Comparison of the Ultrastructural Basis of the Contractile Process in Heart and Skeletal Muscle

The ultrastructure has been compared in both heart and skeletal muscle relative to the length-tension curve. Similar sarcomere band patterns consistent with constant myofilament lengths were obtained along the ascending limb of the length-tension curve for both types of muscle. In shortened muscle, the appearance of these central A contraction bands, which are inversely proportional to sarcomere length, lends strong support to a purely sliding mechanism for muscular contraction for both heart and skeletal muscle. Differences in band patterns were encountered in these two forms of striated muscle along the descending limb of the length-tension curve. This may be related to a stiffer effective parallel elastic structure in heart muscle, which limits sarcomere extension. The general cytology of heart muscle cells is also briefly reviewed.