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In the presence of several different ions, the equilibrium of the cell depends on the relative permeability of the ions. For this, we use the Goldman-Hodgkin-Katz equation:


Permeability of an ion is dependent on a number of factors such as the size of the ion, its mobility, etc. During rest in the squid giant axon, the permeabilities have the ratio so that

Since dominates, this is close to During an action potential the ratio is so that

Later on we will approximate the GHK equations by a linearized version:

where the conductances g are proportional to the permeabilities.


  1. Suppose the external potassium in a mammalian cell is increased by a factor of 10. What is the new value of ?
  2. At a cell contains 80 mM sodium inside and has only 100 mM sodium outside. What is the equilibrium potential for sodium?
  3. Using the same permeabilities for the mammalian cell as were used for the squid axon, compute using the table at the back of these notes.

Bard Ermentrout
Mon Jan 5 13:18:36 EST 1998