446 ANNALS NEW YORK ACADEMY OF SCIENCES 



D. The Brief Period of Low Resistance 



This is in addition to that attributable to catelectrotonus, during the 

 initial, "active" phase of the end-plate potential.'*^ The postulated 

 local response of the end-plate region of the muscle would produce 

 just such an additional fall of resistance, running the same time-course 

 as the active phase.* 



E. Slow Catelectrotonic Potentials in the Pre-Synaptic Fibers^* ^^ 



Such potentials have only been observed with synapses in the central 

 nervous system (the dorsal root potentials). The present hypothesis 

 has been extended to explain the production of these potentials, by 

 making the additional assumption that the terminal region of the pre- 

 synaptic fiber resembles the post-synaptic area, in being specialized to 

 give local responses. The catelectrotonic focus provided by the local 

 response of the post-synaptic membrane sets up the current, which ex- 

 cites the pre-synaptic terminal to give a local response, which, in turn, 

 acts as a focus, setting up the spreading catelectrotonus of the dorsal 

 root potential. ^° 



7. DIFFICULTIES OF THE HYPOTHESIS 



The electrical hypothesis encounters difficulties in explaining the fol- 

 lowing experimental observations, but possible lines of reconciliation 

 are suggested. 



A. Synaptic Block Produced by Curarine in Skeletal Muscle 



and Ganglia 



Curarine acts as a specific depressant of the excitatory responses 

 evoked in motor end-plates and ganglion cells by acetylcholine 

 and related substances.^' ''' ^^' "^' ^^' ^* Thus, the acetylcholine hypothe- 

 sis provides an obvious explanation of the synaptic blockage produced 

 by curarine. Now curarine causes such blockage by depressing the 

 synaptic potential. ^^' ^^' ^^' ^^ Hence, according to the electrical hy- 

 pothesis, the simplest explanation of the blockage would be, that there 

 is depression of the local response set up by the cathodal focus (cf. 

 FIGURES 6d and 7c) ; i.e., that curarine depresses the electrical excita- 

 bility of the post-synaptic membrane, as well as its acetylcholine ex- 

 citability. In a recent attempt to test this, by electrical stimulation 

 of the motor end-plate in the isolated nerve-muscle fiber preparation, 

 before and after curarization, the initiation of propagated muscle im- 



♦ Cf. Katz, B." FiGtniB 11. 



