known to exist-that is that there is a
|> potential difference accross the membrame of nerve cells of about -60
|> mV.
|>
|> >all makes sense, that sodium pumps are legitimate ad hoc
|> >stratagems to allow for ion currents which are purportedly
|> >electrical.
|>
|> Ion pumps are electrogenic if there is unequal charge
|> transfer eg the sodium-potassium pump. If they are blocked with a
|> toxin the membrane potential changes. The current that they generate
|> is as electrical as it gets.
|>
|> >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.
|>
|> I think that you would find some basic physical chemistry
|> more beneficial than a new textbook. The first few chapters of Kandel
|> and Schwartz would also be a good source of reading material.
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