  Complex interactions between cellular systems and their surrounding
extracellular matrices are emerging as important mechanical regulators of cell
functions such as proliferation, motility, and cell death, and such cellular
systems are often characterized by pulsating acto-myosin activities. Here,
using an active gel model, we numerically explore the spontaneous flow
generation by activity pulses in the presence of a viscoelastic medium. The
results show that cross-talk between the activity-induced deformations of the
viscoelastic surroundings with the time-dependent response of the active medium
to these deformations can lead to the reversal of spontaneously generated
active flows. We explain the mechanism behind this phenomenon based on the
interaction between the active flow and the viscoelastic medium. We show the
importance of relaxation timescales of both the polymers and the active
particles and provide a phase-space over which such spontaneous flow reversals
can be observed. Our results suggest new experiments investigating the role of
controlled pulses of activity in living systems ensnared in complex
mircoenvironments.
