112 Inside the Living Cell 



providing mechanism is not greatly different to that of other cells 

 (see p. 47) and that the energy produced by the oxidation of sugar 

 becomes available to the muscle in the form of the adenosine triphos- 

 phate (atp) — another example of how the biochemical mechanisms 

 are closely related to each other, since the same basic energy yielding 

 cycle is used both for protein and nucleic acid synthesis and for giving 

 the energy required by the muscles. 



The chemical composition of muscle has also been studied for a 

 long time. Kiihne in 1864 extracted a soluble protein called myosin. 

 Nearly eighty years later, Straub in the laboratory of Szent-Gyorgyi 

 found another component in muscle which he called actin. The com- 

 pound of actin and myosin (actomyosin) had marked contractile pro- 

 perties, and as myosin itself was found to be a powerful agent for 

 decomposing atp and as the latter caused the contraction of syn- 

 thetic actomyosin threads, it appeared that the most important agents 

 in the muscle mechanism had been isolated. 



t" — ^ — 1 / h*^ r^'~^ 



Myosin y* 



filament jctin r ^^A^ H S- filament 



filament 



FIG. 19. Structure of striated muscle fibres (Huxley) 



More recently attention has been given to the structure of the 

 fibres in the intact muscle. The existence of striations or banding in 

 many muscle fibres had been observed by microscopists during the 

 nineteenth century. The striations consist of alternate zones having 

 higher and lower refractive index and were sometimes called dark 

 and light bands. The spacing of the bands is about 2-5/i, i.e. between 

 1/100 and 1/1000 of a millimetre. Their existence has been confirmed 

 by electron microscope pictures of thin sections of muscle (see 

 Plate 14). Numerous attempts have been made to identify the sub- 

 stances present in the bands by dissolving out the different com- 

 ponents separately. Finally, Dr H. E. Huxley and Miss Hanson on 

 one hand and Drs A. F. Huxley and R. Niedergerke both proposed 

 in 1954, on the bases of their observations, the picture shown in 

 Fig. 19. 



The continuous threads running through the muscles are actin fila- 

 ments, which are held together at intervals by a kind of spacing plate 

 (the Z line). The myosin filaments run side by side with these, but 



