Structure and function of the normal myocardium1
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The myocardium is composed of individual striated muscle cells (fibres), io to i5 ,um. in diameter and 30 to 6o ,um. in length. Under the light microscope each fibre contains multiple cross-banded strands (myofibrils), which run the length of the fibre and are composed of a serially repeating structure, the sarcomere. The remainder of the cytoplasm, lying between the myofibrils, contains other cell constituents such as the single centrally located nucleus, numerous mitochondria, and intracellular membrane systems. The sarcomere, the fundamental structural and functional unit of contraction, is delimited by two adjacent dark lines, the Z lines (Fig. I). The distance between Z lines varies with the degree of contraction or stretch of the muscle and ranges between I-5 and 2-2 ,tm. Within the confines of the sarcomere alternating light and dark bands are seen, giving the myocardial fibres their striated appearance under the light microscope. At the centre of the sarcomere is a broad dark band of constant width (i5 ,um.), the A band, which is flanked by two lighter bands, the I bands, which are of variable width. The sarcomere of heart muscle, like that of skeletal muscle, is made up of two sets of myofilaments. Thicker myofilaments, composed of the protein myosin, traverse and are limited to the A band. The myosin filaments are about iooA in diameter, with tapered ends, and they measure i 5 to i -6 ,tm. in length. Thinner myofilaments, composed primarily of actin, course from the Z line through the I band into the A band. The thin filaments are approximately 5o A in diameter and i o ,um. in length. Thus there is overlapping of thick and thin filaments only within the A band, while the I band contains only thin filaments (Fig. I). Bridges extend between the myosin and actin filaments within the A band. The 'sliding' model for muscle rests on the fundamental observation that both the thick and thin contractile filaments are constant in
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