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Smooth and Cardiac Muscles

In general, the mechanisms of contraction of both smooth and cardiac muscles resemble those of skeletal muscles. Several important differences in both structure and function, however, are apparent. Smooth muscles contain both actin and myosin filaments, but they are more randomly arranged and are not positioned into sarcomeres. Smooth muscle cells also:
  • Lack transverse tubules.
  • Have only a single nucleus
  • Have poorly developed sarcoplasmic reticula.
  • Contain the protein calmodulin instead of troponin which serves essentially the same function by binding to calcium.
  • Are stimulated only by the autonomic division of the nervous system.
  • Can recognize the neurotransmitter norepinephrine (noradrenalin) as well as acetylcholine. In addition, smooth muscles are often stimulated to contract by hormones.
Cardiac muscle is located exclusively in the heart, and is unique in being the only muscle that is both striated and involuntarily controlled. Its contraction is virtually identical to that of skeletal muscle, as might be expected from the presence of sarcomeres. However, the ends of the muscle cells of the myocardium are connected by intercalated disks, which help hold the cells together and allow muscle impulses to travel rapidly from cell to cell. Unlike skeletal muscle, cardiac muscle is self exciting, and when one portion of the muscle is stimulated, the depolarization quickly travels to other fibers, causing the entire muscle to contract as a unit. The initial stimulation for a single “heartbeat” comes from the sinoatrial node, or “pacemaker”.

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