4 PHYSIOLOGICAL TRIGGERS 



or amount of feedback). These examples are chain reactions, which are an im- 

 portant family though not the only one in this subclass. We may regard these 

 as three primary subclasses of triggers, and call them respectively mechanical, 

 molecular and kinetic critical points. 



On other criteria and in a less fundamental way we can classify triggers ac- 

 cording to whether the system is self-restoring or is restorable, or is actually 

 refractory for some time after an event, before another cycle can be initiated. 



A further distinction can be made between these instances where the energy 

 released after the critical point is passed, is stored by the pull of the trigger 

 itself or is previously stored and held for long periods in potential form. Ob- 

 viously the distinction between these cases will not always be easy, and de- 

 pends upon the process to which we give the name 'pulling the trigger.' But 

 very commonly the difference between the process of storing the energy and the 

 process of pulling the trigger is clear (camera shutter which requires cocking) 

 and in other cases they are obviously the same process (light switch). 



Triggers differ among themselves greatly in stability, that is, in the energy 

 required to pull the trigger; put in another form, the probability of random 

 firing. For certain purposes and studies this distinction will be useful or en- 

 lightening. Although the difference may only be clear in extreme cases, we can 

 recognize that to control the blasting charge in road-building operations, a 

 plunger requiring large energy input and very unlikely to be set off by accident 

 is used, whereas in other situations the discharge of energy may be inexpensive 

 or harmless and we are more concerned with sensitivity, as in a thermostat 

 which may unavoidably chatter with heavy footsteps. 



Triggers differ from one another in their requirements with respect to the 

 time course of pull, especially in their sensitivity to very slow pull. Some triggers, 

 especially electronic ones (e.g., those controlling the raster pattern in a televi- 

 sion tube) are AC-coupled and are quite insensitive to slow pull, or, in the ter- 

 minology of the physiologist, have a significant accommodation. Others are 

 protected from fast pulls, like a chattering thermostat to which we connect 

 a large condenser so that only slow or maintained pulls will trip the trigger. 



Various accessories may be attached to triggers; for example, to require a 

 unidirectional pull, or to arm the trigger only under certain conditions, or to 

 protect against accidents, or to channel the form of energy to which the trigger 

 is sensitive. 



Triggers made or selected for a use commonly are of a type involving a 

 sequence of different events. Thus a gun, when loaded, represents at least two 

 step functions in series — the release of the hammer and the explosion of a charge. 

 Some cameras and many electronic devices trigger several events seriatim. 

 We shall see that this is particularly characteristic of biological triggered 

 systems. 



