Smooth muscle provides the contractile force for most of our internal organs, including the digestive tract, urinary bladder, uterus, and blood vessels. Structurally, smooth muscle cells are usually long and spindle-shaped and, unlike skeletal muscle, each cell has only a single nucleus. The actin and myosin filaments in smooth muscle are not arranged in a regular pattern like that in skeletal muscle, and the cells therefore have a "smooth" appearance, rather than the striated appearance of skeletal muscle, when viewed under a microscope.
In many organs, smooth muscle cells are arranged in sheets, with individual cells in electrical contact with one another through gap junctions. As a result, an action potential generated in one smooth muscle cell can quickly spread to all the cells in the sheet. Thus, the cells in the sheet can contract in a coordinated fashion. In the digestive tract, a coordinated, spreading wave of smooth muscle contraction will push the contents through its central lumen. This process is called peristalsis.
An interesting property of smooth muscle cells is that stretching alone can depolarize the smooth muscle membrane, activating the contractile mechanism. This property is important for organs such as the digestive tract, where food entering the tract will stretch the muscle cells, thereby triggering a wave of peristalsis that can push the food along the hollow core of the tract. The same rhythmic contractile properties also act to control the flow of urine from the bladder, and help expel the baby out of the uterus during childbirth.
Textbook Reference: Concept 33.4 Many Distinctive Types of Muscle Have Evolved