Smooth Muscle

Smooth muscle is often called involuntary muscle because its activity arises spontaneously or through activity of the autonomic nervous system. Smooth muscle contractions

are slower and more sustained than skeletal or cardiac muscle contractions.

Organization and Structure. Smooth muscle cells are spindle shaped and smaller than skeletal muscle fibers. Each smooth muscle cell has one centrally positioned nu­cleus. Z bands or M lines are not present in smooth mus­cle fibers, and the cross-striations are absent because the bundles of filaments are not parallel but crisscross ob­liquely through the cell. Instead, the actin filaments are attached to structures called dense bodies. Some dense bodies are attached to the cell membrane; others are dis­persed in the cell and linked together by structural proteins (Fig. 4-26).

The lack of Z lines and of regular overlapping of the contractile elements provides a greater range of tension development. This is important in hollow organs that un­dergo changes in volume, with consequent changes in the length of the smooth muscle fibers in their walls. Even with the distention of a hollow organ, the smooth muscle fiber retains some ability to develop tension, whereas such distention would stretch skeletal muscle beyond the area where the thick and thin filaments overlap.

Intermediate filament bundles attached to dense bodies

Relaxed

RE 4-26 Structure of smooth muscle showing the dense bodies. In smooth muscle, the force of contraction is transmitted to the cell membrane by bundles of intermediate fibers. (Cormack D.H. [1993]. Essential histology [p. 229]. Philadelphia: J.B. Lippincott).

Smooth muscle is usually arranged in sheets or bun­dles. In hollow organs, such as the intestines, the bundles are organized into the two-layered muscularis externa con­sisting of an outer, longitudinal layer and an inner, circu­lar layer. A thinner muscularis mucosa often lies between the muscularis externa and the endothelium. In blood ves­sels, the bundles are arranged circularly or helically around the vessel wall.

Smooth Muscle Contraction. As with cardiac and skeletal muscle, smooth muscle contraction is initiated by an in­crease in intracellular calcium. However, smooth muscle differs from skeletal muscle in the way its cross-bridges are formed. The sarcoplasmic reticulum of smooth muscle is less developed than in skeletal muscle, and no transverse tubules are present. Smooth muscle relies on the entrance of extracellular calcium and its release from the sarcoplas­mic reticulum for muscle contraction. This dependence on movement of extracellular calcium across the cell mem­brane during muscle contraction is the basis for the action of calcium-blocking drugs used in treatment of cardio­vascular disease.

Smooth muscle also lacks the calcium-binding regula­tory protein, troponin, which is found in skeletal and car­diac muscle. Instead, it relies on another calcium-binding protein called calmodulin. The calcium-calmodulin com­plex binds to and activates the myosin-containing thick filaments, which interact with actin.

Types of Smooth Muscle. Smooth muscle may be di­vided into two broad categories according to the mode of activation: multiunit and single-unit smooth muscle. In multiunit smooth muscle, each unit operates almost in­dependently of the others and is often enervated by a single nerve, such as occurs in skeletal muscle. It has little or no inherent activity and depends on the autonomic nervous system for its activation. This type of smooth muscle is found in the iris, in the walls of the vas defer-ens, and attached to hairs in the skin. The fibers in single-unit smooth muscle are in close contact with each other and can contract spontaneously without nerve or hor­monal stimulation. Normally, most muscle fibers contract synchronously, hence the term single-unit smooth muscle. Some single-unit smooth muscle, such as that found in the gastrointestinal tract, is self-excitable. This is usually associated with a basic slow-wave rhythm transmitted from cell to cell by nexus (i.e., gap junctions) formed by the fu­sion of adjacent cell membranes. The cause of this slow wave is unknown. The intensity of contraction increases with the frequency of the action potential. Certain hor­mones, other agents, and local factors can modify smooth muscle activity by depolarizing or hyperpolarizing the membrane. Smooth muscle cells found in the uterus and small-diameter blood vessels are also single-unit smooth muscle.