EDWARD O. BOETTIGER II5 



This behavior of fibrillar muscle cannot be fully explained in the present 

 slate of our understanding of muscle physiology. The agent controlling the 

 activity of the contractile element operates continuously throughout the cycle 

 and in relation to the events of the cycle. The agent cannot exert its effect 

 through the diffusion of a substance to and from the active sites, for the com- 

 plete cycle of deactivation and reactivation can occur in less than 0.5 milli- 

 second. Sotavalta (13) has noted a frequency of 2200 muscle cycles per second, 

 and this high frequency must be accounted for in any theory of mechanism. 

 The controlling mechanism must be held structurally in close association with 

 the active sites of the contractile protein. This could be accomplished by a new 

 fibrous protein system paralleling the actomyosin chains. The tension-sustain- 

 ing chemical bonds within the contractile protein could be neutralized by 

 bonding laterally with the new protein. Rapid shifts in these bonds result in 

 the cycle. The large size of the fibrils, averaging almost exactly double that 

 of comparable non-fibrillar muscles (10), is at least consistent with this idea. 



As the change produced by the agent appears as an alteration in the tension- 

 sustaining ability of the muscle, it can be described as a change in the active 

 state. The change in active state is viewed here, however, not as a reversal of 

 the activation process by which the active state is set up but as a direct inde- 

 pendent effect upon the contractile mechanism. 



The question posed by the title of this paper may now be answered. The 

 individual changes in length of activated fibrillar muscle are not triggered, but 

 are rather phases of a process that is continuous throughout the cycle. Once 

 the muscle is set into the active state and allowed to shorten with a load, 

 oscillations are initiated. With regard to triggering the active state, as with 

 other properties, fibrillar muscle behaves as a typical striated muscle. 



REFERENCES 



1. BoETTiGER, E. G. AND E. FuRSHPAN. Observations on the flight motor of Diptera. Biol. 

 Bull. 99: 346, 1950. 



2. BoETTiGER, E. G. AND E. FuRSHPAN. The mechanics of flight in Diptera. Biol. Bull. 

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3. BoETTiGER, E. G. AND E. FuRSHPAN. The response of fibrillar muscle to rapid release 

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4. BoETTiGER, E. G. AND E. FuRSHPAN. Mechanical properties of insect flight muscle. J. 

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5. BoETTiGER, E. G. .A.ND F. McCann. Single fiber action potentials in insect fibrillar muscle. 

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6. Gasser, H. S. and a. V. Hill. The dynamics of muscular contraction. Proc. Roy. Soc. 

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7. Hocking, B. The intrinsic range and speed of flight of insects. Tr. Roy. Enlomol. Soc. 

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8. McEnroe, W. Tension-length curves of insect fibrillar muscle. Federation Proc. 11: 104 

 1952. 



