Elasticity and dynamics of cytoskeletal filaments and networks of them

Fundamental to any understanding of the complex cytoskeleton is a quantitative model for the structure, interactions, and response of networks such as the actin cortex. Unlike conventional polymer networks and gels, however, these networks have been clearly shown to possess properties that cannot be modelled by existing polymer theories. These properties include anomalously large shear moduli, strong signatures of a nonlinear response, and unique dynamics. In a very close and active collaboration between theory and experiment over the past decade or so, a standard model of sorts for the material properties of such networks has emerged. Central to such a model has been the semiflexible nature of the constituent filaments, which is both a fundamental property of almost any filamentous protein and a clear departure from conventional polymer physics, which has focused on flexible and rod-like limits. This chapter begins with an introduction to the models of the response and dynamics of single filaments. Since cytoskeletal filaments are often the most important structural components in cells, a quantitative understanding of their mechanical response to bending, stretching, and compression is crucial for any model of the mechanics of networks of these filaments. It shows how the fundamental properties of the individual filaments can explain many of the properties of solutions and networks of biopolymers. Finally, some of the nonequilibrium aspects of cells and reconstituted cytoskeletal networks are examined.