THEODORE H. BULLOCK 7 



depends on the suggestions which future workers find in chapters outside their 

 own specialties. 



Thus, possibly the greatest point of interest growing out of this comparison 

 of events of sudden onset is that so many of them turn out not to be inherently 

 all-or-none triggers, but graded or continuously controlled processes. Even ferti- 

 lization of the egg and initiation of contraction in insect fibrillar muscle are 

 placed in this category by the present authors. Other cases are the hormonal 

 alterations in permeability of certain target cells, the nervous control of some 

 endocrine glands, and the transducer phase of sensory and postjunctional 

 neuronal response, prior to initiation of impulses. 



These cases are not, in their status as non-triggered events, equivalent. For 

 example, the egg and the muscle are denied trigger status for different reasons 

 and — illustrating one of the thought-provoking difiliculties of the concept — ■ 

 both debatable. Experimentally, an egg can be activated partially; but a typical 

 trigger may, by experimental manipulation, be altered into a graded device 

 and an inherently graded device may appear to be all-or-none because the 

 minimum normal stimulus is already maximal. One or both of these may be as 

 true of fertilization of the egg as of the initiation of infection by a virus particle 

 or bacterium (not the time course of the infectious process). This triggering by 

 quantal stimuli is different from processes which are continuous functions of 

 their normal input, like the work output of mechanically stimulated insect 

 fibrillar muscle and the microphonic response of the cochlea. 



The vibrating insect fibrillar muscle delivers work as a continuous function 

 of the phase difference between its tension and its length. But under normal 

 conditions there is presumably a discontinuous difference between the actively 

 contracting state of a given fibril and the relaxed states The change from one to 

 the other may be said to be triggered; but the adequate stimulus for this change 

 is at the same time one of the conditions determining the magnitude of the ten- 

 sion and hence does not lose control, experimentally. Not only is the high fre- 

 quency contraction in vivo a modified step function but also the permissive 

 state of the muscle, under control of the few all-or-none nerve impulses per 

 second, is apparently a step function but probably with sloping steps. 



Whether other cases, e.g., junctional transmission at synapses, are normally 

 based on quantal input or on graded input is a question which has not been 

 adequately recognized and we possess little information; but it seems probable 

 that some junctions receive graded activity in presynaptic terminals and others 

 all-or-none prespikes. Similarly, in endocrine-controlled events the present 

 knowledge suggests a picture of some which are all-or-none, as in metamorphosis 

 and diapause, while others are normally at least somewhat graded, as in uterine 

 changes. "At least somewhat" means that even though the normal event is as 

 ungraded as the proverbial "slight case of pregnancy," constituent processes 

 may be continuously under the control of their hormonal inputs. 



Each example could be similarly examined and at least a number of the 



