Membrane potential buffering by permeable ions

Membrane potential buffering by permeable ions

均勻分布電荷於無限延伸之平板上對平板外任意點產生之電場如下

If there is a membrane with equal space inner & outer to membrane, which is freely permeable to [A-],[B+],impermeable to [Na+]. When we give more [Na+] to outside, without [A-],[B+] around two side, inner potential(V) become negative, which proportional to ([Na+]i-[Na+]o)

When we give more [Na+] to outside, same half [A-],[B+] around two side, V become negative, and then permeable cations get in membrane, permeable anions get out of membrane, both counteract the negative voltage created by ([Na+]i-[Na+]o) alone. As a result, same ([Na+]i-[Na+]o) makes less transmembrane potential due to buffering effect of permeable ions. On the other words, it takes more |[Na+]i-[Na+]o| change to make same potential change.

The exact formula derive from Nernst equation is shown below

It's somewhat like the pH buffering effect of buffer solution.
http://crtzengweb.blogspot.tw/2013/07/buffer-solution.html