Smooth muscle tissue differs structurally from the other two types of muscle tissue in several ways. Most obviously, the actin and myosin filaments are organized in a way that does not create a striated appearance. Because these muscle cells are nonstriated, they are termed smooth muscle. Smooth muscle cells are small in comparison to the other types of muscle. They have an elongate, spindle-shaped appearance with a central nucleus. Smooth muscle cells typically form thin sheets or small individual bundles and can contract in unison or individually. Smooth muscle contractions are slower than those of striated muscle cells, but they are able to maintain forceful contractions for long periods with minimal use of energy. Like cardiac muscle, smooth muscle is considered to be involuntary muscle because it is under unconscious control. Smooth muscle is found in the walls of the digestive tract, the urinary system, the blood vessels, some respiratory passageways, and certain reproductive organs. Contractions of the smooth muscle within the walls of these organs help control the movement of the contents through these organs.