Smooth muscle is primarily found in the walls of hollow structures and some visceral organs, including the walls of the vasculature, GI, respiratory, and genitourinary tracts. Smooth muscle contracts more slowly and is regulated differently than skeletal muscle. Smooth muscle can be stimulated by nerve impulses, hormones, metabolic factors (like pH, CO2 or O2 levels), its own intrinsic pacemaker ability, or even mechanical stretch. Whatever the stimulus is, it results in an increase in sarcoplasmic Ca levels. This Ca results in a phosphorylation of myosin, which activates it, allowing the myosin to interact with the actin. In smooth muscle, the actin is attached to cytoskeletal proteins located throughout the sarcoplasm and cell membrane known as dense bodies. Therefore, when the myosin pulls on the actin, the actin pulls on the dense bodies, causing the entire cell to â€œscrunchâ€ up and contract.