Figure 16–36 The control of skeletal muscle contraction by troponin. (A) A skeletal-muscle-cell thin filament, showing the positions of tropomyosin and troponin along the actin filament. Each tropomyosin molecule has seven evenly spaced regions with similar amino acid sequences, each of which is thought to bind to an actin subunit in the filament. (B) Reconstructed cryoelectron microscopy image of an actin filament showing the relative position of a superimposed tropomyosin strand in the presence (dark purple) or absence (light purple) of calcium. (A, adapted from G.N. Phillips, J.P. Fillers and C. Cohen, J. Mol. Biol. 192:111–131, 1986. With permission from Academic Press; B, adapted from C. Xu et al., Biophys. J. 77: 985–992, 1999. With permission from Elsevier.) composed of sheets of highly elongated spindle-shaped cells, each with a single nucleus. Smooth muscle cells do not express the troponins. Instead, elevated intracellular Ca2+ levels regulate contraction by a mechanism that depends on calmodulin (Figure 16–37). Ca2+-bound calmodulin activates myosin light-chain kinase (MLCK), thereby inducing the phosphorylation of smooth muscle myosin on one of its two light chains. When the light chain is phosphorylated, the myosin head can interact with actin filaments and cause contraction; when it is dephosphorylated, the myosin head tends to dissociate from actin and becomes inactive.
The phosphorylation events that regulate contraction in smooth muscle cells occur relatively slowly, so that maximum contraction often requires nearly a second (compared with the few milliseconds required for contraction of a skeletal muscle cell). But rapid activation of contraction is not important in smooth
Figure 16–37 Smooth muscle contraction. (A) Upon muscle stimulation by activation of cell-surface receptors, Ca2+ released into the cytoplasm from the sarcoplasmic reticulum (SR) binds to calmodulin (see Figure 15–29). Ca2+-bound calmodulin then binds myosin light-chain kinase (MLCK), which phosphorylates myosin light chain, stimulating myosin activity. Non-muscle myosin is regulated by the same mechanism (see Figure 16–39). (B) Smooth muscle cells in a cross section of cat intestinal wall. The outer layer of smooth muscle is oriented with the long axis of its cells extending parallel along the length of the intestine, and upon contraction will shorten the intestine. The inner layer is oriented circularly around the intestine and when contracted will cause the intestine to become narrower. Contraction of both layers squeezes material through the intestine, much like squeezing toothpaste out of a tube.
(C) A model for the contractile apparatus in a smooth muscle cell, with bundles of contractile filaments containing actin and myosin (red) oriented obliquely to the long axis of the cell. Their contraction greatly shortens the cell. Only a few of the many bundles are shown. (B, courtesy of  Gwen V. Childs.) muscle: its myosin II hydrolyzes ATP about 10 times more slowly than skeletal muscle myosin, producing a slow cycle of myosin conformational changes that results in slow contraction.
The heart is the most heavily worked muscle in the body, contracting about 3 billion (3 × 109) times during the course of a human lifetime (Movie 16.6). Heart cells express several specific isoforms of cardiac muscle myosin and cardiac muscle actin. Even subtle changes in these cardiac-specific contractile proteins—changes that would not cause any noticeable consequences in other tissues—can cause serious heart disease (Figure 16–38).