THEODORE H. BULLOCK 



TRIGGERS IN BIOLOGICAL SYSTEMS 



These considerations mean that if we recognize the class of trigger phenomena 

 and then ask the question whether they are represented in physiological 

 systems, we can immediately recognize a series of questions of intrinsic interest, 

 such as the following. Can we identify the components of a trigger whose exist- 

 ence we know of only from the step-function of input and output (fig. i)? Can 

 we identify the necessary and sufiicient form of the adequate input (stimulus)? 

 Which type of critical point determines the firing of this trigger, mechanical, 

 molecular or kinetic? What are the properties of the trigger system with respect 

 to accommodation, adaptation, refractoriness, sensitivity, and the source of 

 energy released? The most useful question in many cases will be "Is this in fact 

 a trigger?". As will be seen in the papers in this symposium, it is quite possible 

 that a striking or classical case of an abrupt change may turn out, on experi- 

 ment, to be graded at least in part. 



Biological phenomena which appear to satisfy the conditions set forth are 

 not difiicult to find. We may think of cases like the activation of the egg by 

 spermatozoa or other means; the initiation of flowering in higher plants; the 

 initiation of infection, muscle contraction, sensory discharge, metamorphosis; 

 the onset and termination of diapause in insects; the initiation of hormone 

 secretion controlling phasic events as well as the initiation of the cellular 

 events in the target organ by the hormone; mitosis; nerve impulses; discon- 

 tinuous luminescence. These and many other examples come to mind. 



Consideration of the present state of knowledge of these cases is likely to 

 lead to some interesting general conclusions. For example, it seems likely that 

 none of these biological triggers involves the type of critical point which we 

 called mechanical (Boettiger has elsewhere described certain insects which use 

 a mechanical snap mechanism in their flight motor) and it seems very doubtful 

 whether they involve the class of molecular critical points exemplified by 

 boiling point, freezing point, and the like. This last conclusion is very likely to 

 need correction in the light of newer knowledge of protein behavior in cell 

 membranes and similar critical loci, but at present it seems likely that at least 

 most physiological triggers will depend for their critical points on the kinetic 

 principle, that is to say, on a critical ratio between two simultaneous rates. 



Another generalization is that inanimate triggers are generally degrading, 

 resulting in a more probable distribution of matter and energy, whereas at 

 least many biological triggers significantly increase complexity or initiate a new 

 state which requires more information to describe it than the last state. There 

 is always an energy hump which must be surmounted, but in the inanimate 

 triggers, after passing this hump the new state is typically, like that of the ham- 

 mer and spring in a gun, at a lower energy level than the cocked state. The same 

 may be true of many biological triggers, but others, like the activation of the 



