Also called visceral muscle in vertebrates and almost always under the control of the autonomic nervous system, it is contractile tissue lacking transverse striations of skeletal muscle and any other obvious ultrastructure. Having slow, rhythmical contractions, it functions to produce peristaltic movement of food along the gut, and blood along contractile vessels as well as in the regulation of blood pressure (at least in vertebrates). In fact, it is involved in the functioning of all internal organs, except the heart that is controlled by cardiac muscle that is like striated muscle, but which acts like single-unit smooth muscle. Single-unit smooth muscles are found in the gastrointestinal system, small blood vessels and the uterus, and produce waves of spontaneous muscle contraction. In contrast, multiunit smooth muscles are to be found in the eye, hair follicles, and large arteries, and are normally inactive, but which respond to hormones and neural stimulation.
See Autonomic nervous system (ANS), Bronchi, Cardiac muscle, Hormones, Myoglobin, Oxytocin, Peristaltic, Proteins, Striated (or striped or voluntary) muscle, Vagus (or pneumogastric) nerve