APA Dictionary of Psychology

any muscle that is not striated and is under the control of the autonomic nervous system (i.e., it is not under voluntary control). Smooth muscles are able to remain in a contracted state for long periods of time or maintain a pattern of rhythmic contractions indefinitely without fatigue. Smooth muscle is found, for example, in the digestive organs, blood vessels, and the muscles of the eyes. Also called involuntary muscle. Compare cardiac muscle; skeletal muscle.