ABSTRACT
The mechanistic bases of electrical signaling in neurons can be understood by considering a simple spherical
cell (Figure 7.2(a)) with a membrane that is semi-permeable (i.e., a membrane that passes ions, but only slowly,
so that the internal and external concentrations of the ions can be assumed constant). Two factors influence
the movement of ion X through the membrane. First, any concentration gradient of X across the membrane
will drive a diffusive flux, according to Fick’s first law. Second, because the ion has charge, its movement will be
influenced by any electrical field across the membrane. The Nernst equation describes the equilibrium
condition for ion X, in which these two sources of ionic flux cancel each other out, resulting in zero net flux:
In Equation (7.1), E
is the equilibrium or Nernst potential, the value of membrane potential (inside minus
outside, by convention) at which this single-ion system is at equilibrium. R is the molar gas constant,
8.31 J/(mol K). T is absolute temperature in degrees Kelvin. z
is the valence of X (þ1 for Na
and K
). F is
Faraday’s constant, 96,500 C/mol. ½X
and ½X
are the external and internal concentrations of X, typically
expressed in mM. It is often more convenient to convert the Nernst equation to log
, and to assume a value of
temperature. At 37
C, the Nernst equation reduces to
where E
has units of mV.
