48 



D. M. NEEDHAM 



VOL. 4 (1950) 



correspondence with the behaviour of the "A" substance described above is striking. 

 These observations may be summarized as follows : 



It seems that there must be some intimate connection between the three classes 

 of substance mentioned in the 3rd. column; whether the same substances are actually 

 responsible for the staining and the electron-scattering phenomena we do not know. 



Besides these localizations which have been recognized, and which must have 

 significance for contraction it seems likely that there may be much localization still 

 unknown. In particular it is to be expected that the soluble protein fractions, myogen 

 and globulin X, including most of the enzyme equipment of the muscle, instead of being 

 merely dissolved in the sarcoplasm, will show pattern. 



CONCLUSION 



If one is to make any sort of tentative picture of the mechanism of contraction, 

 one must, under present conditions, be allowed a bias towards one side or the other 

 in answering the question "Is relaxation of the fibril an active process, requiring pro- 

 vision of free energy?" The writer would like to take the standpoint that an affirmative 

 answer best fits the observed physiological behaviour during relaxation and that obser- 

 vations on the relations of heat production and on the effect of work on heat production 

 are not at variance with this view. 



One can make the basic assumptions that, in the stimulated muscle, chemical 

 reaction becomes possible between groups situated along the protein chain; that this 

 reaction goes on with production of free energy and that in the resting muscle there 

 is some configurational hindrance to its taking place. Further, one can assume that 

 the number of these groups which can react together will depend upon the length which 

 the muscle is made to assume, being fewer at greater lengths and increasing in number 

 as the muscle shortens. It is known that during a twitch the amount of "shortening 

 heat" production is proportional to the degree of shortening of the muscle, while the 

 rate of "shortening heat" production is dependent on the speed with which the muscle 

 shortens, (A. V. Hill^^). Thus for shortening a given distance, the "shortening heat" 

 production is the same, whether the shortening is slow or fast. But the rate of shortening 

 depends on the load, being slower the greater the load; thus at slower rates of shortening 

 between two given lengths, more work is done and more energy must be produced, 

 since the heat remains the same. If this energy production is the result of the interaction 

 of the same groups at different rates of shortening, we must suppose that, at the slower 

 rates, repeated interaction takes place. When speculations are made as to the timing 

 of 'ATP breakdown, it is usually supposed that this is confined either to the contraction 

 phase or to the relaxation phase (in the latter case its energy being used to restore 

 energy-rich protein linkages). If we suppose that, when work is done, before a pair 

 References p. 4g. 



