ECCLES: ELECTRICAL THEORIES OF TRANSMISSION 435 



(also FIGURE 3c, for distal arm of double salt bridge) shows that this 

 occurs under the converse conditions to those above, and that effects 



C2.^-~ ~ N 



e^^?^^ 



Figure 4a. Converse diagram to figure 3a, showing current flow at zone of separation of two 

 previously contiguous fibers, fi) Impulse approaching bifurcation gives Ai effect; (ii) impulse at 

 bifurcation gives C2 effect. 



(i) 



(ii) 



t'^l ""^' ^>i^ ^^^, 





r^W^ 







3E 



&K 



Figure 4b. (i) Two impulses approaching a collision give Ai effect on resting fiber; and (ii) 

 at collision of impulses, 02 effect on resting fiber. 



Ci and A2 are prevented, the action being due to effects Ai and C2.* 

 At synaptic regions, a similar electrical action would be exerted by the 

 pre-synaptic impulse on the post-synaptic membrane (cf. figure 5), 

 since the surrounding conducting medium provides a pathway for cur- 

 rents generated by the approaching impulse. There is, of course, no 

 departing impulse {boutons de passage excepted). In this connection, 

 Arvanitaki's results^ are of especial interest, for it was only in ephaptic 

 situations giving A1C2 effects that she observed appreciable local re- 

 sponses of the resting fiber. She concluded that, in all other condi- 

 tions, the terminal A, effect suppressed any active response of the rest- 

 ing fiber. 



The approximate time-course of the penetrating current at a synapse 

 may be derived from the monophasic spike potential at the pre-synaptic 

 terminal, by considering the flow of current, much as Katz and Schmitt*^ 

 did for two parallel fibers. The longitudinal current flowing in the 

 external circuit is proportional to the first derivative of the monophasic 

 potential, dP/dt. Immediately proximal to the end of the pre-synaptic 

 fiber, all the longitudinal current is provided by the current penetrating 

 the terminal end. Hence, this penetrating current is also proportional to 

 dP/dt,t and not to d'P/dt^ as occurs along the length of the fiber. A 



* Cf. Arvanitakl, A.^ Figure 4 IT. 



t Cf. Marrazzl, A. S., & XI. Iiorente de JSf6^: 89. 



