110 INVERTEBRATE PHYSIOLOGY 



The Mechanism of Deactivation 



In Pringle (1954a) I discussed briefly the possible relationship of the 

 phenomenon of deactivation by release in insect fibrillar muscle to the me- 

 chanical properties of vertebrate striated muscle. It is well known that a 

 sudden release given to an excited muscle leads, if its amplitude of release 

 is sufficient, to the complete disappearance of tension, followed by its re- 

 development at a rate comparable to that at the beginning of excitation. 

 A. V. Hill has interpreted this and other results in terms of a "series elastic 

 component" in the muscle which goes slack when there is a sudden release. 

 In the experiments of Gasser and Hill ( 1924) on the frog sartorius muscle, 

 a release of 10% of the resting length was required to produce a tension 

 drop to zero, but Hill ( 1950) states that a smaller figure would have been 

 obtained if due allowance had been made for the elasticity of the suspen- 

 sion. This disappearance of tension on quick release resembles that found 

 with the cicada tymbal muscle, but the fact that the tymbal muscle is there- 

 upon re-extended to its initial length without immediate reappearance of 

 the original tension makes it impossible to explain the result simply in 

 terms of a series elastic component. The deactivation process here occurs 

 with a release of 1.5% of the resting length, showing that any series elastic 

 component present is very small, a result perhaps understandable in terms 

 of the different histology of the muscle attachments in insects and verte- 

 brates. 



Any hypothesis about the nature of deactivation by release must take 

 into account the normal features of muscular contraction (Wilkie, 1954), 

 the extensive studies of glycerinated muscle fibers (Weber and Portzehl, 

 1954; Morales, Botts, Blum, and Hill, 1955), and the pecularities of 

 fibrillar muscle (Tiegs, 1955), including the enzymatic properties of the 

 large sarcosomes (Watanabe and Williams, 1951). In the absence of any 

 general agreement on the nature of the biochemical and biophysical ma- 

 chinery involved in contraction, in spite of intensive current study, hypo- 

 theses are particularly vulnerable but may nevertheless be attempted in 

 the hope that the properties of this tissue may throw light on the general 

 problem. 



A generalized if somewhat naive view would be that contraction and 

 tension development occur in a muscle because excitation in some way 

 allows access to certain sites in the actomyosin complex of a high-energy 

 substance liberated by the sarcosomes. It is already clear that maintenance 

 of tension in striated muscle involves the continuous expenditure of energy, 

 and the sites are therefore presumably occupied and vacated cyclically, with 

 breakdown of the high-energy substance. The fact that the additional heat 

 of shortening is proportional to the distance shortened could be explained 



