"We examined in models the influence of Na channel activation, inactivation, slow voltage-gated channels and synaptic conductances on spike threshold.
We propose a threshold equation which quantifies the contribution of all these mechanisms.
It provides an instantaneous time-varying value of the threshold, which applies to neurons with fluctuating inputs.
We find that spike threshold depends logarithmically on Na channel density, and that Na channel inactivation and K channels can dynamically modulate it in an adaptive way: the threshold increases with membrane potential and after every action potential.
" See paper for more.
Platkiewicz J, Brette R (2010) A threshold equation for action potential initiation. PLoS Comput Biol 6:e1000850 [PubMed]