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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 1 



STIMULUS 

 Subthreshold Thretshold 



FIG. 31. Ephaptic excitation of squid giant axon. Two nerves 

 are arranged as siiown in diagram. I. Contact between nerves 

 in sea water. A weak stimulus (/(/O evokes a local response of 

 the pre-ephaptic fiber (seen in trace .4). This is not propagated 

 to the ephapse and has no effect on the latter (trace B). When 

 a pre-ephaptic spike was evoked by a stronger stimulus (.4'), 

 the post-ephaptic nerve generated a locaJ response (B'). .^head 

 of it is seen the electrotonic pick-up of the pre-ephaptic spike. 

 II. Excitability of the axons was increased by removing calcium 

 ions from the medium. The weak stimulus still could not evoke 

 activity in the post-ephaptic fiber (B), since conductile activity 

 was lacking in the pre-ephaptic unit QA). When a stronger 

 stimulus evoked a spike (.4') the postephaptic fiber also pro- 

 duced a spike (B')- This arose on a step which is the local 

 response of the postephaptic fiber (seen in isolation on the 

 lowest trace). [From Arvanitaki (10).] 



be absent if ionic processes leading to hyperpolariz- 

 ing and depolarizing p.s.p.'s were equally balanced. 

 Despite this, however, the transducer actions ini- 

 tiated by the excitants of depolarizing and hyper- 

 polarizing synaptic membrane would still take place, 

 and the ionic transports of the transducer action 

 would still occur. Thus, ionic, metabolic and other 

 biochemical effects might be produced in the ap- 

 parent absence of electrical activity (96, 97). 



EPH.APTIC EXCITATION 



Electrical Modes of Transmission 



In the course of efforts to validate the theory 

 of electrical transmission many attempts were made 



to confirm Kiihne's dictum that "a nerve only throws 

 a mu.scle into contraction by means of its currents 

 of action." In 1882, Hering (i 10) found that a nerve 

 \-olley initiated in one distal branch of the frog 

 sciatic nerve and coursing centrally in the whole 

 nerve trunk could set up activity in another branch 

 when the impulses arrived at the centrally transected 

 stump of the nerve. The current flow generated by 

 the active fibers must have stimulated the previously 

 inactive fibers. The effect has been confirmed many 

 times (cf. 149) but nowhere more clearly than in a 

 preparation involving two squid giant axons (lo). 

 It must be emphasized that specially favorable ex- 

 perimental conditions are required to produce this 

 ■ model' of transmission which is termed an ' ephap.se' 

 (false synapse). In the squid giant fiber (fig. 31) the 

 electrical excitability of the ephaptic region is 

 heightened by depriving the medium of calcium. 

 The extrinsic current of the spike in the pre-ephaptic 

 terminal is then capable of acting as a sufficiently 

 strong electrical stimulus to evoke a postephaptic 

 spike. As a weaker stimulus, it can elicit a graded 

 local response. In more complex geometrical con- 

 ditions between active and inactive cells, the direc- 

 tions of the extrinsic or field currents may produce 

 hyperpolarizations as well as depolarizations (figs. 

 I, 32). The activity travelling in one fiber generates 

 extrinsic current fields in contiguous parallel fibers 

 which have a triphasic sequence (126) that suc- 

 cessively produces hyperpolarization and depressed 

 excitability, then depolarization and heightened 

 excitability, followed again by hyperpolarization 

 and depression (fig. 32). 



A weakness of ephaptic transmission as a model 

 of synaptic activity lies in the fact that basically it 

 does not offer a mechanism for polarized transmis- 

 sion. Thus, in figure 31 the ephaptic excitation might 

 very well have taken the opposite direction, from 

 nerve B to nerve A. Special geometric properties 

 were invoked by du Bois-Reymond and by Eccles 

 (figs. I, 32), and tlie latter also introduced the special 

 electrophysiological rectifying effects of anodal and 

 cathodal currents (fig. 32). These conditions might 

 account for polarized transmission with an electrical 

 mechanism; and, as will be described below, a high 

 degree of rectification recently discovered in one kind 

 of junction (83) does polarize conduction. However, 

 the crucial distinction is whether or not current 

 flow in a presynaptic terminal, or current flow im- 

 posed through the synaptic junction, can excite the 

 activity of the latter. The an.swer, illustrated in this 

 chapter with a number of examples (e.g. figs. 6, 19), 



