Introduction

Smooth muscle has a wide distribution in the body, and its functional specializations are extraordinarily varied. Smooth muscle is chiefly located in the wall of hollow organs, where it occurs in broad and thin sheets, in arrays of bundles, or, in the case of the taeniae, as conspicuous long cords. Other cordlike smooth muscles connect the ovaries, duodenum, and rectum to the posterior abdominal wall, and link hair follicles to the surrounding connective tissue. In mammals, a ring of smooth muscle lies close to the pupillary edge of the iris. Individual smooth muscle cells are scattered within connective tissues or close to epithelia in many organs. The total amount of musculature in the body is difficult to calculate. On the basis of the estimates shown in Table 1, smooth muscle may represent 2% of human body weight.

Similarities and dissimilarities with striated muscles are obvious. Smooth muscles are made of small, elongated, uninucleated cells, embedded in abundant extracellular material with a large fibrous component (Figure 1): intercellular cooperativity and role of extracellular material are greater than in striated muscles. A smooth muscle cell has no transverse striations in spite of its actin and myosin filaments, has no T tubules, has numerous caveolae, and has an extensive insertion of the contractile apparatus on its cell membrane over the entire cell length. Its contractile machinery, based on actin and myosin filaments and on a dominant role of calcium (as in striated muscle), has an architecture and consequent mechanical properties that are substantially different from those of striated muscles.