Molecular mechanisms of contraction

Abstract

Mechanical phenomena in muscle are frequently interpreted by invoking cross bridge mechanisms. These interpretations imply that tension development and shortening are brought about by the action of physical cross links formed transiently between the interdigitating thick and thin filaments that comprise the contractile apparatus. There is now substantial support for the mechanism of force generation by physical cross links; however, incontrovertible evidence for cross bridge attachment during contraction in living muscle still does not exist. Thus it seems worthwhile to entertain other proposals provided these are demonstrably consistent with the same body of data adduced to support the cross bridge theory. Some of the more plausible schemes have been based on electrostatic mechanisms, but most have not shown broad consistency. On the other hand, the theory proposed by Iwazumi shows sufficient consistency. Because this theory is less well known than the cross bridge theory the authors discuss it. The two theories differ in three fundamental ways: tension is brought about by physical links in the former and electric fields in the latter; in the cross bridge theory tension depends rather directly on the degree of overlap of thick and thin filaments whereas in the field theory it is unrelated to overlap, but depends on the calcium level in the region of the tip of the thin filaments; and the structure postulated for the cross projection is totally different in the two theories. Given these striking differences, one might expect the evidence supporting one theory to be inconsistent with the other, but this is not necessarily the case.