The Trigger Concept in Biology ^ 



THEODORE HOLMES BULLOCK 



Department of Zoology, University of California, Los Angeles, California 



MANY BIOLOGISTS IN MANY CONTEXTS have uscd the word trigger. Many 

 others have objected because the word was not clearly defined. We may 

 examine the question whether there is at least heuristic value in formulating 

 a concept of triggers in biology and, if so, what sorts of questions it raises. 



At the outset we may recognize some justification for the desire to consider 

 what might be called discontinuous rate processes, or events of relatively sudden 

 onset. So many of our symposia quite appropriately have concentrated on the 

 steady state processes of living systems, that inevitably some have raised the 

 question, "What about the sudden changes in living systems?". Events are as 

 characteristic of life as the steady state, especially improbable events. As a 

 tentative beginning, we may ask whether there is a class of events to which the 

 concept of triggers can be properly applied and, if so, what common denomina- 

 tors define them. The conclusion that such a class exists and can be defined is 

 the starting point of this essay. Given this, the assumption is made that there 

 is at least some value for some people in recognizing the distinguishing char- 

 acters, the variables and the possible subdivisions of triggers. By spelling out 

 the defining features of the class and several types of criteria for recognizing 

 subclasses, it is expected that at least formal comparisons can be made be- 

 tween different cases. The stage of knowledge in specific instances will be more 

 clearly recognized and new measurements may be more obvious. 



DEFINITION 



The dictionary defines a trigger as a piece connected with a catch or detent 

 as a means of releasing it. We may define a trigger as an arrangement of parts 

 in a system, or an energy state in a system, such that application of an in- 

 crement of energy (commonly but not necessarily small) will precipitate a 

 change, whose time course (commonly but not necessarily rapid) and magnitude 

 are essentially independent of those of the energy increment. This definition 

 admits a wide class of phenomena: the trigger of a gun (it is not important 

 whether there is a bullet in position or not, but simply that the hammer be 

 triggered) ; a lever pushing a rock ofif a cliff (or down a gentle slope) ; the release 

 of a trap, or of a bomb rack in a plane; a light switch; an electronic circuit with 

 two stable positions, or, indeed, one in which neither position is stable but lasts 

 a long time relative to the transit time between these two positions, (a so-called 

 multivibrator or flip-flop circuit); even a relaxation oscillator. 



