ECCLES: ELECTRICAL THEORIES OF TRANSMISSION 



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C. Reactions of Ephapses (Artificial Synapses) 



The double axon preparations^' ■**' *^ are particularly relevant to 

 electrical action across synapses. Since there has been excellent corre- 

 lation between the effects predicted by the "local current" theory of 

 nerve conduction and the effects observed, it may be concluded that 

 these effects are caused by electrical current flow across the ephapse. 

 48, 49, 64 According to the geometry of the ephaptic contact, three main 

 types of effect are exerted on the resting fiber by an impulse in the ac- 

 tive fiber :^ 



(i) At regions where fibers are contiguous for some distance on 

 either side — for maximum effect, at least half a wave length. Here 

 the currents generated by the impulse have, in turn, anodal, cathodal, 

 and anodal action on the resting fiber.*^* ''^ Figure 2 shows that the 



Figure 2. Diagram of two contiguous fibers, showing the current flow generated by impulse in 

 active lower fiber and its penetration of the resting fiber (cf. Katz & Schmitt, figure 2''*). As the 

 impulse (shown above) propagates along the active fiber, any point on the resting fiber is sub- 

 jected, in turn, to effects A1C1C2A2. Active part of impulse shown by hatched area in this and 

 subsequent figures. 



cathodal phase is really double, being due to currents generated by the 

 membrane battery of the active fiber, at first ahead (Ci), and then in 

 the wake, of the impulse (Ca). Thus, the sequence of action is 

 A1C1C2A2, as the impulse sweeps past a point on the resting fiber. As 

 Katz and Schmitt pointed out, the current penetrating and acting on 

 the resting fiber is virtually a mirror image of the penetrating current 

 of the active fiber, and hence, has an intensity-time course correspond- 

 ing to the second derivative of the monophasic action potential, with 

 respect to the time coordinate (d^P/dt^). Since the curvature of the 

 wave front of the impulse is at least twice as sharp as that of its wake, 

 Ai and Ci will be at least twice as large as C2 and A2, 

 will be correspondingly longer in duration.* 



• Cf. Katz, B., ft O. H. Sdunltt.*^ Ftgijees 5Aji and 6, Curve 1. 



