Sarcomere length non-uniformity in relation to tetanic responses of stretched skeletal muscle fibres.

Abstract

At sarcomere lengths above the plateau region of the length-tension diagram, it has been found that isometric tetanic tension is proportional to the amount of thick and thin filament overlap. This finding has been questioned recently and is reinvestigated here. Central segments of single frog skeletal muscle fibres were held at constant length during contractions at various sarcomere lengths above those associated with the plateau region. Tension records showed little or no creep, and the tetanic tensions measured at 0 and 20°C were inversely proportional to sarcomere length. These results extend and substantiate earlier findings. In contrast, when a stretched fibre had only its ends fixed during a tetanus, a different tension response was observed. The tension rise was initially very rapid but soon slowed to a gradual upward creep as stimulation was continued. This was followed by a tension decline. These tension phases were correlated with large decreases in sarcomere length at the fibre ends, while sarcomeres in the middle were extended a small amount. This tetanic tension response can be explained using the sarcomere length-tension relation and the force-velocity properties of muscle. These results strongly support the sliding filament, cross-bridge theory of muscle contraction.