Active Stresses and Stress Relaxation in Cytoskeletal Networks*

Living systems must operate out of thermodynamic equilibrium at the molecular scale. This lack of equilibrium results in directed fluxes through chemical states or phase space, corresponding to violations of the principle of detailed balance at the molecular scale. We explore consequences of such active, nonequilibrium dynamics at the meso and macro scales, with particular emphasis on cytoskeletal networks, which are driven by a variety of internal stresses. These stresses are often due to molecular motors, such as myosin that generates tension in the actin networks. These stresses can result in active remodeling of the networks and self organization of stress, particularly in networks near marginal stability. Active processes can also govern stress relaxation in such networks, through treadmilling and severing of filaments. We show how severing can lead to a surprising molecular weight independent relaxation in actin gels. We also show how transient crosslinking, together with prestress can lead to very slow, glassy relaxation in actin gels. Moreover, transient nonequilibrium (un)binding can also generate contractile stress in the absence of molecular motors.