  The inactivation of a Na+ channel occurs when the activation of the charged
S4 segment of domain DIV is followed by the binding of an intracellular
hydrophobic motif which blocks conduction through the ion pore. The voltage
dependence of Na+ channel inactivation is, in general, dependent on the rate
functions of the S4 sensors of each of the domains DI to DIV. If the activation
of a single voltage sensor that regulates the Na+ channel conductance is
coupled to a two-stage inactivation process, the rate functions for
inactivation and recovery from inactivation, as well as the time dependence of
the Na+ current in terms of the variables m(t) and h(t), may be derived from a
solution to the master equation for interdependent activation and inactivation.
The rate functions have a voltage dependence that is consistent with the
Hodgkin-Huxley empirically determined expressions, and exhibit saturation for
both depolarized and hyperpolarized clamp potentials.
