Smooth muscle forms the contractile portion of the wall of the digestive tract from the middle portion of the esophagus to the internal sphincter of the anus. It is found in the walls of the ducts in the glands associated with the alimentary tract, in the walls of the respiratory passages from the trachea to the
alveolar ducts, and in the urinary and genital ducts. The walls of the arteries, veins, and large lymph vessels contain smooth muscle as well.
Smooth muscle is specialized for slow and sustained contractions of low force. Instead of having motor units, all cells within a whole smooth muscle mass contract together. Smooth muscle has inherent contractility, and the autonomic nervous system, hormones and local metabolites can influence its contraction. Since it is not under conscious control, smooth muscle is involuntary muscle.
Smooth muscle fibers are elongated spindle-shaped cells with a single nucleus. In general, they are much shorter than skeletal muscle cells. The nucleus is located centrally and the sarcoplasm is filled with fibrils. The thick (myosin) and thin (actin) filaments are scattered throughout the sarcoplasm and are attached to adhesion densities on the cell membrane and focal densities within the cytoplasm. Since the contractile proteins of these cells are not arranged into myofibrils like those of skeletal and cardiac muscle, they appear smooth rather than striated.
Smooth muscle fibers are bound together in irregular branching fasciculi that vary in arrangement from organ to organ. These fasciculi are the functional contractile units. There is also a network of supporting collagenous tissues between the fibers and the fasciculi.