38 The Physiology of Sense Organs 



where different regions of a single nerve cell are concerned with, 

 respectively, the absorption of stimulus energy, the generation of 

 graded slow potential changes, and the initiation of conducted 

 impulses; (2) non-neural sensory cells exhibiting graded activity, 

 and making functional contact with true second-order neurons 

 which initiate conducted impulses. In receptor systems of the 

 latter type there are at least two distinct possible locations for 

 graded electrogenesis — the sensory cell itself, and the region of 

 functional contact between the primary and secondary cells. 



Fig. 15. Extracellular electrical record from a frog 

 stretch receptor. Upper trace indicates duration of 

 applied stretch. Lower trace records the response, 

 which consists of dynamic and steady depolarizing 

 phases (downward deflection) and a brief hyper- 

 polarizing phase following removal of the stimulus. 

 (From Katz,^® Fig. i.) 



Unless the precise location of an extracellular electrode is known 

 with respect to these two loci, it is difficult to justify positive 

 identification of graded changes in potential due to stimulus 

 application as reflecting particularly either the receptor potential 

 itself or a secondary synaptic potential arising at the junction 

 between the two cells. Without specifying which of these two 

 sources or which combination thereof was involved, Bernhard, 

 Granit and Skogland^^ used the term ' generator potential ' to 

 designate the graded depolarization responsible for impulse 

 triggering in the visual system of Dytiscus. After its introduction, 

 at which time this term was clearly defined in an operant manner, 

 it appears to have been widely used to designate generally all 

 graded potential changes in receptor systems, often having very 

 little similarity with respect to functional morphology and point 

 of impulse origin. In a review concerning sensory receptor 



