Fig. 1. Contraction of acto-myosin networks. Actin filaments (green) and myosin filaments (orange) interact to generate contractility. (A-C) Large boxes illustrate the structure of the acto-myosin network; small boxes illustrate the overall network before (top) and after (bottom) contraction. The black arrows indicate a contractile stimulus, for instance, ATP addition. Muscle sarcomeres (A) are highly ordered and contract uniformly. (B) Cortical networks are randomly organized; contraction results in local densities of contractility and local breaks in the network. (C) Contractile interactions do not exist in a solated network. (D) A uniform acto-myosin network (as depicted in B) exhibits local contractions throughout the network (left box), but the local downregulation of myosin activity or the severing of actin filaments (circle, left box) can lead to a global asymmetric contraction of the entire network (right box).