Microfilaments are the thinnest filament of a eukaryotic cell's cytoskeleton. The linear microfilament chains are composed of actin proteins, which are flexible and relatively strong. A cell's cytoskeleton is a cellular scaffolding consisting of these actin microfilaments, along with intermediate filaments and microtubules.
As explained by Michael W. Davidson of Florida State University, eukaryotic cells rely heavily on the integrity of microfilaments to survive the many stresses they endure. When first produced by the cell, actin appears in a globular form as G-actin. These globular monomer subunits pair together in a double-helix polymerized chain. When intertwined in long chains, actin subunits appear in filamentous form as F-actin.
Microfilaments can be organized as bundles or networks. The bundling protein fimbrin organizes microfilaments into tightly packed parallel bundles. In networks, microfilaments are arranged in a web-like form that often spans out from the cell's nucleus. Microfilaments found directly beneath the cell's plasma membrane are considered part of the cell cortex, as it regulates the shape and movement of the cell's surface. Assembly and disassembly of actin microfilaments are tightly regulated by cell signaling mechanisms. In muscle tissue, actin interacts with a myosin motor protein to form the contractile actin-myosin structure that generates movement.