The smooth muscle is found in the walls of many hollow organs such as stomach, intestines, urinary bladder, and in the walls of passageways, like the veins and arteries of the circulatory system, and the tracts of the urinary, respiratory and reproductive system. It is an involuntary non-striated muscle. Smooth muscles are divided into two subgroups: the single-unit smooth muscle which is much more common, and multiunit smooth muscle. Single-unit muscle contracts as a single unit because their muscle fibers joined by gap junctions. This type of smooth muscle is also called visceral muscle as it is present in the walls of all visceral organs except the heart. Multiunit smooth muscle fiber rarely has gap junctions so the contraction is confined to the cell originally stimulated. Smooth muscle fibers are spindle-shaped and each has a single centrally located nucleus. Unlike skeletal muscle and heart muscle, smooth muscle fibers do not have striations and sarcomeres. A variety of target molecules expressed on smooth muscle tissues are applied to the detection of related disorders.
