The two major types of smooth muscles are multiunit and visceral. Multiunit smooth muscles are found in the walls of blood vessels and the iris of the eye. These fibers are separate rather than organized into sheets, and they contract only in response to stimulation by nerve impulses or selected hormones. Visceral smooth muscles, which are more common, are composed of sheets of spindle-shaped cells in close contact with one another. They are found in the walls of organs such as the stomach, intestine, uterus, and urinary bladder. Fibers of visceral muscles can stimulate each other and also display rhythmicity or repeated contractions.