"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)
|| >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
| 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,
| equilibrium potential).
|| The real distinction occurs with the numerous synapses where a
| allows several types of ion through. For example, the vertebrate
| neuromuscular junction nicotinic receptor/channel allows both Na+
| K+ through. So the reversal potential is neither EK, the potassium
| equilibrium potential nor KNa, the sodium equilibrium potential,
| some value approximately halfway between these two. At the
| 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
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
"Schmitd! Schmitd! Ve vill build a Shapel!"