INTRODUCTION

In the last chapter attention was concentrated mainly on the various types of evidence which contributed to the knowledge of the fine structure of the myofibril at rest and contracted. The generally preferred conception emerged of the interaction of two types of filament, one consisting mainly of myosin, the other mainly of actin, intermittently linked by bridges from the myosin; and of contraction as depending not on shortening of the filaments, but on the degree of overlap (thus on the degree of possibility of interaction) of the two sets of filament. The manner in which such a conception could fit with such well-known facts as the effects of ATP on actin/myosin association, or energy provision by actomyosin-catalysed ATP hydrolysis, was explored in a general way. Now we have two tasks – first that of considering more specific theories attempting to explain how the sliding could take place and derive its energy; secondly and mainly that of describing experimentation of recent years which, in various ways, has set out to throw light on possible conformational changes in the proteins during contraction, or under conditions which might obtain during contraction, or which might resemble such conditions. In some cases the possibility that these changes might be relevant is simply stated; in other cases, the results are assembled to support certain detailed ideas concerning the mechanism of sliding and the provision of energy for it.