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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