192 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



Dorsal Root Reflex 



Acting upon electrically excitable components, 

 however, field currents might still affect central 

 nervous functioning. For example, small depolariza- 

 tion of a cell by field current might facilitate its 

 discharge by an otherwise subliminal depolarizing 

 p.s.p. Likewise, presynaptic terminals close to an 

 active synaptic focus might be subjected to con- 

 siderable potential change (51), an action which 

 may account for the dorsal root reflex (188). This 

 prolonged centrifugal discharge of dorsal root fibers 

 is evoked with a latency of some milliseconds after 

 the same or other dorsal root fibers cany a volley 

 centripetally. The dorsal root reflex is enchanced 

 by low temperatures as is the motor root reflex (29, 

 89). The latency, temperature effect and prolonged 

 discharge of the dorsal root reflex indicate that it is 

 produced by a synaptic activity in the spinal cord, 

 and yet there is no histological evidence of synaptic 

 inflows to the dorsal root collaterals. In the absence 

 of the latter, ephaptic excitation may be invoked, 

 but will involve synaptic pathways also. This could 

 result from the field effects generated in the dorsal 

 root terminals by the activity of some interneuronal 

 pools. The activity of these cells, being evoked by 

 synaptic transfer, would account for the apparent 

 synaptic properties of the dorsal root reflex, but its 

 final development would be by ephaptic excitation. 



Ephaptic Transmission in Annelid and 

 Crustacean Nerve Cords 



In many species of these invertebrates there occur 

 junctions (septa) between anatomically distinci 

 elements, the segments of the septate giant axons. 

 Across the septa considerable electrotonic current 

 flow can take place and ephaptic electrical transmis- 

 sion is then possible (125; Kao, C. Y. & H. Grund- 

 fest, manuscript in preparation). The junctional 

 membranes of these functional ephapses must there- 

 fore be fundamentally different from those of 

 synapses, across which only insignificant electrotonic 

 current flow occurs. However, the anatomical data 

 to account for this difference are still unsatisfactory. 

 The transverse sheaths which separate abutting seg- 

 ments of the septate giant axons appear to be identi- 

 cal with the sheaths that invest the axis cylinders 

 (cf. 1 25). On the other hand, the junction between 

 the medial giant axon and the motor giant fiber 

 of crayfish seems to be formed by processes from the 

 postjunctional motor nerve which penetrate the 

 Schwann sheaths to make intimate contact with the 



cell membrane of the prejunctional fibers Ci74)- The 

 junctions between two motor giant axons are also 



similar. 



a) unpol.\rized EPH.'SiPTic JUNCTIONS Thcsc havc 

 been studied with intracellular recordings in the 

 .septate giant axons of earthworm (125) and cray- 

 fish (Kao, C. Y. & H. Grundfest, manuscript in 

 preparation). The septa, sometimes called 'un- 

 polarized macros\napses' (cf. 30, 125), appear to be 

 merely the boundaries demarcating the multiple 

 origins of the .septate giant axons from a number of 

 segments of the animal. Activity in one segment of 

 the axon causes electrotonic potentials in the neigh- 

 boring segments large enough to excite the latter. 

 Thus, transmission is by local circuit excitation, es- 

 sentially as in other axons. As in the latter, the 

 ephaptic transmission of the septate axons is un- 

 polarized, capable of propagating an impulse in 

 either direction. 



b) pol.-^rized eph.-vptig transmission. One system 

 recently described (83), the junction between cord 

 giant fibers and efferent motor giant axons of cray- 

 fish, may be classified in this category. Current flow- 

 ing outward from the depolarized prefiber can enter 

 the junctional membrane of the postfiber, causing 

 large depolarization in the latter (fig. 33.-1) and its 

 ephaptic excitation. However, when the postfiber is 

 depolarized (fig. 335) the electrotonic effects in the 

 prefiber are small. Likewise when the prefiber is 

 hyperpolarized current flow in the postfiber is hind- 

 ered (fig. 33.4), while hyperpolarizing the postfiber 

 causes large electrotonic changes in the prefiber 

 (fig. 33^). The junctions thus exhibit rectification, 

 with conductance in one direction (that tending to 

 depolarize the postfiber) about 20 times greater than 

 in the opposite direction. Thus, in the case of the 

 motor giant fiber ephapse, the low electrical resist- 

 ance in one direction and high resistance in the other 

 makes for polarized ephaptic transmis.sion. 



Since the junction meets the criteria of anatomical 

 discontinuity and transmissional polarization, it 

 fits the definition of synapse extant since Ramon y 

 Cajal and Sherrington. However, though it may be 

 called an 'electrically excitable synapse' (83), it 

 probably differs profoundly from the electrically 

 inexcitable synapses discussed in this chapter. The 

 distinction between ephaptic junctions which have 

 low electrical resistance and synapses which have 

 high resistance helps to make the classification more 

 precise. Thus, experiments similar to those shown in 



