Nernst equations - Can anyone help me with this?

Rifle River jstream at girch1.med.uth.tmc.edu
Mon May 23 11:25:02 EST 1994


In article <Rq2Ndh9.gokelly at delphi.com>, GREGORY C.O'KELLY
<gokelly at delphi.com> wrote:
>  
>   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

No, it is not.  Electricity is a force exerted between charged particles. 
It could be any charged particle like an electron or a potassium ion.  You
need to read a basic Physics text.  Basic Chemistry and Biology may also
help your understanding.

> 	The Nernst equation does not directly translate into the 
> potential difference of electricity.

The Nernst potential describes only one ion with one permeability.  There
are multiple ions in biological systems with differing permeabilities. 
Therefore, the Nernst equation is inadequate in describing membrane
potential and the GHK equation is much more useful. 

>  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.

No, it does not.  Different channels have different permeabilities. 
Therefore, the relative contribution from each Nernst potential varies
dependent upon which ion is the most permeable.

>   It was then hypothesized that observed values 
> differed from theoretical values because of the permeability of the 
> membrane to other ions.

Right.

>  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.

See Kandel and Schwartz for the Goldman-Hodgkin-Katz equation.  This will
help you understand what's wrong with this statement.
 
> In other words, Vm would go from Ek 
> to Ena as the action potential passed and Na+ flowed across the 
> membrane.

Vm never reaches Ek or ENa because the membrane is never permeable to just
one ion (which is what it would take to reach Eion).

> 	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.

No, it was not based merely on the fact that Vm went from neg. to pos. 
Evidence for the movement of specific ions came from the drugs/chemicals
which happen to block some channels, not others.  When the only information
you have is that the cell became positive, you can't know whether it was a
Cl eflux, K influx or Na influx based on that infromation alone.

>  According to the 
> Nernst equations, if the concentration of Na+ intracellularly is 
> increased to more nearly what it is outside,

Greg, you continually ask us to listen to what you have to say.  However,
you refuse to listen to what I have said.  I sent you email explaining that
concentrations do not change during the firing of an action potential.
 
>  sodium was replacing potassium 
> intracellularly (in which case, according to the Nernst equations, 
> Ena should have been far smaller than +55mV).

Yes, it would have been 0mV instead of approaching 55mV.  However, this
"replacement" does not occur.

> 	I suspect that the conflation between electrical potential 
> differences and Nernst potential differences falsely equates ion gradients >with 
> voltage.

You have not adequately supported this point as refuted above.  However, 
your understanding might become clearer if you look up the GHK equation. 
Once you understand how the Nernst potential relates to the GHK equation
and how the GHK relates to membrane voltage this may clear up some of your
misunderstanding.  I suggest this because I know you have Kandel and
Schwartz.

>  I am told I don't know what I am talking about, and that this 
> all makes sense,

You don't know what you are talking about.  At the same time, basic facets
of neuroscience seem wrong to you *because* you don't understand
neuroscience.

> that sodium pumps are legitimate ad hoc 
> stratagems to allow for ion currents which are purportedly 
> electrical.

Now you demonstrate that you clearly do not understand.  Pay attention: 
For the firing of one action potential and the subsequent repolarization of
the membrane, no pumps are needed.  In other words, electrical ion currents
can occur in the absence of pumps.
   
>  What I am questioning is the equating of Ex and Vm

Ex is not equated with Vm in a biological system.  You REALLY need to see
the GHK equation.

> the insistence that Nernst equations tell us the latter too;  that Ek 
> and Ena cannot exist simultaneously across the same membrane wall

That is simply the definition of the Nernst potential.  It is an
experimental construct that is not achieved in a biological system.  See
GHK...
 
>  that Na+ displaces K+;

Na does not displace K.  Only a few ions move at a time.

>  and that Em must 
> be one or the other.

No, it's a combination:  hence, the GHK equation.

> 	Can anyone shed some light on this matter for my own 
> enlightenment?  I will admit to being 
> a beginner in this area, so maybe there is something the textbook did 
> not cover.

Kandel and Schwartz is a thorough textbook.  I think it is unreasonable to
expect that you should retain everything even after several consecutive
readings.  I suggest you go back to the chapters on membrane potential.  If
you send me email demonstrating that you have read (and thought) about the
GHK equation then I will explain it to you or answer any further questions
regarding membrane potential.  I think that you will probably be able to
answer your own questions, though.  

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