Relation of ultrastructure to function in the intact heart: sarcomere structure relative to pressure volume curves of intact left ventricles of dog and cat.

• The sarcomere is the basic ultrastructural unit of contraction in striated muscle. Huxley and Hanson have shown that the alternating band patterns of the sarcomere in skeletal muscle derive from the longitudinal disposition of two sets of interdigitating contractile protein filaments. On the basis of these and other studies, the sliding filament hypothesis for muscle contraction was proposed.The ultrastructure of heart muscle has also been examined and its sarcomeres shown to be similar in structure to those of skeletal muscle. More recently sarcomere structure has been studied in relation to the length-tension curve of both skeletal" and cardiac° muscle, and despite previous objections," it has been demonstrated that the sliding filament hypothesisexplains the band pattern changes at varying muscle lengths as well as the relation between active tension and muscle length for both types of striated muscle.The relevance, however, of sarcomere length-tension relations for linear samples of myocardium (papillary muscle) to physiological performance of the intact left ventricle has not