  Spontaneous directed motion, a hallmark of cell biology, is unusual in
classical statistical physics. Here we study, using both numerical and
analytical methods, organized motion in models of the cytoskeleton in which
constituents are driven by energy-consuming motors. While systems driven by
small-step motors are described by an effective temperature and are thus
quiescent, at higher order in step size, both homogeneous and inhomogeneous,
flowing and oscillating behavior emerges. Motors that respond with a negative
susceptibility to imposed forces lead to an apparent negative-temperature
system in which beautiful structures form resembling the asters seen in cell
division.
