Engulfment of material is facilitated by the actin-myosin contractile system. This starts the depolarization and eventual action potential in contractile cells. It inhibits cytoplasmic division by blocking the formation of contractile microfilaments. These are organized into sarcomeres, the fundamental contractile units of muscle cells. The pupillary dilator consists of a spokelike arrangement of modified contractile cells called myoepithelial cells. The actin filaments of contractile units are attached to "dense bodies". There are two types of cardiac muscle cells&colon; autorhythmic and contractile. This local generation of new actin filaments is important for the contractile ring formation. During contraction, there is a spatial reorganization of the contractile machinery to optimize force development. Usually only the macronucleus and sometimes cilia and contractile vacuoles are visible in the cyst. They are associated with the contractile smooth muscle via gap junctions. Muscles are bundles of contractile fibers that are attached to bones by tendons. It is also found in non-muscle cells in contractile bundles called stress fibers. Muscles contain special proteins called contractile protein which contract and relax to cause movement. Calcium is pumped away from the contractile apparatus and the muscle relaxes.