The two types of filaments comprising the sarcomeres are thin filaments, actin, and thick filaments, myosin, though actin and myosin are not only present in muscle cells but perform structural and mechanoenzyme functions in other types of eukaryotic cells. Actin, in fact, is the most ubiquitous structural protein of the eukaryotic cytoskeleton, involved in many kinds of intracellular movements.