SYNAPTIC AND EPHAPTIC TRANSMISSION 



'93 



Vtt 



MV 



hypcrpolorizotion 



8 



5H 



'9^ 



^'post 



i^ 



hypcrpolarlzotion 



depolarizotion 



-5 



'pre 



•-lO 

 mV 



FIG. 33. Rectification at the junction be- 

 tween a cord giant fiber and a motor giant 

 axon in crayfish results in polarized ephaptic 

 transmission. A: Current was allowed to flow 

 through a microelectrode in the prejunctional 

 cord giant axon. The changes in the membrane 

 of the same fiber were recorded with another 

 microelectrode and are shown on the abscissa. 

 The ordinate indicates the membrane voltage 

 recorded at the same time with a micro- 

 electrode in the postjunctional fiber. When 

 the prefiber was depolarized, a steeply rising 

 depolarizing change also took place in the 

 postfiber. As an extrinsic local circuit change 

 was produced by a spike in the prefiber, it 

 would lead to an electrically excited response 

 of the postfiber. When the prefiber is hyper- 

 polarized (left side of .-1), only small changes 

 in potential develop in the postfiber. The ratio 

 of current flowing in the two directions is about 

 20:1. B: In this experiment current was 

 applied to the postfiber, the abscissa shows the 

 change in membrane potential of this fiber 

 and the ordinate the change in membrane 

 potential of the prefiber. When the postfiber is 

 hyperpolarized, there is a considerable current 

 How into it from the prefiber, causing some 

 clectrotonic hypcrpolarization of the latter. 

 When the postfiber is depolarized, little 

 current flows into the prefiber and it there- 

 fore cannot be stimulated by a spike in the 

 postfiber. Electrical excitation across the 

 junction is thus transmitted only from the 

 pre- to the postfiber. [From Furshpan & 

 Potter (83).] 



figure 33 were done on the squid giant axon synapse 

 by Tasaki. He "could not detect any recognizable 

 spread of clectrotonic effects across the synapse in 

 either direction" (personal communication). It is 

 likely that pharmacological data and various other 

 criteria of the constellations listed in table i will 

 distinguish the two types of transmission systems 

 further. 



Several properties of the polarized ephaptic junc- 

 tion may be deduced from the available data and 

 from general considerations. Rectification is ex- 

 hibited by the membranes of many, though not all, 

 cells, although not to the same large degree (cf. 

 Tasaki, Chapter III). Where found, it is manifested 

 by a higher membrane resistance to inward current 

 than to outward flow. In the present case two mem- 

 branes are involved and, if both are rectifiers, then 

 they must each act in opposite polarity to the other. 

 On the other hand, only one of the two membranes 

 need show rectification and this situation is the more 



probable. It also seems most likely that this property 

 resides in the surface of the prefiber for in that case 

 the membrane would permit outward current flow 

 and restrict inward as in other cells. The membrane 

 of the postfiber would then need have no rectifier 

 properties but would resemble that of the septa in 

 its low nondirectional resistivity. 



As may be seen from figure 34, current probably 

 flows outward across the prejunctional membrane 

 during the ephaptic transmissional process whereas 

 in the rest of the active region the membrane current 

 is inward. Furthermore, excitation of the postfiber 

 must occur at membrane sites where the local circuit 

 current flows outward, not at the ephaptic region 

 where it flows inward. Therefore, neither junctional 

 membrane of this polarized ephapse takes part in 

 the active responses of the junction. Like the mem- 

 branes at the septa they therefore need not be ex- 

 citable. 



