  Cytoskeletal networks are the main actuators of cellular mechanics, and a
foundational example for active matter physics. In cytoskeletal networks,
motion is generated on small scales by filaments that push and pull on each
other via molecular-scale motors. These local actuations give rise to large
scale stresses and motion. To understand how microscopic processes can give
rise to self-organized behavior on larger scales it is important to consider
what mechanisms mediate long-ranged mechanical interactions in the systems. Two
scenarios have been considered in the recent literature. The first are systems
which are relatively sparse, in which most of the large scale momentum transfer
is mediated by the solvent in which cytoskeletal filaments are suspended. The
second, are systems in which filaments are coupled via crosslink molecules
throughout. Here, we review the differences and commonalities between the
physics of these two regimes. We also survey the literature for the numbers
that allow us to place a material within either of these two classes.
