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

[Kalman Rubinson]

On Tue, 29 Apr 2003 08:30:05 -0400, r norman <rsnorman_ at _comcast.net>
wrote:
>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.


Yup.  That's what we try to teach our students but, as you point out,
they find it quite subtle.

Kal