THE MOLECULAR BASIS OF MUSCLE CONTRACTION 



285 



thin filaments gives rise to density gradients which appear to us through the 

 light microscope as the bands (Figure 10-12 (b) and Figure 10-13). 



The motive power is provided by the inherently contractile molecular 

 actomyosin complex, a complicated protein condensation product of two 

 complex units, actin and myosin — the former apparently primarily a struc- 

 tural support and the latter an enzyme which catalyzes the hydrolysis of 

 ATP. There is evidence that myosin is contained principally in the thick 





* : %V 









(a) 



Figure 10-13. Huxley's Famous Electron Micrographs of Intermeshing Arrays of Thick 

 and Thin Filaments of Striated Muscle Fibers, (a) Side view (longitudinal section). 

 Note how the light H-band is formed by a discontinuity in the thin filaments. Note also 

 the direct evidence for cross-bonds between thick and thin filaments (300,000 x). 

 (b) End view (cross-section) (170,000x). (Courtesy of H. E. Huxley, Laboratory of 

 Molecular Biology, Cambridge University.) 



