The Structure of Cardiac and Smooth Muscle
The structure of heart myocytes is different from that of skeletal muscle fibers. Heart myocytes are approximately 50 to 100 ¡m long and 10 to 20 ¡m in diameter. The t-tubules found in heart tissue have a fivefold larger diameter than those of skeletal muscle. The number of t-tubules found in cardiac muscle differs from species to species. Terminal cisternae of mammalian cardiac muscle can associate with other cellular elements to form dyads as well as triads. The association of terminal cisternae with the sarcolemma membrane in a dyad structure is called a peripheral coupling. The terminal cisternae may also form dyad structures with t-tubules that are called internal couplings (Figure 17.31). As with skeletal muscle, foot structures form the connection between the terminal cisternae and t-tubule membranes.
In higher animals, large percentages of the terminal cisternae of cardiac muscle are not associated with t-tubules at all. For SR of this type, Ca2+ release must occur by a different mechanism from that found in skeletal muscle. In this case, it appears that Ca2+ leaking through sarcolemmal Ca2+ channels can trigger the release of even more Ca2+ from the SR. This latter process is called Ca2+-induced Ca2+ release (abbreviated CICR).
The myocytes of smooth muscle are approximately 100 to 500 ¡m in length and only 2 to 6 ¡m in diameter. Smooth muscle contains very few t-tubules and much less SR than skeletal muscle. The Ca2+ that stimulates contraction in smooth muscle cells is predominantly extracellular in origin. This Ca2+ enters the cell through Ca2+ channels in the sarcolemmal membrane that can be opened by electrical stimulation, or by the binding of hormones or drugs. The contraction response time of smooth muscle cells is very slow compared with that of skeletal and cardiac muscle.
Vertebrate organisms employ smooth muscle myocytes for long, slow, and involuntary contractions in various organs, including large blood vessels; intestinal walls; and, in the female, the uterus. Smooth muscle contains no troponin complex; thin filaments consist only of actin and tropomyosin. Despite the absence of troponins, smooth muscle contraction is dependent on Ca2+, which activates myosin light chain kinase (MLCK), an enzyme that phosphorylates LC2, the regulatory light chain of myosin. Contraction of smooth muscle is initiated by phosphorylation of LC2, and dephosphorylation causes relaxation of smooth muscle tissue.
FIGURE 17.31 • Electron micrograph of a dog heart muscle. The terminal cisterna of the SR (TC-SR) is associated with the t-tubule (TT) by means of foot structures (FS), forming a dyad junction. MF indicates the location of myofilaments. LT-SR signifies the longitudinal tubule of the SR. (From Fleischer, S, and Inui, M, 1989. Annual Review of Biophysics and Biophysical Chemistry 18:333-364.)
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