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

r norman rsnorman_ at _comcast.net
Tue Apr 29 07:30:05 EST 2003

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.

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