Smooth muscle forming the contractile wall of inner hollow organs and vessels is composed of bundles or sheets of fusiform cells that measure in length between 20 μm in blood vessel walls and approximately 200 μm in the wall of the intestine; smooth muscle cells can reach lengths of 500 μm in the wall of the uterus during pregnancy. The cells usually possess one nucleus of elongated shape and tapering ends in central position. Excluding the nuclear pole regions, where most organelles and compartments of the biosynthetic system are concentrated, the contractile apparatus occupies the vast majority of the cytoplasm. It consists of myosin II, actin, and associated proteins such as tropomyosin and caldesmon, which block the myosin binding sites. Calmodulin, a Ca²⁺-binding protein, regulates the intracellular concentration of Ca²⁺; binding to caldesmon, it effects its phosphorylation and release from F-actin. Unlike the onset of contraction in striated muscle where the troponin-tropomyosin complex has a pivotal role, contraction in smooth muscle is regulated by the Ca²⁺-calmodulin/myosin light chain kinase system and initiated by phosphorylation of one of the myosin light chains in the head domain. Ultrastructurally, the thin actin filaments dominate. Via alpha-actinin, they are anchored in dense bodies and plaques attached to the plasma membrane, which are assumed to be part of an anastomozing network extending from the cell surface into the interior of the cells. The dense bodies and plaques are analogs of the Z disks in the striated muscle. Intermediate filaments consist of desmin or vimentin, in the case of vascular smooth muscle cells. Gap junctions permit communication between adjacent cells.

Panels A and B show smooth muscle cells in the small intestinal wall of the rat in a survey electron micrograph and at higher magnification, respectively. In panel A, smooth muscle cells are visible in longitudinal sections in the lower part and cross-sectioned in the upper part of the picture. The bulk of the cytoplasm is filled by the contractile filaments. Mitochondria (M) are interspersed and may be lined in the cell periphery but, together with abundant ribosomes and endoplasmic reticulum, are particularly crowded in cytoplasmic areas extending from the regions close to the nuclear poles, as shown in the micrograph in panel A in the uppermost smooth muscle cell sectioned longitudinally. Dense areas and dense bodies providing the platforms for the anchoring of actin filaments are conspicuous in the interior of the cells (asterisks) and are attached to the plasma membrane. Each smooth muscle cell is sheathed by a basal lamina (arrowheads in panel B), the constituents of which, such as type IV collagen, laminin, and proteoglycans, are products of the muscle cell. Both micrographs show groups of caveolae that occupy large areas of the smooth muscle cell surfaces. In smooth muscle cells, caveolae may be closely apposed to membranes of the endoplasmic reticulum (cf. panel A in Fig. 63). It is assumed that there may be analogies to the T- and L-systems in striated muscle fibers. Studies with vascular smooth muscle of the ferret aorta have revealed that the muscle cell contractility is regulated by caveolin-1.