2l8 HANDBOOK OF PHYSIOLOGY ^^ CIRCULATION I 



-band 



I 

 Z-line 



A-band 

 >c 



l-band 



II 1 



l-band 



A-band 



l-band 



Z-l.ne 



120% R.L. 



100% R.L. 



90% R.L. 



75% R.L. 



60% R.L. 



FIG. 17. Diagrammatic representation of band pattern 

 chcmges during contraction. [From H. E. Huxley (iio).] 



pears to exist in solution in tlie interstices of the 

 myofibril, particularly in the I bands. Chemical 

 analysis of the isolated rabbit myofibril (191) indi- 

 cates that the protein composition of the myofibril 

 is as follows: myosin 50 to 55 per cent; actin 20 to 

 25 per cent; tropomyosin 10 to 15 per cent; other 

 proteins 5 to 10 per cent. The identity of the protein 

 of the Z membrane is unknown, although it is esti- 

 mated to represent about 5 per cent of the myofibrillar 

 protein. 



In skeletal muscle the myosin filament is lOoX 



o 



15,000 A which would accommodate aijout 400 

 molecules of skeletal myosin. The thin filaments could 

 be composed of a double or triple strand of F-actin. 

 According to Huxley (106), the interaction of the 

 filaments in the contractile process is made possible 

 by the presence of elastic side pieces on the myosin 

 filament which can combine with reactive sites on 

 the active filament. Actual '"feet" protruding from 

 the thick filaments have been seen in the electron 

 microscope which could be a double strand of myosin 



Z-line 



FIG. 18. Diagram showing behavior of actin and myosin 

 filaments during changes of muscle length. [From H. E. Hux- 

 ley (no).] 



Myosin 

 f ilQmenJ 



Actin 

 filameni 





Equilibrijm pos'lion 

 of M site 



FIG. 19. Diagram illustrating a mechanism of actin-myosin 

 combination in the sliding model. The part of a fibril which is 

 shown is in the right-hand half of an A band, so that the actin 

 filament is attached to the Z line which is out of the picture to 

 the right. The arrow gives the direction of the relative motion 

 between the filaments when the muscle shortens. [From A. F. 

 Huxley (106).] 



peptide emerging from the ijody of the macromolecu- 

 lar filament. 



It is postulated that the linkages between the 

 sliding members are formed spontaneously, but are 

 i;)roken only by the input of energy from metabolic 

 sources. This is not difficult to imagine if it is sup- 

 posed that the reactions are catalyzed by an enzyme 

 which is fixed to one of the filaments or perhaps by 

 enzymatic activity, intrinsic to one of the contractile 

 proteins as ATPase is to myosin. 



Figure 19 shows a schematic diagram of the sliding 

 model in which the contractile elements shown are 

 thought of as lying in the righthand half of an A band 

 .so that the nearest Z line to which the actin filament 

 is attached is off the picture to the right. During 

 shortening, the actin filament moves to the left rela- 

 tive to the mvosin filament. The distance from A, the 



