108 INVERTEBRATE PHYSIOLOGY 



sarcostyles, a sarcoplasm of low viscosity and with nvimerous large sarco- 

 somes. Perhaps the most striking case, for which we have at present only 

 histological evidence, is in the delphacid Perkinsiclla (Tiegs, 1955), where 

 a tergal abdominal muscle appears to have been drawn into the complex of 

 flight muscles as a wing levator and to have become fibrillar in structure. 

 It seems clear that in insects, at least, there is a potential deactivation-by- 

 release mechanism present in all striated muscle. Is this true of all muscles 

 in the animal kingdom ? 



It is important to emphasize first of all that this type of myogenic rhythm 

 has little in common with the well-known myogenic rhythm of the verte- 

 brate heart. In insect fibrillar muscle, the rhythm derives from a property 

 of the contractile fibrils in the interior of the muscle fiber, and there is little 

 or no synchronous change in the permeability properties of the external 

 membrane as measured by electrical potentials across the cell surface. In 

 heart muscle, on the other hand, the rhythm of mechanical activity is 

 accompanied by a rhythmic change in membrane potential, and it seems to 

 be established beyond reasonable doubt that the rhythm is determined by 

 these membrane properties. Del Castillo and Katz (1955) and Hutter and 

 Trautwein (1955) have independently demonstrated that the cardio- 

 inhibitor and cardio-accelerator fibers in the vagus and sympathetic nerves 

 produce their effect by modification of the membrane potential of the heart 

 muscle fibers in the usual directions ; the spikes of activity originate at a 

 critical level of spontaneous slow depolarization during diastole. At least 

 in the mammalian heart it is also clear that the mechanical stimulus of 

 stretch produced by an increased venous return does not affect the fre- 

 quency of beat in the denervated heart ; control of frequency of beat 

 is a reflex phenomenon from mechanoreceptors in the great veins 

 and aorta. 



In some vertebrate smooth (short-fibered) muscles we also have a myo- 

 genic rhythm (Bozler, 1948), and here mechanical conditions do affect 

 the frequency of contractions. Our knowledge of the physiology of these 

 tissues has lagged seriously behind that for striated muscle, but it seems 

 probable that here, as in the heart, the rhythmicity derives ultimately from 

 rhythmic properties of the cell membranes ; the form of action potentials is 

 similar to those of heart muscle. Conduction of excitation can take place 

 through the "muscle net" without the involvement of nerves, but is not all- 

 or-none as in the heart ; whether the effect of mechanical stimuli is mainly 

 on the membrane properties of the individual muscle cells, or on the inter- 

 cellular conduction mechanism, remains to be elucidated. 



Myogenic rhythms also occur in the hearts of some arthropods (Krijgs- 

 man, 1952) and moUusks (Krijgsman and Divaris, 1955) ; we have no 

 knowledge of the membrane potential changes in these tissues, but their 



