The molecular mechanism of contraction

Abstract

From the physical point of view, muscle is an engine for the conversion of chemical energy into mechanical work, operating by means of reversible elastic strains in a solid working substance, the muscle proteins. In a system of this kind, only two types of cycle are possible, one involving changes in potential energy, and the other changes of entropy. In general, the evidence seems to be in favour of an entropy cycle, although there are some serious objections. One important difference between the two types is that in an entropy cycle the strength of intermolecular attractions should decrease during contraction, and in a potential energy cycle they ought to increase; observations on whole muscles suggest that they do in fact decrease. The action of ATP on muscle proteins in vitro agrees with this; as Professor Weber has remarked, it reduces the viscosity of solutions and liquefies gels. The most important objection to an entropy theory lies in the sign of the observed heat exchanges in active muscle. Professor Hill has pointed out that if the contraction of muscle were analogous to the contraction of stretched rubber, heat should be absorbed instead of being given out during shortening. I think this difficulty arises from the fact that to compare muscle to rubber implies that it functions as a heat engine, which is unlikely. It seems more probable that muscles convert chemical energy directly into work, and are capable of operating at constant temperature; if so, the type of entropy cycle we ought to consider is one which does not involve changes of temperature.