The Depolarizing Nature of the Trigger 25 



particularly pertinent alternative possibilities could be sug- 

 gested concerning the origin of the observed graded changes 

 in potential: (i) since the receptor muscle was included between 

 the active and indifferent electrodes, it could be argued that 

 intrinsic potential changes generated by the muscle fibers them- 

 selves accounted for the recorded activity; and (2) the sustained 

 potential variations observed during stretch of the receptor, with 

 the accompanying impulse trains, could represent an electrical 

 summation of residual after-potentials from the individual im- 

 pulses. Experimental evidence produced by Katz quite clearly 

 militates against both of these possibilities. Short-circuiting the 

 nerve, by laying it on the muscle itself, consistently abolished 

 stretch-induced potential changes. Crushing the sensory axon 

 at its point of entry into the muscle also had a similar effect. 

 Neither of these procedures would have interfered with electro- 

 tonically-recorded activity originating in the muscle tissue itself; 

 but they certainly would be expected to prevent the detection of 

 current flow from a source of potential in the nerve-ending. Two 

 other arguments are relevant to the second objection. Firstly, 

 most observations showed a negative shift in the potential baseline 

 which occurred before the generation of the first action potential, 

 thus eliminating the possibility that it was some form of after- 

 potential. In the second place, it was shown that the generator 

 potential remained virtually unchanged in both amplitude and 

 duration, after all impulse activity had been suppressed by the 

 addition of procaine to the bathing solution. The remarkable 

 records shown in figure 8 thus allowed direct observations to be 

 made on the relationship between the strength of the stimulus, 

 its time-course, and rate of application to the amplitude and 

 duration of the graded generator potential. In all cases, these 

 observations were from the superimposed electrical interference 

 of an impulse discharge. Some of these findings, in particular the 

 quantitative relationship between stimulus intensity and receptor 

 potential amplitude, will be discussed in Chapter 3. Perhaps a 

 more important observation (utilizing the diflFerential effect of the 

 procaine) was the demonstration of the separate physiological 

 nature of the membrane regions which generated, respectively, the 

 action and the graded receptor potentials.'^ Other major differ- 

 ences between these two types of membrane response may be 

 S.O.— C 



