Abstract
The current idea that the sliding between thick and thin filaments in muscle is caused by the "rotation" of the myosin cross-bridge head is critically reviewed, together with the hystorical backgrounds having led to the development of the head rotation model of muscle contraction. The head rotation model involves two basic assumptions, i.e. (i) the rotation of the cross-bridge head and (ii) the presence of the elastic component in the SII portion of the cross-bridge. In the review, emphasis is particularly placed on the examination about the two basic assumptions.
Our recent study with glycerinated rabbit psoas fibers showed that most of the muscle compliance in rigor fibers originates in the cross-bridges and another our study on the effect of the chemical cross-linking of the SII portion onto the surface of the thick filaments in rigor fibers on the muscle stiffness showed that the SII portion is not compliant. Since there has been no evidence for the head rotation in rigor fibers when the fibers are stretched as well as in active muscle fibers, we suppose that the proximal domain of the cross-bridge head (SI portion) is compliant. The implication of the compliance in the proximal domain of the cross-bridge head in the mechanisms of the muscle force generation is discussed.
