MUSCLE STRUCTURE 73 



^— A- 



1 H ' ' H ' ' H 



Fig. 33. A diagram of the apparent arrangement of actin and myosin mole- 

 cules in a striated muscle. The thin lines represent the actin molecules; the 

 thick lines the myosin molecules. When the actin molecules come together 

 end-to-end, the muscle is presumed to shorten maximally, with the disappear- 

 ance of the H-band. The maximum shortening on this model would be 40%. 



of up to 80%. Further, this shortening of 80% may also be observed in 

 smooth, unstriated muscles which presumably are very differently con- 

 structed. Furthermore, the synthetic actomyosin filaments, when in- 

 cubated with ATP, have been observed to contract more than 40%, and 

 in this case there is no possibility of a "sliding" structure. 



It is therefore of some weird interest that electron microscopy of 

 muscles has revealed an internal arrangement which dovetails neatly 

 with the sliding structure model. The structure is represented diagram- 

 matically as in Fig. 33. What we see are thick myosin filaments and thin 

 actin filaments. These long protein molecules are placed so that the dark 

 parts of the A-band contain both kinds of molecules, the clear H-band 

 contains only myosin molecules, and the I-band contains only actin 

 molecules. As the sliding structure model would have it, the actin mole- 

 cules of the I-band slide past the myosin molecules until, as sketched in 

 the next figure, they have gone as far as they can. Thus, the I-band has 

 shortened by as much as the H-band. The evidence for these diagrams 

 derives from studies of cross sections in the I-, A-, and H-regions. As 

 shown in Fig. 34, there are only thick filamentous elements in the H- 

 band. In the dark part of the A-band, there is the same number of thick 

 elements, and twice as many thin elements. In the I-band there are only 

 this doubled number of thin elements. Some clever chemistry has demon- 

 strated that the thick elements are myosin, and that the thin elements 

 are mainly actin. 



As may be deduced from the discussion thus far, the structural basis 

 of muscle contraction is simply not known, despite a wealth of detailed 

 information about the nature and arrangement of the materials making 

 up the muscle. Either some vital information is lacking or else there is 

 some misinformation in w r hat we think w r e already have established. 



