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There are three types of muscle, each with a different structure and function. Skeletal muscle produces the most powerful contractions and is essentially involved in performing the body's movements. Skeletal muscle is under conscious control and is therefore described as voluntary. It is composed of cells, elongated up to twelve inches in length, containing many minute myofibrils and nuclei. It can contract and relax rapidly, but is easily fatigued. Smooth muscle is found in the arteries and in the digestive and urinary tracts, and performs the slow, long-term contractions needed to maintain these systems. Smooth muscle is made up of elongated cells arranged in bundles, each with a single nucleus. Within the intestines an inner circular band produces ring-like constrictions and an outer longitudinal band produces wavelike motions. Controlled by the autonomic system, smooth muscle contraction is involuntary. Cardiac muscle, found exclusively in the heart, continuously generates very strong contractions and rarely tires in its task of pumping blood throughout the body. It has a built-in rhythm of contraction, but is regulated by the heart's pacemaker. Cardiac muscle forming the heart is made up of branched cells containing many nuclei. It occurs as thick spiral bands around the ventricles of the heart.

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