FINE STRUCTURAL CHANGES IN HEART MUSCLE IN RELATION TO THE LENGTH-TENSION CURVE

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Abstract
The ultrastructure of cat papillary muscle has been studied in relation to the resting and active length-tension curve. Sarcomere structure has been analyzed at known increments in muscle length. It is observed that: Increasing muscle length increases sarcomere length from 1.5 [mu] at L0, where resting and active tension approach zero, to 2.2 [mu] at the apex of the active length-tension curve. The apex of the active length-tension is attained with a 45% increase in muscle length above Lq. With further increments in muscle length, active tension declines, approaching again with an extension of 100% over L0. The I band is absent at L0 and increases as a function of sarcomere length. No change in the H band occurs over the entire length-tension curve. Similarity, the A band is constant. These observations support a "folding" myofilament model for myocardial contraction and are not consistent with a purely "sliding" model.