2 PHYSIOLOGICAL TRIGGERS 



All these are examples from the inanimate world which would be embraced 

 under this formulation. Of course it is possible, if one prefers not to be so in- 

 clusive, to rephrase the definition to exclude a number of these cases. For ex- 

 ample, if it is deemed essential that the increment of energy be of relatively 

 small amount compared to the energy released after the critical point, then we 

 can exclude the relaxation oscillator, the light switch and other cases where the 

 energy released is simply that which is put into a spring by the pull upon the 

 lever. 



MAGNITUDE OF INPUT 



Fig. I. Diagrammatic input-output relations; a is the relation defining a trigger, the others 

 are various classes of linear and nonlinear continuously input-controlled relations, with (b, 

 d, e) or without (c) a threshold. 



In either case it is apparent that our formulation leaves out of account the 

 nature and quantity of energy which may be released as a consequence of the 

 change. For example, the amount of current which flows after the light switch 

 is flipped or the amount of dynamite with which a cap or a bomb may have been 

 charged, or even the presence of such energy, is unimportant. The essential 

 common denominators in this class of phenomena are: a) a pull, that is to say 

 an addition of energy to a system, at a rate which may vary within wide limits, 

 b) a critical point, beyond which c) there is released a store of energy, d) in 

 a manner which is independent of the time course and energy content of the pull 

 and beyond the control of that input, i.e., is all-or-none. 



