EXCITATION, INHIBITION, AND THE CONCEPT OF THE STIMULUS 325 



When it is recognized that excitatory agents (or neurons) can cause inhibi- 

 tion as soon as their influence is removed, and that inhibitory agents (neurons) 

 can cause excitation as soon as their action is terminated, it becomes an 

 important matter of definition what one is to call a "stimulus". If the muscle 

 element of a stretch receptor organ is stretched, we can call that a stimulus; 

 and if we remove an inhibitory substance from an inhibited cell, we can call 

 this also a stimulus. 



A stimulus can thus be something that is applied or something that is 

 removed. The common denominator between these two types of events is 

 the change in environmental conditions (mechanical, chemical, thermal or 

 electrical). The direction of the change determines whether inhibition or 

 excitation results from the change. 



In the previous discussion I was careful to avoid the term "stimulus" and 

 to use instead the term "stimulation". I would like to suggest to reserve the 

 term "stimulus" for any change in environmental conditions from a state A 

 to a state B, which leads to either excitation or inhibition in the responding 

 cell. The term "stimulation" should be used for the whole complex of events 

 which includes the change from state A to state B, and the maintenance of 

 state B for a certain length of time (thus permitting accommodation). If 

 repetitive stimuU result in a shift of membrane potential which is maintained 

 even during the intervals between stimuli, the series of stimuli should be 

 referred to as stimulation. 



With this terminology two things are achieved: (1) Recognition is given to 

 the fact that the term stimulus implies the reaction of a cell to an environ- 

 mental change and that it is not a substitute name for the agent applied. 

 (2) It is possible to describe physiological phenomena in unambiguous terms. 

 For instance, it is possible clearly to differentiate the meaning of "duration 

 of stimulation" and "duration of the stimulus". One can now make the 

 statement: the end of inhibitory stimulation can act as an excitatory stimulus 

 provided the duration of the stimulation was long enough to permit accom- 

 modation of the responding cell. 



REFERENCES 



Eyzaguirre, C. and Kuffler, S. W. (1955) Process of excitation in the dendrites and in 



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39 : 87-119. 

 Florey, E. (1956) Adaptationserscheinungen in den sensiblen Neuronen dti Streck- 



receptoren des Flusskrebses. Z. Natiirforsch. lib : 504-513. 

 Florey, E. (1957) Chemical transmission and adaptation. J. Gen. Physiol. 40 : 533-545. 

 Kuffler, S. W. (1953) Discharge patterns and functional organization of mammalian 



retina. /. Neiirophysiol. 16 : 37-68. 

 Terzuolo, C. a. and Bullock, T. H. (1956) Measurement of imposed voltage gradient 



adequate to modulate neuronal firing. Proc. Nat. Acad. Sci. U.S. 42 : 687-694. 



