70 F. O. SCHMITT VOL. 4 (1950) 



is the axial periodicity demonstrated both by small-angle X-ray dil^raction^^ and by 

 electron microscopy*. This period has a value of about 400 A in uncontracted fibres and 

 appears to be characteristic of muscle generally, for Bear has observed it not only in 

 vertebrate striated muscle but also in various invertebrate muscles which are generally 

 regarded as being of the smooth type. In electron micrographs the filaments have a 

 beaded appearance which gives rise to a fine banding of the myofibril, the distance 

 between bands being about 400 A. Draper and Hodge^^ have shown the period very 

 strikingly in electron micrographs of platinum-shadowed preparations. In their prelimi- 

 nary note they state that the axial period varies inversely with the degree of shortening 

 of the muscle. Variations in the 400 A period with fibre length were also noted by 

 Bennett^^ who believes to haye shown that the filaments have a helical structure. If 

 these points are satisfactorily documented and confirmed we shall have visual evidence 

 of the contractile phenomenon at the near-molecular level. 



Actually the relation between the 400 A axial period demonstrated by X-ray 

 diffraction and the pseudo-period of about the same value seen in electron micrographs 

 is not clear. The largest meridional spacing observed in the X-ray patterns is about 

 27 A which is an order of the larger period. If the situation is similar to that of para- 

 myosin^*' ^^ one might expect that the period which might be observable as cross bands 

 in the electron microscope, would have a value of about 27 A ; the larger period of about 

 400 A would be manifested as a geometric pattern of discontinuities within the bands. 

 However, depending on the type of geometry of the intraperiod structure, discon- 

 tinuities at a spacing larger than 27 A may appear in electron micrographs. The solution 

 of this problem will have to await a more detailed X-ray analysis and attainment of 

 very considerably increased electron microscope resolution of the structure of the 

 filaments. 



AsTBURY, Perry, Reed, and Spark^^ have observed a spacing of 54 A in fibrous 

 actin. At large angles the pattern is not that of an alpha protein. This led the authors 

 to the conclusion that the large-angle pattern of muscle is due to myosin while the small- 

 angle pattern is due to actin; the full muscle pattern derives from actomyosin which 

 exists as a complex in muscle. While this may prove to be the case, the diffraction evi- 

 dence is not yet sufficiently detailed to require this conclusion. 



The electron microscope investigation of contractility might be facilitated by 

 examination of in vitro models such as the actomyosin-ATP system described by Szent- 

 Gyorgyi^'. This would be true if such systems permitted higher resolution than could 

 be achieved in the myofibril and, particularly, if the essential properties of such a system 

 faithfully portray those of muscle. Recently Szent-Gyorgyi^^ has found that muscles 

 thoroughly extracted with 50% glycerol at low temperatures are capable of contraction 

 when treated with ATP and produce the same tension as the intact muscle when maxi- 

 mally excited. Differences in the behaviour of this model as compared with intact muscle 

 are attributed to the fact that the model may lack some of the proteins, lipids and other 

 substances with which the actomyosin is normally associated in muscle. From studies 

 of this model, as from the previous one of Varg.\^^, the conclusion was reached that 

 contractile substance is composed of functional units, "autones", and that contraction 

 represents an all-or-none equilibrium reaction of these units ; contraction and relaxation 

 are two distinct allotropic states of the autones. 



Unfortunately, as admitted by Szent-Gyorgyi^^ and as amplified by Sandovv^" 

 none of the partial systems and models thus far proposed fully displays the essential 

 References p. 76I77. 



