Reversal potential/Equilibrium potential Definitions! (should be easy for you guys)

r norman rsnorman_ at _comcast.net
Wed Apr 30 08:43:51 EST 2003


On Wed, 30 Apr 2003 00:25:12 GMT, "KP-PC"
<k.p.collins at worldnet.att.net%remove%> wrote:

>Hi, Richard.
>
>"r norman" <rsnorman_ at _comcast.net> wrote in message
>news:5nrsavgildi2i904qud2hmed640iv08sbm at 4ax.com...
>| On 28 Apr 2003 08:43:05 -0700, chrismin at bigpond.com (chrissy)
>wrote:
>|
>| >Just a simple question (hopefully):
>| >What's the difference between the reversal potential and the
>| >equilibrium potential for ion flow in neurons?
>| >Both have to do with balancing electrical potentials, don't they?
>|
>| This is always a subtle distinction for students.
>|
>| If a channel is permeant to a single ion, then the reversal
>potential
>| is, in fact, the equilibrium potential for that ion.  At membrane
>| potentials more negative, the ionic currents flow one way and at
>| membrane potentials more positive, they flow the other way.  At the
>| specific value, the currents reverse (reversal potential) and the
>| diffusion and electrical forces cancel (no net change in delta G,
>the
>| equilibrium potential).
>|
>| The real distinction occurs with the numerous synapses where a
>channel
>| allows several types of ion through. For example, the vertebrate
>| neuromuscular junction nicotinic receptor/channel allows both Na+
>and
>| K+ through.  So the reversal potential is neither EK, the potassium
>| equilibrium potential nor KNa, the sodium equilibrium potential,
>but
>| some value approximately halfway between these two.  At the
>reversal
>| potential, neither ion is in equilibrium.  Still, the influx of Na
>| just balances the effluxs of K so the current is zero.
>
>But the "zero" isn't actually the same thing twice.
>
>Please come back, if I didn't establish this 'point' in my reply to
>"New Zelander".
>
>Your 'point' in your 2nd paragraph is some of it - it's not only
>"some value approximately halfway between these two", that
>'combinatory'-value is, itself, never the same [regardless of what
>the meter says :-]
>
>[I'm taking advantage of folks' focus to [hopefully] lift folks'
>understanding up to the 'level' of integrated nervous system
>function.]
>
>ken

Ken, I think your questioning the nature of "zero" is really about the
following point.

The real distinction between equilibrium potential and reversal
potential is the true subtlety that students fail to appreciate. That
is: the reversal potential is an empirically determined value that you
discover by performing an experiment.  Clamp the membrane potential,
systematically varying the value. Measure the synaptic current.  Plot
current vs. potential.  You get a plot that crosses the x-axis (zero
current) at some potential.  That is the reversal potential.  To the
left (more negative potentials) the current is outward. To the right
(more positive potentials) the current is inward.  There is no problem
with defining "zero" or interpreting what it means.  Zero means no
(additional) current flows through your clamp equipment caused by the
opening of synaptic channels.  What is actually happening at the
synapse is a matter of interpretation. You are right in that this zero
is really the cancellation of two ongoing processes, both subject to
fluctuation from other circumstances (that may alter ionic
concentrations, for example).  That is, it is not an "absolute" or
fixed point.

The equilibrium potential is another story completely.  It is a
theoretical value based on physical chemistry and the Nernst
equilibrium.  It is not at all something that you measure.  However,
according to the ionic theory of membrane behavior (i.e., the membrane
contains channels that allow ions to move solely according to the laws
of diffusion and electricity) then under certain very special
conditions, you should expect an actual measured potential to be equal
to that conceptual or theoretical value. The special conditions are
that the membrane be permeable to only one ion (or if it permeable to
more than one, that they all obey the Donnan condition so they can all
be in equilibrium at the same time).

The fact that the reversal potential for a particular synapse matches
the equilibrium potential for a particular ion is the experimental
evidence that the synaptic channel is specific for that ion.  A more
throrough experimental test would be to vary the equilibrium potential
by changing the ionic concentration and verifying that the reversal
potential changed in exactly the same way.

Ken, you have a second extremely valid point that you consistently
emphasize -- that the nervous system is a very complex, integrated,
holistic entity.  Everything influences everything else and a purely
molecular, physical chemical approach does not lead to a real
understanding of what is happening.  Again, that is very true.

However, you also have your own theories about how to deal with the
situation, and that is another story, entirely.





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