Spatial lay-out of various smooth muscles.

The characteristic mechanical activities of the smooth muscles found in all organs of the body are highly variable and depend mainly on the spatial arrangement of the muscle cells and the stroma: mass, orientation, relationships, links, constraints, which are deployed in various configurations. These structural features are examined here for their mechanical relevance, in light and electron microscopic views of several muscles of viscera and blood vessels, in a selection of mammalian species. Smooth muscles are incompressible and therefore maintain constant volume. They do not have available space and any movement of a part requires displacement of another part. Most of them have no terminations or points of attachment, and in hollow organs such as intestines, blood vessels and uro-genital tract they usually form structures closed onto themselves, such as rings or bag-like containers In these situations, changes in the size of the lumen is achieved very efficiently by a concentric inward enlargement that accompanies muscle contraction. The longitudinal arrangement of collagen blocks an elongation of small blood vessels upon contraction, further enhancing the efficiency of lumen reduction. In other muscles, links between layers and special arrangements of the stroma allow both shortening and elongation of a tubular organ to occur. The mechanics of smooth muscles has many characteristic features (some unique, some shared with those of hydrostats, some at variance with other muscles) and histological data are a contribution to our understanding of these properties.