I04 PHYSIOLOGICAL TRIGGERS 



might, therefore, be accounted for by assuming that the action potentials of the 

 fibrillar muscle were of a frequency to induce tetanus. Upon being quick- 

 released by the click mechanism, the tension in the tetanized shortening muscle 

 would fall. A subsequent quick stretch by the antagonist on the return stroke 

 would bring the muscle to full length before the tension redeveloped (i). To 

 explain the work cycle the muscles must exert higher tensions on shortening 

 than while lengthening. 



Experimental work was undertaken in our laboratory to test this theory. 

 McEnroe (8) demonstrated that tension was maintained during tethered flight 

 in the fibrillar muscle of flies. In these experiments, one end of the muscle was 

 attached to a stiff recording lever while the other end, through which the muscle 

 was innervated, remained in situ. Typical twitches and tetanus resulted upon 

 electrical stimulation of this preparation (9). The shortening possible in these 

 muscles is extremely small, 0.02-0.04 millimeters, and had escaped previous 

 notice. The processes of excitation, coupling and activation are, therefore, not 

 modified in fibrillar muscle and the physiological trick by which the muscle 

 performs its function is in the contractile mechanism itself. Pringle (12) ob- 

 served similar behavior in a more favorable preparation, the fibrillar tymbal 

 muscle in the song-making apparatus of certain cicadas. He used the term "de- 

 activation' to describe what happens in the muscle when suddenly unloaded by 

 the click mechanism of the tymbal. Gasser and Hill had explained the mo- 

 mentary loss of tension in frog muscle as due to a more rapid shortening of the 

 elastic element than of the contractile element rather than as a basic change in 

 the contractile mechanism. 



The deactivation of the tymbal muscle was considered by Pringle to be of 

 very short duration, the muscle being restretched by the recoil of the tymbal 

 before reactivation could occur. This explanation is not adequate to account 

 for the observed behavior of the flight mechanism. A number of records ob- 

 tained by us showed the muscle cycle suddenly blocked with one muscle short 

 and the other long for a period equivalent to the duration of several cycles. 

 When movements again began, the longer muscle shortening and stretching 

 the shorter muscle, it was evident that the tension had not returned in the 

 muscle whose restretching had been delayed. One may alter the frequency of 

 movement in the same insect from 40 per second to 330 per second, by chang- 

 ing the wing load. The deactivation-reactivation cycle must have quite a dif- 

 ferent duration at these extreme frequencies. It is necessary, therefore, to postu- 

 late a reactivation by stretch as well as the deactivation by release. That 

 stretching a quick-released fibrillar muscle does increase the rate of rise in 

 tension was shown experimentally by Boettiger and Furshpan, (3, 4). 



Recent experiments to be described here have demonstrated oscillatory move- 

 ments in fibrillar muscle in the absence of a click mechanism. As background 

 for the discussion of these results, it will be necessary to define some of the 

 properties of a vibrating system. 



