Smooth Muscle: An Ultrastructural Basis for the Dynamics of Its Contraction

Electron micrographs of vertebrate and invertebrate smooth muscle indicate that the myofilaments are oriented obliquely to the long axis of the muscle fibers containing them and insert along the sides of the fibers. As a result, a greater proportion of the contractile elements are in parallel with one another and a smaller proportion are in series than would be possible if the myofilaments were strictly parallel to the fiber axis. From this ultrastructural organization it is possible to predict several well-known, but previously unexplained, physiological properties of smooth muscle.