Microfilaments are thin fibers that function as cooperative members of the cytoskeleton. Single fibers usually group together to perform various functions. They form a thin skeleton just inside plasma membranes called the cortical cytoskeleton to provide stiffness, structure, and shape to the membrane.
Microfilaments are polymers of a 42 kD protein called actin, of which there are several tissue-specific variants and are ropes of actin monomers, and look like two single chains of monomers twisted together. The diameter of the fiber is 6-9 nm depending on the twist.