  The cellular cytoskeleton is an active material, driven out of equilibrium by
molecular motor proteins. It is not understood how the collective behaviors of
cytoskeletal networks emerge from the properties of the network's constituent
motor proteins and filaments. Here we present experimental results on networks
of stabilized microtubules in Xenopus oocyte extracts, which undergo
spontaneous bulk contraction driven by the motor protein dynein, and
investigate the effects of varying the initial microtubule density and length
distribution. We find that networks contract to a similar final density,
irrespective of the length of microtubules or their initial density, but that
the contraction timescale varies with the average microtubule length. To gain
insight into why this microscopic property influences the macroscopic network
contraction time, we developed simulations where microtubules and motors are
explicitly represented. The simulations qualitatively recapitulate the
variation of contraction timescale with microtubule length, and allowed stress
contributions from different sources to be estimated and decoupled.
