The FitzHugh-Nagumo model. Speed variation of travelling waves

Excitation cI(t) with period T at the left border is added to the system
    t u = DΔu + F(u, v) = DΔu + u(1 - u)(u - a) - v + c I(t)δ(x),
    ∂t v = H(u, v) = ε (bu - v),
    I(t) ~ [cos(πt/T)]500,     δI(t)dt = 1.

Below you can see that travelling wave speed depend on recovery state. The first impulse is the fastest. Next impulses are slower because v > 0 for them.
a b ε Δt Δx D T c

delay Ymin Ymax it fps
Heart rhythms     updated 3 Dec 2011