Motor proteins exert active stresses on polar actin filaments, generating self-organized flows and defect dynamics. Toner-Tu theory describes flocking; Kruse-Jülicher-Joanny-Prost theory describes contractile/extensile active gels.
Active gels: ζ>0 = extensile (pushes fluid along axis, e.g. MT/kinesin), ζ<0 = contractile (pulls inward, e.g. actomyosin). Instability at |ζ|>ζ_c generates spontaneous flows and ±1/2 topological defects in the orientation field. Ramachandran-Joanny: defect unbinding controls active turbulence.