46 D. M. NEEDHAM VOL. 4 (1950) 



protein) with the sulphydryl reagents iodoacetate and porphyrexid causes decreased 

 shortening when ATP is subsequently added. The interpretation of these results is not 

 immediately obvious for, when sulphydryl reagents are added to actomyosin sols in 

 0.5 M KCl, there is a decrease in viscosity as would be expected if the reagent, like ATP, 

 broke the link between the actin and the myosin. The effect is slower than with ATP 

 itself, probably because the reagents react also with other -SH groups (for example, in 

 the actin) while the ATP reacts specificially with the connecting groups. In the case of 

 the threads, there seems no reason why -SH reagents should inhibit the splitting of the 

 myosin from the actin; possibly the attachment of ATP to the myosin, as well as the 

 presence of free -SH groups, is involved in the further changes in state of aggregation 

 and it is these wh'ch become impossible. 



It seems doubtful whether these phenomena of synaeresis are connected with the 

 mechanism of contraction. If the removal of water is intermoiecular, this would lead 

 in vivo to a narrowing rather than a shortening of the fibres, since both myosin and actin 

 are known to be arranged with their long axes parallel to the fibre axis. Perry et al}^ 

 have remarked on this and also pointed out that though the loss of water associated 

 with volume contraction is very rapid, the reverse process (which might be analogous 

 to relaxation) is slow and there is little information as to its degree. 



the localization of substances in the striated fibre 



The anisotropic (A) band seems to have had, from the early days of work on muscle 

 fibres, a particular interest for observers. In spite of the many variations and discrepan- 

 cies of description of the h'stological appearance of contracted muscle (depending on 

 the different sources of the muscle ; differences in preparation, whether fresh or fixed and 

 stained; differences in optical set-up; and differences in degree of contraction) there has 

 been a widespread if by no means unan'mous opinion that it is the A-band which be- 

 comes shorter in appea^-ance on contraction of the muscle while the I-band may show 

 little change or even become longer. 



This conclusion was probably based partly on the formation of contraction bands 

 (see below) in strong contraction ; a condition where the position of the staining mate- 

 rial has actually become reversed with respect to the A- and I-bands; but studies 

 like those of Buchthal et al.^^ on single living fibres do show a decrease in the A/I 

 ratio in early conti action. The work of Buchthal et al. was quantitative and showed, 

 in short isometric tetani, a decrease in length of the A-band of 18%, an increase in 

 the I-band of 28%. 



The visible changes in length of the A- and I-bands have often been taken as indi- 

 cating that the actual contractile process was limited to the A-bands; the I-bands, 

 thorgh not necessarily considered as passive, being the seat of less important changes. 

 The conception of mo^e recent yea^-s took the form that in the A-bands the protein 

 micelles undergo folding while this process is much less or gives place to unfolding in 

 the I-bands. 



The idea that the protein of the A-band actually differed in kind from that of the 

 I-band was given up as more accurate estimates of the myosin content of the muscle 

 became available. For several years the view was then prevalent that the fibril consists 

 of collections of myosin chains, a^'ranged in the anisotropic bands with their long axes 

 parallel to the fibril axis, but in the isotropic bands having much less orderly arrange- 

 References p. 4g. 



