Smooth Muscle

This muscle type is found as layers in the walls of most organs where it provides for involuntary motility. Smooth muscle consists of layers or bundles of non-striated, spindle-shaped fibers that contain one centrally located nucleus per fiber. The actin and myosin filaments are organized as a lattice-like network that is connected to the sarcolemma via adhesion complexes called dense bodies. This organization allows the cells to twist as they contract, sometimes giving them a distinctive corkscrew appearance in specimens as seen in the diagram and image below. In addition to lacking striations, smooth muscle cells differ from striated muscle cells because they also lack T-tubules.

Examine smooth muscle cut transversely and longitudinally in the muscularis of the esophagus and small intestine, and in the myometrium of the uterus. Identify fibers, fascicles, and the connective tissue layers, comparing these features to those of skeletal muscle. The CT layers are generally not named in smooth muscle. The muscularis of the gastrointestinal tract is frequently organized as inner circular and outer longitudinal layers of smooth muscle with a nerve plexus located in between the layers as seen in the image at the right.

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