npps.md

+# NPP expression
+
+In the model of Mauritz() the time dependent permeability due to NPPs was described by:
+
+P = P_max * (1 + exp[(t_12 - t) / s])^-1
+where P_max is the maximal permeability for an ion, t_12 is the timw of half-maximal permeability and s is the steepness of the permeability change.
+
+Here we use this function to describe the expression of NPPs.
+We describe it with the time derivative as we use ODEs:
+
+dNPP/dt = NPP_max * exp((t_12 - t) / s) / (s * (1 + exp((t_12 - t) / s))**2)
\ No newline at end of file