  We consider the coaction of two distinct noise sources on the activation
process of a single and two interacting excitable units, which are
mathematically described by the Fitzhugh-Nagumo equations. We determine the
most probable activation paths around which the corresponding stochastic
trajectories are clustered. The key point lies in introducing appropriate
boundary conditions that are relevant for a class II excitable unit, which can
be immediately generalized also to scenarios involving two coupled units. We
analyze the effects of the two noise sources on the statistical features of the
activation process, in particular demonstrating how these are modified due to
the linear/nonlinear form of interactions. Universal properties of the
activation process are qualitatively discussed in the light of a stochastic
bifurcation that underlies the transition from a stochastically stable fixed
point to continuous oscillations.
