Smooth Muscle is an involuntary, non striated muscle found in the gastrointestinal tract and in the walls of blood vessels.  It is made up of spindle shaped fibers, which are small with only a single nucleus and are arranged in bundles or sheets.  Smooth muscle is classified as either single unit ( "visceral" ) smooth muscle , or multiple-unit.  Single unit (Visceral) smooth muscle is the most common type.  It is poorly innervated and the fibers are grouped together so that they contract synchronously.  These fibers are found in the walls of the digestive tract.   Multiple-unit smooth muscle fibers are not grouped together and are well innervated.  It is found in large airways and large arteries (Ex.  Aorta).

This slide is a longitudinal cut of smooth muscle.  Notice the spindle shape of the fibers and the single nuclei (N). The next slide is transverse section of smooth muscle.

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