  Cells and tissues have the remarkable ability to actively generate the forces
required to change their shape. This active mechanical behavior is largely
mediated by the actin cytoskeleton, a crosslinked network of actin filaments
that is contracted by myosin motors. Experiments and active gel theories have
established that the length scale over which gel contraction occurs is governed
by a balance between molecular motor activity and crosslink density. By
contrast, the dynamics that govern the contractile activity of the cytoskeleton
remain poorly understood. Here we investigate the microscopic dynamics of
reconstituted actin-myosin networks using simultaneous real-space video
microscopy and Fourier-space dynamic light scattering. Light scattering reveals
rich and unanticipated microscopic dynamics that evolve with sample age. We
uncover two dynamical precursors that precede macroscopic gel contraction. One
is characterized by a progressive acceleration of stress-induced
rearrangements, while the other consists of sudden rearrangements that depend
on network adhesion to the boundaries and are highly heterogeneous. Our
findings reveal an intriguing analogy between self-driven rupture and collapse
of active gels to the delayed rupture of passive gels under external loads.
