194 



HANDBOOK OF PH'lSKJLOGV 



NEUROPHYSIOLOGV I 



Intrinsic 



\ 



Local Circuits 

 -> 1 I ^ 



Transjunctional 



Ephaptic Junction 



FIG. 34. Diagiam showing the current flows that probably 

 take place at a polarized ephaptic junction. In the prejunc- 

 tional fiber membrane current flow is inward in the region of 

 activity. Longitudinal current flow takes place behind this 

 region as part of the intrinsic local circuit within this fiber. 

 Current flows outward through the membrane recovering from 

 previous activity. Outward current also flows in the prejunc- 

 tional membrane of the ephapse. This enters the postcphaptic 

 cell at its junctional membrane and flows out through adjacent 

 regions of membrane, exciting the latter. Note the profound 

 difference between the current flows postulated for ephaptic 

 transmission in this diagram and the hypothetical situation al 

 synaptic junctions shown in fig. i . 



Evolutiormry Aspects of Ephaplii Transmissum 



In their transverse divisions the septate axons bear 

 the sign of their segmental origin. The processes of a 

 number of neurons at a nerve cord segment fuse to 

 produce a short length of giant axon. End-to-end 

 apposition of the segmental fibers then forms a long 

 axonal pathway. To the extent that the septa dis- 

 appear or that their resistance is low the segmented 

 axons approach the nonsegmented giant axons in 

 efliciency as through conduction pathways, excited 

 by local circuit action. 



The septate axons, however, combine with through 

 conduction, another feature which is absent in the 

 nonsegmented giant fibers (Kao, C. Y. & H. Grund- 

 fest, manuscript in preparation). They make elaborate 

 local synaptic connections, both efferent and afferent, 

 with other fibers of the nerve cord. Although the 

 anatomy of these connections is not as yet clear, the 



synaptic properties of the septate axons probably 

 derive from their segmental origin of the fibers. The 

 septate giant axons therefore play a much greater 

 role in the integrative activity of the nervous system 

 than can the nonseptate axon which lack these synap- 

 tic connections (103, 125). 



On the basis of the interpretation given in the pre- 

 vious paragraphs, the polarized, electrically excitable 

 ephaptic junction may be derived from the septal 

 junctions by the addition of rectifier property to 

 one of the junctional membranes. Two other features 

 further strengthen the resemblance between septate 

 and motor giant fibers. The two motor axons of a 

 segment make unpolarized connections with each 

 other. In this case, too, electron microscopy has not 

 as yet revealed essential details (cf. i 74). Also, like 

 the septate axons, the motor giant fiber combines 

 'chemically mediated synapses' with an ephaptic 

 junction (83). The former presumably are electrically 

 inexcitable. 



Thus, it appears likely that motor giant fibers of 

 the crayfish bear a close functional similarity to the 

 septate axons but with a significant modification away 

 from the latter. It remains to be seen whether ephap- 

 tic polarized transmission made possible by rectifica- 

 tion is a fairly common evolutionary variant. Another 

 interesting correlation, whether or not this transmis- 

 sion scheme developed only in those animals that 

 have septate unpolarized ephapses, might give fur- 

 ther clues to their evolutionary origin. 



Qjiasiartificial Synapses 



The excitation of giant nerve fillers in annelid 

 nerve cords by activity in other giant axons is well 

 documented (31) and may be an ephaptic phe- 

 nomenon In Protida the sites of transfer vary from one 

 occasion to another and have been termed quasi- 

 artificial synapses. These systems have not yet been 

 studied with intracellular recording. The latter could 

 help to determine whether the transmission is ephap- 

 tic or whether it is associated with complex synaptic 

 phenomena such as have been found in earthworms 

 (125)- 



REFERENCES 



1. Albe-Fessard, D. and C. Chagas. J. physiol., Paris 46: 



823. '954 



2. Albert, A. Ergehn. Physiol. 49: 425, 1957. 



3. Altamirano, M. .and C. VV. Coates. J. Cell. & Comp. 

 Physiol. 49: 69, 1957. 



4. Altamirano, M., C. W. Coates and H. Grundfest. 

 J. Gen. Physiol. 38: 319, 1955. 



5. Altamirano, M., C. W. Coates, H. Grundfest and D. 

 Nachmansohn. J. Gen. Physiol. 37: 91, 1953. 



6. Altamirano, M., C. W. Coates, H. Grundfest and D. 

 Nachmansohn. Biochim. el biophys. acta 16: 449, 1955. 



