144 



PHYSIOLOGICAL TRIGGERS 



at least part of this time is occupied by release and diffusion of transmitter 

 from the nerve terminals to the post-junctional membrane. 



1) Other Interrelations of Pre- and Post-Junctional Units. The foregoing 

 discussion has focussed on the nature of the electrogenic triggering of junc- 

 tional transmission. Pre- and post-units cooperate in other ways. Profound 

 chemical and morphological changes occur in many, if not all, post-junctional 



'A 



7^ 



Fig. 9. Mode of transfer of excitation from the synaptic to the electrically excitable mem- 

 brane, i) A graded neural stimulus produces increasing depolarization via increases of p.s.p. 

 (A-C). This initiates a local response and spike in the eel electroplaque (D, E). 2) Another 

 electroplaque in which a maximal neural volley did not discharge the cell (//). The depolariza- 

 tion was not purely a p.s.p., however, as seen by applying the neural volley at different times 

 during recovery from a conditioning direct stimulus. (.4, B): The neural stimulus produces 

 a small distortion of the falling phase of the spike, when the electrically excitable membrane 

 is absolutely refractory. Early during relative refractoriness it generates a local response (C) 

 which grows and arises earlier {D, E) to fuse with the p.s.p. (F, G). Calibration 10 mv and 1 

 msec, above, and 100 mv, i msec, below (from ref. 4). 



cells when they are deprived of the jjre-junctional supply. The reactions of 

 denervated muscle (cf. 154) and the transneuronal degenerative changes in 

 nerve cells are well established. Complex metabolic and pharmacological 

 changes have been demonstrated in denervated autonomic ganglia (e.g. 157, 

 158). These phenomena indicate that electrogenic transducer activity triggered 

 by presynaptic action is probably not the only function of the specialized post- 



