The functional unit of visceral (smooth) muscle tissue is called the “smooth muscle cell” or “smooth muscle fiber.” Unlike skeletal muscle, which is made up of long, multinucleated muscle fibers, smooth muscle cells are smaller and spindle-shaped with a single nucleus. These cells contract rhythmically and involuntarily to perform various functions within the body, such as propelling substances through hollow organs like the digestive tract, regulating blood flow in blood vessels, and controlling the diameter of airways in the respiratory system. The coordinated contractions of smooth muscle cells allow for the movement and regulation of various physiological processes.