Muscle whose activity is under the control of the autonomic (involuntary) nervous system. Smooth muscle comprises long spindle-shaped cells without striations and thus has a ‘smooth’ rather than a striated appearance. These cells occur singly, in groups, or as sheets in the skin, around hair follicles, and in the digestive tract, the respiratory tract, the 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. The thin (actin) and thick (myosin) filaments of smooth muscle interdigitate but are not organized into regular bands, as in the sarcomeres of skeletal muscle. Instead, the thin filaments are anchored to dense bodies within the cell or to attachment plaques on the inside surface of the plasma membrane. Single-unit smooth muscles are typically small elongated cells that taper at each end. They are connected by gap junctions, which transmit spontaneous (myogenic) electrical depolarizations rapidly from cell to cell, thereby stimulating a wave of contraction, as in the peristaltic contractions of the intestinal tract. By contrast, the cells of multi-unit smooth muscles act independently and contract only when stimulated by autonomic nerves, as in the walls of blood vessels. Neurotransmitter is released from swellings (called varicosities) in the autonomic axons, which each activate numerous muscle cells. Compare cardiac muscle; skeletal muscle.