On 24 Sep 2003 04:39:28 GMT, lucillepaul at aol.com (LucillePaul) wrote:
> Hello all:
>>>I have a question about different membrane potentials and how it affects
>movement of Na+ and K+. I know that at -70mv, Na+ is moving into the cell and
>K+ is moving out. However, what about if the membrane potential increased or
>decreased? The hard part I am having is that I know that opposites attract, so
>if, say the membrane potential was +5mv, then both Na+ and K+ would be move out
>of the cell to balance out. However, there is the concentration gradient to
>consider and how strong it is. Wouldnt K+ be moving into the cell at -130mv
>because the charge is so much greater than the concentration gradient?
>>Thanks!
>>Lucy
There are two different "forces" acting on ions across the membrane --
diffusion and electrical. Diffusion always causes ions to move from
higher concentration to lower. Electrical forces always cause a
positive ion to move from a more positive potential to a more negative
one. Sometimes the two "forces" act in the same direction and add,
sometimes they act in opposite directions and tend to cancel. There
is one specific electrical potential where the two tendencies to move
(what I call "forces") cancel so the ion doesn't move at all. That is
the Nernst equilibrium.
At rest, diffusion for Na+ is inward and diffusion for K+ is
outward, assuming reasonably normal ion concentrations inside the cell
and outside. At rest, the electrical force for both Na+ and K+ is
inward because the inside of the cell is negative. As a result, the
two forces add for Na+ making a very strong inward force. The two
forces tend to cancel for K+ but normally outward diffusion is
slightly greater than inward electrical so K+ tends to move out.
If you depolarize the cell (make it less negative), you reduce the
inward electrical force. So K+ would leave the cell faster and Na+
would enter the cell more slowly. If you depolarize to zero voltage,
then the electrical force disappears completely and only diffusion
remains. You still have inward Na+ and outward K+. If the cell
becomes inside positive, the electrical force pushes Na+ and K+ both
out. Still, at +5mV, the inward diffusion on Na+ is stronger than the
outward electrical and Na+ still enters the cel. It is not until you
pass ENa, the Na equilibrium potential (Nernst potential for Na) that
the situation reverses so that Na actually leaves the cell.
These same ideas happen when you hyperpolarize, making the cell more
negative. Now you are increasing the inward electrical force. At
some point (EK, the K equilibrium potential) the two forces on K+
cancel and K+ doesn't move. When you hyperpolarize even further, the
inward electrical force on K+ becomes stronger than the outward
diffusion and K+ will enter the cell.
The key to understanding which way an ion will move when confronted
with two conflicting forces is to consider the Nernst potential. For
a positive charge, if the membrane potential is above the Nernst
potential, the ion will move inward. If the potential is below the
Nernst potential, the ion will move outward. For negative ions (Cl-),
the movement is reversed but the reveral point is still the Nernst
potential.
