Functional Implications of the Resting Sarcomere Length‐Tension Curve in Living Heart Muscle

The sarcomere pattern and tension of atrial trabeculae isolated from frog hearts have been monitored. The sarcomere length at zero tension varied with the size of the trabeculae but was never less than 1.88 pm, at which length the ends of the thin filaments are at the centre of the A band. Resting tension became large at sarcomere lengths greater than 2.3 m̈m. It was difficult to stretch the trabeculae to produce sarcomere lengths greater than 2.7 m̈m and doing so generally resulted in irreversible changes. Sarcomeres as long as 3.2 m̈m were seen, however, in cells in series with spontaneously contracting fibres. Broadening of the A band at larger sarcomere lengths was interpreted as indicating misalignment of thick filaments and suggests that thick and thin filaments interact in the resting heart. The entire change in length of the central undamaged half of the trabeculae during stretching could be accounted for by the change in sarcomere length.