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involuntary muscle (smooth muscle) Muscle whose activity is not under the control of the will; it is supplied by the autonomic nervous system. Involuntary muscle comprises long spindle-shaped cells without striations. These cells occur singly, in groups, or as sheets in the skin, around hair follicles, and in the digestive tract, respiratory tract, urinogenital tract, and the circulatory system. The cells contract slowly in spontaneous rhythms or when stretched; they may show sustained contraction (tonus) for long periods without fatigue. Compare voluntary muscle.
involuntary muscle One of three types of muscle in the body, so called because, unlike skeletal muscle, it is not under the conscious control of the brain but stimulated by the autonomic nervous system and by hormones. Smooth muscle is the muscle of the alimentary canal, blood vessels, and bladder. Cardiac muscle powers the heart.