The cells of smooth muscle, which forms the walls of many internal organs as well as the walls of arteries and veins, have one nucleus each and lack striation. The individual cells or fibers of smooth muscle are long, thin, and pointed at their ends. The fibers are generally interwoven to form sheets instead of bundles as in striated muscle. Smooth muscle is stimulated by the autonomic nervous system. Each cell is excited by two types of nerve fibers: One type of nerve fiber transmits impulses that elicit contraction while the other transmits impulses that inhibit contraction. Smooth muscle is also able to contract without nervous stimulation as in the peristaltic movement of the intestine tract, for example. Smooth muscle carries out adjustments to an organism's internal environment.
Cardiac muscle forms the tissue of the heart and shares characteristics from both striated and smooth muscle: Both cardiac fibers and the fibers of striated muscle are striated, but cardiac fibers can be stimulated by the autonomic nervous system as are the fibers of smooth muscle. Specialized regions of cardiac muscle spontaneously contract in the absence of nervous impulses, though such impulses may modify the automatic rhythm. Under normal circumstances, the impulses that cause cardiac muscle contraction originate from specialized cardiac muscle cells, not in the nervous system. The junction of two closely fused cell walls is the intercalated disc.
