6 PHYSIOLOGICAL TRIGGERS 



egg by the spermatozoan, may be said to initiate a train of events which, while 

 releasing previously stored energy, create greater complexity of distribution 

 of energy and structure. 



This feature may be regarded as the consequence of another generalization 

 about biological triggers: they commonly occur in chains, each the adequate 

 stimulus for the next step, which is likely to be a qualitatively different event 

 and may be either a true trigger or a graded process. 



It will be apparent from reviewing the state of knowledge of particular physi- 

 ological triggers that in many instances we are still seeking the components of 

 the trigger; that is, identifying them anatomically (hypothalamic control of 

 pituitary, Sawyer), or physiologically (cell mechanism altered by hormones, 

 Szego; activating substance in Paramecium, Metz; steps in synaptic transmis- 

 sion, Grundfest; linkage between action potential and contraction in muscle, 

 Botts). In other cases the problem is at the stage of searching for the identifica- 

 tion of the adequate input (infectious vs. noninfectious virus particles, Rappa- 

 port et al.; control of luminescence, McElroy et al.; intermittent CO2 release 

 in insects. Buck; chemical, electrical and light triggers in algal cells. Blinks). 

 In virtually every case we still have to solve the problem of the nature of the 

 critical point, except in the analytical picture of the axonal membrane as de- 

 scribed by Cole where a kinetic relationship which may well be the prototype 

 of biological triggers in formal terms, is developed. In many cases the inquiries 

 of current interest involve secondary properties such as refractoriness (break 

 of diapause, Schneiderman; block to polyspermy, Metz), sensitivity (kinetins, 

 Skoog), the source of stored energy (cochlear microphonic, Davis), and the time 

 course of its release (high frequency insect muscle, Boettiger). 



The trigger concept has one further feature which deserves attention. Trigger 

 mechanisms are compared and analyzed on the basis of functional relations 

 between their components rather than on the structural nature of those com- 

 ponents. This means that we can consider cases at different levels of structural 

 complexity from the molecular to the tissue and organ levels. We can hope to 

 establish at least formal connections and contrasts between the types of de- 

 pendencies, of various examples at different levels, upon the functions of their 

 constituent elements. 



SOME COMPARISONS AND CONTRASTS 



This is the background and point of departure for this collection of papers. 

 They are a selected sample of cases where some kind of progress has recently 

 been made in elucidation of phenomena which at least superficially appear to 

 be triggered. Those who decry analogical thinking will find here a horrible 

 example, for there is no thread or common denominator except the possibility 

 of functional comparison through the concepts sj)elled out above. The several 

 instances represented illustrate quite different aspects of the problem of un- 

 ravelling discontinuous rate process and, put together, their special value 



