The Depolarizing Nature of the Trigger 39 



mechanisms which was published in 1961, Davis^* attempted to 

 set standards for the usage of the two terms, in order to clear up 

 some of the ambiguity prevalent within the literature. He pro- 

 posed that the term ' receptor potential ' be used, in the sense 

 first employed by Gray,^® to designate the first electrical change 

 in a primary sense cell directly attributable to the absorption of 

 stimulus energy. This concept seems a useful one and is in 

 keeping with the experimental observations. Thus, the receptor 

 potential has a specific causative agent, an absolute magnitude 

 and time-course, and a fixed locus of origin based upon structural 

 modifications and the spatial position of the sensory region of the 

 cell. Moreover, Gray's definition does not specify (or imply) that 

 the receptor potential necessarily be a depolarization of the 

 membrane. 



A * generator potential ' can be any depolarizing potential 

 change which generates conducted impulses. The term is thus 

 ambiguous. Davis suggested that it should be used to identify 

 (depolarizing) potential variations in the impulse-generating 

 regions of a sensory system. In primary sensory neurons, this 

 would be identical with the receptor potential and the current 

 sources for both would be the same. In a second-order sensory 

 neuron the generator potential would presumably be equivalent 

 to the depolarizing synaptic (or electrotonic) potential arising 

 ac ross tHe post-junctional membrane and generating impulses at 

 that point. As discussed above, in some instances the receptor 

 potential may be hyperpolarizing in sign and hardly capable of 

 directly generating action potentials. In other cases, as in the 

 abdominal photoreceptor neurons of crayfish, ^<'' ^2 the cells 

 involved serve as tactile interneurons as well as primary sensory 

 cells for light stimuli. In this preparation conducted impulses are 

 initiated not only in response to light stimuli, but also by con- 

 ventional postsynaptic potentials evoked as a result of activity in 

 primary mechanosensory neurons. The depolarizations which 

 generate impulses in the abdominal photoreceptor neurons not 

 only may arise from different membrane regions, but also are due 

 to quite different physiological processes. It is thus essential that 

 usage of the term ' generator potential ' should be accompanied 

 by adequate qualifications; it should never be assumed by the 

 reader to be synonymous with the receptor potential, unless the 



