MYOGENIC RHYTHMS 111 



if the rate of vacation of the sites is directly proportional to the distance 

 through which the muscle has shortened ; in addition to the number re- 

 quired in any unit of time to maintain tension, a further number of high- 

 energy molecules would then be needed to achieve shortening. 



One hypothesis to account for deactivation by release can be stated as 

 follows. If a sudden shortening occurs in a muscle due to the sudden re- 

 lease of tension, a large number of sites on the actomyosin complex will 

 be suddenly vacated. Supply of high-energy molecules by diffusion from 

 the sarcosomes may be inadequate to provide for their re-occupation imme- 

 diately following this large demand, and the state of the muscle would then 

 become temporarily similar to the inexcited condition. This would produce 

 deactivation by release. Whether or not the deactivation phenomenon will 

 occur depends, according to this hypothesis, on the relative rates of vaca- 

 tion of active sites by rapid shortening and of resupply of high-energy 

 molecules by diffusion from the sarcosomes. If the maximum possible rate 

 of vacation is low and the diffusion pathway small, complete deactivation 

 can never occur. If the rate of vacation is large and the diffusion pathway 

 relatively long, deactivation may last for sufficiently long to allow the 

 myofibrils to be re-extended to their initial length and a myogenic rhythm 

 of activity becomes a possibility. A rough calculation of probable ATP 

 diffusion rates over the distances involved shows that a time lag of a few 

 milliseconds in re-establishing the local concentration in the myofibrils is 

 a possibility. 



Consistent with this hypothesis is the histological observation that the 

 "fibrils" are thick in insect muscles showing a myogenic rhythm. Tiegs 

 ( 1955) has shown that the fibrils which can be isolated from these fibrillar 

 muscle are, in fact, sarcostyles composed of several myofibrils, but this does 

 not alter the fact that the large sarcosomes are situated between the sarco- 

 styles with a relatively long diff'usion pathway. The diameter of the sarco- 

 styles in fibrillar muscles of Hymenoptera, Coleoptera, and jassids is from 

 3 to 5/x, by contrast with values of less than 1/a for orthopteran wing 

 muscles, insect limb muscles, and vertebrate striated muscle. This differ- 

 ence and the large sarcosomes are the only known histological features 

 consistently associated with the myogenic rhythm property. 



A hypothesis of this sort demands no sudden change in the basic mech- 

 anism of contractility in the change from the 1 :1 state to the myogenic 

 rh}-thm, and there is thus no difficulty in the occurrence of rhythms in many 

 different evolutionary lines in the insects. It makes no suggestion about 

 the nature of the active state in the muscle fiber during which access to 

 the sites on the actomyosin complex is possible by the high-energy mole- 

 cules. Pringle ( 1954a) showed that the duration of the active state follow- 

 ing the arrival of a single nerve impulse is not markedly different, whether 



