HARRY GRUNDFEST 13! 



Lecithinases inactivate and depolarize axons and muscle fibers (179). Some 

 breakdown products of lecithin act likewise. Other enzymes (e.g. collagenase, 

 chymotrypsin, etc.) have no effect, but Tobias (179) is properly cautious with 

 respect to the implications of his findings. As with the chemical events, the 

 significance of various optical and mechanical changes during activity (e.g. 24) 

 is not yet known. 



Synaptically Excilable Membrane 



a) Synaptic Electrogenesis. A specific response appears to be localized to the 

 regions of the post-junctional membrane immediately accessible to the pre- 

 synaptic stimulus (89). It is designated as the postsynaptic potential (p.s.p.). 

 In most, but not all post-junctional cells, p.s.p. lasts considerably longer than 

 does the spike. It may be in the direction of decreasing the resting potential 

 and therefore excitatory, or oppositely directed and inhibitory. Analysis of 

 several cases has indicated that the mechanism of electrogenesis is different 

 from that causing the spike, and therefore it must involve different types of 

 membrane molecular structures. These differences are also reflected in different 

 pharmacological properties, and particularly in the nature of their initiating 

 stimulus. The preponderance of present evidence supports the view that 

 excitation of synaptic activity is initiated by specialized transmitter agents, 

 released from the presynaptic terminals in the course of their electrical response. 

 The transmitter, and the specialized postsynaptic membrane 'receptor' there- 

 fore constitute the triggers of postsynaptic electrogenic activity. 



Probably only a few workers still adhere fully to the theory that synaptic 

 transmission is mediated by eddy currents of the impulse in the pre-junctional 

 cell flowing through the post-junctional (150). A number of experimenters 

 have found by direct measurement that the membrane depolarization generated 

 in a cell by activity of its presynaptic innervation or that of neighboring cells is 

 too small to initiate excitation (20, 29, 122-124, i77)- Others accept neuro- 

 humoral synaptic transmission for some junctions, but consider that in certain 

 synapses, and particularly in those of the vertebrate central nervous system, 

 transmission is electrical (18, 19). Most investigators, whether adhering to 

 strictly neurohumoral (cf. 63, 71, 78) or to the dualistic view (cf. 19, 81) never- 

 theless appear to accept the possibility that the postsynaptic membrane can be 

 excited by electrical means. Considerable evidence has now accumulated to 

 indicate that this is not the case, and that the postsynaptic membrane probably 

 in all post-junctional cells is one which cannot be excited by electrical stimuli, 

 but is responsive only to specific chemical excitants. 



b) Electroplaques Other Than Those of Eel. At the end of the last century it 

 was found that the electroplaques of Malapterurus (90) and Torpedo (86, 87) 

 cannot be excited directly, but only through their nerves (cf. also 80). The de- 

 nervated electric organ, or the curarized, becomes irresponsive. Their response 

 to neural stimuli is probably like that of the electrically inexcitable Raia 



