SYNAPTIC AND EPHAPTIC TRANSMISSION 



191 



120- 



110- S 



100 



90- 



80- 



Action potential in 

 fibre I 



Excitability chonge in 

 fibre IE 



>A,X»-TC 



Ct1"<AAt 



i':i.|:>iaJ-t 



msec. * — * — 



FIG. 32. Excitability changes caused by field currents. I'pper lejt: A spike was produced by a stim- 

 ulus to one of a pair of crab nerve fibers as in diagram upper right. The electrical excitability of the 

 second fiber is shown (lower left) in relation to the time at which the spike passed the testing region. 

 In the interval before the spike had reached that site, the excitability of the fiber was depressed. 

 During the time that activity resided at the tested level, the excitability was augmented. This was 

 followed by a second depressed phase as the activity propagated out of the tested site. [From Katz & 

 Schmitt (126).] Right: Diagrams of the anodal, cathodal and anodal polarizing sequence generated 

 in the inactive fiber by the spike in an adjoining fiber ilop') and of different field current conditions 

 produced by different geometrical arrangements (bottom). [From Eccles (57).] 



seems to be clear. The current flowing across an 

 active presynaptic terminal and across the post- 

 synaptic membrane appears to be far too small to 

 excite the postsynaptic cell. Furthermore, the proc- 

 esses associated with synaptic activity cannot be 

 initiated by very strong applied currents. 



Role of Field Currents in Central Nervous System 



The activity of masses of cells or fibers in the 

 central nervous system is particularly conducive to 

 development of field currents within the volume of 

 this structure (15). This fact suggested (88, 90) that 

 field effects might play a role in determining the 

 peculiarities of central nervous properties. The 

 hypothesis appeared to have been confirmed by anti- 

 dromic stimulations of motoneurons which altered 

 the responses of contiguous motoneurons to a testing 

 afferent volley (170). That conclusion, however, is 

 invalidated by the subsequent finding (171; cf. 60) 

 that the antidromic stimuli evoked synaptic activity 

 within the spinal cord by means of the recurrent 

 collaterals of the motoneurons. 



Although field currents undoubtedly play some 

 role (cf. 106), their wide significance must now be 



questioned in the light of the evidence that synaptic 

 transmission is not effected by electrical stimuli. 

 Changes in membrane potential produced in one 

 cell by activity of contiguous elements appear to be 

 small (33, 59, 125), although effects may be revealed 

 by tests on electrically excitable membrane (106, 

 126, 185). However, the effects exerted electro- 

 chemically on p.s.p.'s (as described in the section in 

 this chapter on the nature of postsynaptic potentials) 

 are probably insignificant. Thus, electrical ine.x- 

 citability renders the transmissional process insensi- 

 tive to fields of current in the central nervous system 

 (93). Teleologically considered, this is probably an 

 advantage. The fields must shift from moment to 

 moment as the loci of activity shift in the cellular 

 mass of the volume conductor. The effects of these 

 fields must therefore be highly unspecific, now pro- 

 ducing increase, now depression of electrical ex- 

 citability, actions that probably would disturb the 

 precision of organized orderly synaptic transfer. 

 Thus, electrical inexcitability of synaptic membrane 

 removes a major hazard, that irregular effects of 

 electric fields might disrupt the patterned activity 

 of the central nervous system. 



