Smooth muscles are found in the walls of many organs (hollow organs such as the bladder or digestive tract) and blood vessels. The wall is then called a tunic. The function of smooth muscle is to help transport different media within the body, such as blood in the blood vessels or food in the digestive tract.

Smooth muscles are composed of cells with a single nucleus called leiomyocytes or SMCs (smooth muscle cells). Unlike striated or skeletal muscles, smooth muscles contract involuntarily and rhythmically. They are under the control of the parasympathetic nervous system and are stimulated by axonal neuromediators. The contraction/relaxation cycle of smooth muscles requires less energy than that of striated muscles and can therefore produce force over a much longer period. Multiunit smooth muscles are innervated like striated muscles, with one synapse per cell.