(ăk′tĭn)
[Pronunciation]

One of the contractile proteins that make up the sarcomeres of muscle tissue.
During contraction, the actin filaments are pulled toward the center of the sarcomere by the action of myosin filaments, and the sarcomere shortens. Actin is also found in the cytoskeleton of many kinds of cells, where it contributes to cell shape and movement. Polymerized actin is known as filamentous actin; the monomeric form of the protein is known as globular actin