Muscle, Smooth. Involuntary Muscle . Involuntary Muscles . Muscles, Involuntary . Muscles, Smooth . Smooth Muscles . Muscle, Involuntary . Smooth Muscle . Unstriated and unstriped muscle, one of the muscles of the internal organs, blood vessels, hair follicles, etc. Contractile elements are elongated, usually spindle-shaped cells with centrally located nuclei. Smooth muscle fibers are bound together into sheets or bundles by reticular fibers and frequently elastic nets are also abundant. (From Stedman, 25th ed) . 1.00