Sub- and super-threshold brief stimuli in the AFHN model.

(Left) For , indicates the threshold for evoking an action potential. Two trajectories starting near are identified by arrows: (1, left arrow) when the initial condition , returns quickly to the resting potential ; (2, right arrow) when , an action potential is evoked: the system follows a counterclockwise trajectory, quickly approaching the -nullcline, following the right stable branch until reaching the right knee, quickly reaching the left stable branch, and then returning to rest. (Right) When , , the critical value above which an action potential cannot be evoked by a brief perturbation from the steady-state.