MYOGENIC RHYTHMS 



J. W. S. Pringle 

 Cambridge University 



Rhythmic muscular activity is the most characteristic effector action in 

 higher animals. Historically, concentration of attention on the limb muscles 

 of land vertebrates and on other easily isolated muscles in the limbs of 

 arthropods has tended to produce a picture of muscle physiology which 

 may not be valid for the animal kingdom as a whole. This review, while 

 concentrating mainly on insect fibrillar muscle, will concern itself also with 

 other mechanisms by which rhythms of activity can be generated in muscu- 

 lar tissues. 



The most convenient physiological classification of muscles is one based 

 on the ideas of Bozler (1948), who first distinguished "long-fibered" 

 muscles, in which each fiber is innervated by branches of a controlling 

 nerve fiber, and "short-fibered" muscles with more diffuse innervation 

 and the possibility of conduction of excitation from muscle fiber to muscle 

 fiber. Within each class there occur striated and unstriated examples, 

 correlated in general with speed of action. Thus, the somatic muscles of 

 most higher animals fall in the first class, whether they are striated as in 

 vertebrates and arthropods or unstriated as in the byssus retractors of 

 lamellibranch mollusks, and vertebrate visceral muscles fall in the second 

 class, whether they are striated as in the heart or unstriated as in the ureter. 



Until recently the occurrence of a myogenic rhythm of activity (that is, 

 a rhythm in which nervous tissues play no essential role) was recognized 

 only for the class of short-fibered muscles, and in particular for the verte- 

 brate heart. In 1949 I called attention (Pringle, 1949) to the unusual 

 neuromuscular mechanism which appears to be present in the indirect 

 flight and haltere muscles of the higher Diptera, in which a high frequency 

 of muscular contraction is not accompanied by the synchronous muscle 

 potentials which are characteristic of the activity of other somatic striated 

 muscles. The evidence for the existence here of a novel type of rhythmic 

 mechanism was much extended by Roeder ( 1951), and at the same time 

 Boettiger and Furshpan ( 1950, 1952) were demonstrating that the skeletal 

 mechanical system of the fly thorax has by no means the simple lever action 

 described in most entomological textbooks. By the time of the appearance 

 of Chadwick's (1953) chapters on insect flight in Roeder's textbook, it 

 was clear that there was something very unusual in the biophysics of the 

 flight motor in certain higher insects. 



[99] 



