Triggering of the Contractile Process in 

 Insect Fibrillar Muscle' 



EDWARD G. BOETTIGER2 



Department of Zoology, University of Connecticut, Storrs, Connecticut 



IN THE SEQUENCE OF EVENTS that initiates muscle contraction three basic 

 processes are involved: /) excitation, the depolarization of the muscle mem- 

 brane; 2) coupling, or the transfer of excitation to the contractile parts of the 

 cell; and 3) activation, the setting up of the active stage in the contractile ele- 

 ments. Once in the active state the contractile elements shorten, if allowed, at 

 a velocity determined by the external force transmitted to them through the 

 series elastic element. In muscle, therefore, several events are triggered in 

 sequence. 



Into this general picture of the initiation of contraction must be fitted the 

 anomalous behavior of insect fibrillar muscle. Characterized by its large, easily 

 separated fibrils, fibrillar muscle is specialized to produce vibratory movement. 

 It is found as the flight muscle of the higher insects, as the muscle moving the 

 halteres of flies, and as the tymbal muscle of the song-making apparatus of 

 certain cicadas. Pringle (11), using external electrodes, noted that the action 

 potentials of the flight muscle of flies are of a much lower frequency than the 

 wing movements. This surprising independence of action potentials and muscle 

 cell response was confirmed by experiments in which electrodes were placed 

 within single muscle cells (5). Pringle, observing that the indirect flight muscles 

 of flies did not shorten on stimulation, suggested that the action potentials 

 sensitized the muscles to activation by stretch. Upon being stretched by its 

 antagonist, therefore, each muscle would shorten. 



The wing articulation of flies was shown by Boettiger and Furshpan (i) to 

 store elastic energy by straining the thorax at the beginning of each stroke, and 

 to release it at a critical point as an aid in completing the movement. The rapid 

 unloading of the muscle that results from this action should reduce the muscle 

 tension, as in the quick release experiments of Gasser and Hill (6). They had 

 found that when a tetanized, isometrically contracting muscle is allowed to 

 shorten rapidly a small amount, the tension falls to zero and then gradually 

 returns to the level characteristic of the shorter length. The movement cycle 



1 This work was made possible by grants-in-aid from the Institute of Neurological Diseases 

 and Blindness, National Institutes of Health. 



2 The experimental work reported here was done in collaboration with Dr. Edwin Furshpan. 



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