MYOGENIC RHYTHMS 113 



is, so far as I am aware, no other evidence that activation occurs in verte- 

 brate muscle more rapidly than is indicated by the development of tension. 

 Experiments by Weber and others on the glycerinated fiber model are 

 unable to produce evidence about the initial rate of splitting of ATP and 

 the development of tension since the time course of events is here always 

 limited by inward diffusion of ATP into the model. 



The structural peculiarities of fibrillar muscle would be correlated, on 

 this type of hypothesis, with a higher degree of lateral association of muscle 

 elements than in normal muscle, so that plastic behavior is reduced to a 

 minimum ; mechanical events are thus transmitted more completely to the 

 contractile machinery and synchronization of the energy-yielding cycles is 

 more perfect after quick release. Boettiger's evidence ( this symposium) of 

 a high coefficient of elasticity in passive flight muscle may point to the 

 correctness of this correlation. Clarification of the biochemical nature of 

 the deactivation cycle must, however, await further quantitative studies. 



Summary 



( 1 ) Myogenic rhythms of activity have been described in the heart and 

 certain visceral short-fibered muscles of vertebrates, in the hearts of 

 moUusks and some arthropods, in the striated trunk muscles of the dogfish 

 embryo, and in the indirect flight and tymbal muscles of certain insects 

 (fibrillar muscles). They may also occur in the swimming movements of 

 micro-organisms and spermatozoa. 



(2) In vertebrate short-fibered muscles the rhythmic property resides 

 essentially in the surface membrane of the fibers ; potential changes are 

 observed synchronous with the contractions. In insect fibrillar muscles 

 myogenic rhythmicity normally occurs in the presence of a nonlinear elas- 

 tic ("click") mechanism in the muscular coupling, and depends on deacti- 

 vation of the contractile fibrils, with no synchronous potential changes in 

 the fiber membrane. The mechanism of rhythmicity is unknown in dogfish 

 embryo muscle and in the contractile filaments of micro-organisms and 

 spermatozoa. 



(3) Fibrillar structure is correlated with myogenic rhythmicity in in- 

 sects, but has evolved independently several times in the motor systems for 

 flight and sound production. In Homoptera, whose sound-production 

 mechanism in the first abdominal segment may be derived ultimately from 

 movements during copulation, the course of the evolutionary change from a 

 neurogenic to a myogenic rhythm can be understood without difficulty. In 

 the flight system it is suggested that the deactivation phenomenon was first 

 useful as a method of achieving a high-frequency rhythm of neurogenic 

 wing beat and later led to a myogenic rhythm in several different orders. 



