Which of the following is absent in smooth-muscle cells compared to skeletal muscle cells?
The answer is (a).
Smooth muscle is the least specialized type of muscle and contains no troponin. The contractile process is similar to the actin-myosin interactions that occur in motility of nonmuscle cells. In the smooth-muscle cell, actin and myosin are attached to intermediate filaments at dense bodies in the sarcolemma and cytoplasm. Dense bodies contain alpha-actinin and, therefore, resemble the Z-lines of skeletal muscle. Contraction causes cell shortening and a change in shape from elongate to globular. Contraction occurs by a sliding filament action analogous to the mechanism used by thick and thin filaments in striated muscle.
The connections to the plasma membrane allow all the smooth-muscle cells in the same region to act as a functional unit. The sarcoplasmic reticulum is not as well developed as that in the striated muscles. There are no T tubules present; however, endocytic vesicles called caveolae are believed to function in a fashion similar to the T tubule system of skeletal muscle. When intracellular calcium levels increase, the calcium is bound to the Ca2+ -binding protein, calmodulin. Ca2+-calmodulin (answers b and c) is required and is bound to myosin light-chain kinase (answer d) to form a Ca2+ -calmodulin-kinase complex. This complex catlayzes the phosphorylation of one of the two myosin light chains on the myosin heads. That phosphorylation allows the binding of actin to myosin. A specific phosphatase
dephosphorylates the myosin light chain, which returns the actin and myosin to the inactive, resting state. The actin-tropomyosin interactions (answer e) are similar in smooth and skeletal muscle.
Smooth-muscle cells (e.g., vascular smooth-muscle cells) also differ from skeletal muscle cells in that like fibroblasts, they are capable of collagen, elastin, and proteoglycan synthesis.
- a.Troponin
- b.Calmodulin
- c.Calcium
- d.Myosin light-chain kinase
- e.Actin and tropomyosin interactions
The answer is (a).
Smooth muscle is the least specialized type of muscle and contains no troponin. The contractile process is similar to the actin-myosin interactions that occur in motility of nonmuscle cells. In the smooth-muscle cell, actin and myosin are attached to intermediate filaments at dense bodies in the sarcolemma and cytoplasm. Dense bodies contain alpha-actinin and, therefore, resemble the Z-lines of skeletal muscle. Contraction causes cell shortening and a change in shape from elongate to globular. Contraction occurs by a sliding filament action analogous to the mechanism used by thick and thin filaments in striated muscle.
dephosphorylates the myosin light chain, which returns the actin and myosin to the inactive, resting state. The actin-tropomyosin interactions (answer e) are similar in smooth and skeletal muscle.
Smooth-muscle cells (e.g., vascular smooth-muscle cells) also differ from skeletal muscle cells in that like fibroblasts, they are capable of collagen, elastin, and proteoglycan synthesis.
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HISTOLOGY
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