JEAN HOTTS g7 



interfilament distance in an undried preparation (3, 87). Philpott and A. G. 

 Szent-Gyorgyi (78) have -found a 400-A si)acing in L-meromyosin crystals 

 (L-meromyosin being the 'light' fraction of myosin A obtained by short tryptic 

 or chymotryptic digestion). L-meromyosin is presumed to contain no heavy 

 metals, and its periodicity has been attributed to a possible overlap of the ends 

 of basic units or to the accumulation of salts between the ends of particles. If 

 L-meromyosin is longitudinally oriented along a filament, its 400-A periodicity 

 may correspond to the spots where the ends of the interfilament (tropomyosin?) 

 bridges impinge. In this connection it is interesting that, from a comparison of 

 amino acid analyses of actin, myosin A, tropomyosin and the meromyosins, 

 Laki has suggested that L-meromyosin may be largely composed of tropomyo- 

 sin (63; discussed in 71). However this may be, it seems reasonable to suppose 

 that the interfilament bridges correspond to the sites of the bound metal 

 accumulations observed earlier. 



CONCLUDING REMARKS 



Numerous theories of contraction are detailed in current literature (e.g., 4, 

 36, 68, 70, 71, 77, 85, 89, 93). It is recognized that, in order for muscle to do 

 work, chemical energy must ultimately be transformed into mechanical work; 

 but how and at what stage this transformation is accomplished remains contro- 

 versial. 



\'arious steps leading to the development of the active state have been out- 

 lined above. Two possibly triggered processes at the neuromuscular junction are 

 followed by the triggering of the action potential along the fiber membrane. 

 Two related phenomena are associated with the action potential — membrane 

 depolarization and a shift in ion distribution about the fiber membrane. In what 

 way these events may be related to the initiation of contraction is not known. In 

 Fleckenstein's theory, discussed above, it is assumed that the immediate free 

 energy for the work of contraction is derived entirely from the electrochemical 

 processes occurring at the fiber membrane during excitation. Other theories 

 assume further intermediate steps between excitation and contraction and 

 require extensive energy augmentation only at a subsequent stage. In certain of 

 these theories it has been postulated that contractile protein is somehow re- 

 leased from the effect of a 'relaxing factor' and that this release gives rise to the 

 development of the active state. 



Within a tenuous framework of fact and fancy, it is possible to construct a 

 picture of the final steps in the initiation of this active state. Through some un- 

 known mechanism (such as invasion by competing cations), inter-filament 

 bridge protein is forced to release some of its bound calcium. It may be spec- 

 ulated that the latency relaxation preceding contraction is due to the relaxation 

 of these bridges under the effects of an ion shift. If the inter-filament bridges 

 normally impose a slight distortion of the filament surface, an increase in the 



