Stress fiber
Stress fibers are high order structures in cells consisting of actin filaments (aka microfilaments), crosslinking proteins (proteins that bind two or more filaments together), and myosin II motors.

Actin is a globular protein (~43 kDa). It polymerizes, forms into an ordered filament structure which has two protofilaments wrapping around each other, to form a single 'actin filament' also known as a 'microfilament'. Microfilament is an old term that originated before protein composition of the filaments was understood. Actin filament is the typically accepted terminology in scientific circles now.

The myosin motors in the stress fibers move, sliding actin filaments past one another, so the fiber can contract. The process of stress fiber dynamics is currently being rigorously studied and is not yet well understood. In order for contraction to generate forces the fibers must be anchored to something. Stress fibers can anchor to the cell membrane, and frequently the sites where this anchoring occurs are also connected to structures outside the cell (the matrix or some other substrate). These connection sites are called focal adhesions. Many proteins are required for proper focal adhesion production and maintenance. Contraction against these fixed external substrates is what allows the force generated by myosin motors and filament growth and rearrangement to move and reshape the cell.
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