Excitation-contraction coupling and the ultrastructure of smooth muscle

Abstract

The recognition that contraction is initiated in smooth, as in striated, muscle by a rise in cytoplasmic Ca++ and that the source of this Ca++ may be intracellular (for review, Johansson and Somlyo, 1980) inevitably leads to the question of which cellular organelles are the source(s) and sink(s) of activator Ca. Some of the answers to this question that have been obtained by electron microscopy and electron probe analysis are the subect of this Brief Review. The ultrastructure of the contractile apparatus itself has been described elsewhere (A.V. Somlyo et al., 1981a; Bond and Somlyo, 1982; for review, Somlyo, 1980), and it is sufficient to note here that it consists of thick (myosin) filaments that are present both in relaxed and in contracted smooth muscle, thin (actin) filaments, and dense bodies (bound to the plasma membrane or 'free-floating' in the cytoplasm) upon which the actin filaments insert. The cell-to-cell propagation of the action potential and, therefore, the number of cells recruited into the contraction, are thought to depend on the extent of electrical cell-to-cell couplings.