SYNAPTIC AND EPHAPTIC TRANSMISSION 1 87 



FIG. 30. Heterosynaptic facilitatory actions in eel electroplaques from different electric organs. 

 Left: Absence of heterosynaptic facilitation in cells from the Sachs organ. .Activity of one nerve 

 evoked the response seen in A. This response -was preceded by that evoked through another nerve in 

 B to G. Only when the two stimuli (shock artifacts on the left of the records) were less than i msec, 

 apart (_F, G) was there a significant amount of facilitation caused by electrotonic summation of the 

 brief p.s.p. [From .-Mtamirano et al. (5).] Right: Heterosynaptic facilitation in cells of the main organ. 

 A: The response to stimulating one nerve trunk. B: This nerve trunk is used to deliver the con- 

 ditioning stimulus (artifact at the left, upward); the testing stimulus is applied at various intervals 

 later to another nerve trunk (artifact down, superimposed traces). Marked facilitation reached a 

 peak at 10 to 15 msec, and persisted through the end of the record at 25 msec. C: Nerve 2 was cut, 

 and a third nerve trunk was used for the testing stimuli. No facilitation occurred. [From Albe-Fes- 

 sard & Chagas (i).] 



inhibition or facilitation may develop particularly 

 in the more complex varieties of synaptic organiza- 

 tion. The precise effects would depend on the specific 

 pathways and electrical responses involved and can- 

 not be discussed in this chapter (cf. 99-101, 161). 



186). Thus, they can provide sites at which synaptic 

 potentials of both signs may be generated and this 

 electrical summation propagated electrotonically to 

 act upon an electrically excitable membrane distal 

 to the cell body. 



Integrative Utility of Electrical Inexcitability 



The foregoing group of integrative activities de- 

 pends essentially upon graded, algebraically sum- 

 mative potentials of opposite signs which are made 

 available in synaptic transmission by electrical in- 

 excitability. In some neurons, relatively large scale 

 areas of membrane are not electrically excitable and 

 this would appear to aid integrative functions. The 

 superficial cortical dendritic surfaces, richly supplied 

 with synaptic inflows, are an example of this. The 

 synaptic activity that goes on at these dendrites re- 

 sult in algebraically summated potentials. Since 

 these dendrites are not electrically excitable, the po- 

 tentials must be transmitted electrotonically to the 

 electrically excitable membrane of the pyramidal 

 neurons. In each of these the potential can serve to 

 modulate responsiveness to other, more potent 

 synaptic inflows. The soma of lobster cardiac gan- 

 glion cells also are not electrically excitable (33, 109, 



Synaptic Determinants of Different Types of Reflexes 



In the general context of principles, the precise 

 structural and functional complexity of a reflex 

 pathway is of little moment. Therefore, the specific 

 properties of monosynaptic or multisynaptic reflexes 

 need not be dwelt upon since they are finally refer- 

 able to the intensity of synaptic drives upon the final 

 common path. The analysis of synaptic mechanisms 

 in many varieties of reflex response can likewise be 

 simplified by merging all interneuronal activities with 

 that of the final common path, essentially involving 

 a reduction to the monosynaptic case. 



Synaptic organizations involving very strong 

 synaptic drive for depolarizing p.s.p.'s will manifest 

 themselves by large synchronized efferent electrical 

 activity or a twitch-like contraction in response to a 

 single afferent volley. The amplitude of the response 

 will depend upon the proportion of neurons that lie 

 in the discharged zone. The lower the proportion of 



