  For various cell types and for lamellipodial fragments on flat surfaces,
externally induced and spontaneous transitions between symmetric nonmoving
states and polarized migration have been observed. This behavior is indicative
of bistability of the cytoskeleton dynamics. In this work, the Filament Based
Lamellipodium Model (FBLM), a two-dimensional, anisotropic, two-phase continuum
model for the dynamics of the actin filament network in lamellipodia, is
extended by a new description of actin-myosin interaction. For appropriately
chosen parameter values, the resulting model has bistable dynamics with stable
states showing the qualitative features observed in experiments. This is
demonstrated by numerical simulations and by an analysis of a strongly
simplified version of the FBLM with rigid filaments and planar lamellipodia at
the cell front and rear.
