132 PHYSIOLOGICAL TRIGGERS 



clavala (22). The electric organ of Torpedo responds elect rogenically to acetyl- 

 choline (77). 



Twitch muscle fibers. Electrical inexcitability of the end-plate of frog skeletal 

 muscle was inferred by Kufifler (131) and by Fatt and Katz (72) and demon- 

 strated by del Castillo and Katz (37). The neurally evoked end-plate potential 

 (e.p.p.) may develop concurrently with a directly excited spike of the fiber, 

 and the two sum in a manner which indicates that they occur independently in 

 different kinds of fiber membrane, are based on different electrogenic mecha- 

 nisms, and that the e.p.p. is not itself generated by the electrical stimulus of the 

 spike (fig. 5). The exterior of the end-plate membrane responds electrogenically 

 to a jet of less than lo"''^ m acetylcholine (38, 151, 152), but its interior face is 

 not responsive to injections of large quantities (38).^ The resistance of the 

 depolarized and reversely polarized, physiologically non-responsive membrane 

 nevertheless decreases upon application of an acetylcholine jet (39). Thus, 

 although the electrogenic action is abolished because the depolarized fiber no 

 longer provides the battery needed for its expression, the trigger response of 

 the postsynaptic membrane, increased conductance, still remains. 



Eel electroplaques. The p.s.p. neurally evoked in the eel electroplaque is simi- 

 larly independent of the spike generator (4). The p.s.p. cannot be evoked by elec- 

 trical stimuli, nor inhibited by large hyperpolarizing or depolarizing changes in 

 membrane potential It may be elicited while the cell is not responsive to 

 electrical stimuli, either in absolute refractoriness (fig. 2) or when the cell is 

 depolarized (fig. 4). The two responses therefore exhibit different pharmaco- 

 logical and physiological properties. 



Axono-axonal synapses. The segmented giant axons of the earthworm and 

 crayfish, in which the occurrence of axono-axonal synapses appears likely (26), 

 also respond with electrogenic activity (122 124) characteristic of synapses. 

 Their p.s.p. is small, graded and repetitive, and may be superimposed upon the 

 spike or elicited during the absolutely refractory period of the fiber. The mag- 

 nitude of the p.s.p. varies in different parts of the axon, and therefore it is 

 probably localized at synaptic regions. The p.s.p. is absent in the medial giant 

 axons of the crayfish, which are devoid of axono-axonal synai)ses (187). 



Invertebrate and 'slow' vertebrate muscle fibers. (Iraded, only neurally evoked 

 activity is exhibited in some invertebrate muscle fibers which are not electrically 

 e.xcitable (85, 107, 169, 190). These responses appear to be confined to the 

 multiple myoneural junctions (75). As in the eel electroplaque, very strong 

 hyperpolarization of the muscle fiber does not block the neurally evoked p.s.p. 

 (107). Different nerve fibers innervating the same muscle fiber may initiate 

 different forms of response, and these may vary in different i)arts of the same 



7 Microinjection of high concentration of acetylcholine is tolerated by the squid axon, 

 although that of dilute concentration is not (97, 126). The injection into muscle fillers of 

 ATP (70) or Mg+"*" (186), without efTect, appears to he expected on the basis, at least, of one 

 theory (148). 



