SYNAPTIC AND EPHAPTIC TRANSMISSION 



183 



Physiological Factors Determining 

 Transmissional Effectiveness 



a) SYNAPTIC POTENCY AND DRIVE. Just as different 

 parts of the same cell exhibit variations with respect 

 to electrical threshold so also do different cells in a 

 population, that is, the critical firing level may be 

 lower for one cell than for another. In that case, an 

 afferent volley equally effective in generating p.s.p.'s 

 in all the cells may discharge some of these but not 

 others. It is unlikely, however, that the synaptic 

 potency of a given influx is identical for all cells. 

 Even in single multiply-innervated cells, such as the 

 eel electroplaque (4), the p.s.p.'s generated over the 

 large surface are of different amplitudes and always 

 largest at a definite region of the cell surface. Thus, 

 the p.s.p.'s generated in a population of motoneurons 

 would vary in magnitude depending upon the 

 synaptic potency of the afferents to each cell. This 

 variation, added to that of the distribution of elec- 

 trical thresholds, results in a population spread with 

 respect to excitatory effects or synaptic drives. It is 

 ob\'ious that the degree to which the given synaptic 

 inflow also excites hyperpolarizing p.s.p.'s as well as 

 depolarizing, and the relative spatial distributions of 

 the two electrogenic activities will affect the mag- 

 nitude of the synaptic drive. 



The differences in synaptic drive deduced above 

 adequately account for a mass of experimental data. 

 The cells in a population of neurons impinged upon 

 by a sample from a population of innervating nerve 

 fibers will respond with different degrees of depolariz- 

 ing p.s.p.'s. Some of the cells will discharge spikes 

 which can be recorded directly (e.g. fig. 12) or by 

 means of other effects, as for example by their reflex 

 activation of muscle in the case of motoneurons. In 

 other cells excited by the afferent volley the p.s.p.'s 

 alone are generated. 



b) EXCITED AND DISCHARGED ZONES. Thus, an ex- 

 citatory volley causing quantitatively different 

 amounts of synaptic activity also divides the popula- 

 tion of postjunctional cells qualitatively. One group, 

 frequently by far the smaller, falls in the discharged 

 zone, the other in the excited zone (fig. 26). In this 

 distribution the occurrence and influence of hyper- 

 polarizing inhibitory p.s.p.'s may also be considerable 

 but need not be discussed in detail, except in the 

 extreme case when the neural volley generates pre- 

 dominantly or entirely hyperpolarizing p.s.p.'s. In 

 that case spike electrogenesis would not occur and 

 the volley in isolation may produce no overt effects, 

 although direct recording from the cells would dis- 



INNERVATED FACE 



EXCITED CELLS 

 DISCHARGED CELLS 



LEAD A 



LEAD B 



FIG. 26. Discharged and excited zones in a row of eel electroplaques on maximal stimulation of 

 their three different nerve supplies. Cells 6 to 10 were excited by Nerve I as evidenced by their p.s.p.'s 

 and long-lasting homosynaptic facilitation, but did not develop spikes to a single testing stimulus. 

 Nerve II caused discharge of spikes in cells 9 to 11, but in addition excited cells 6 to 8 and 12 and 

 13. Nerve III discharged cells 12 and 13, exciting also cells 10, 11, 14 and 15. The diagrammatic 

 representation of the recording leads shows the method that was used to test this population of cells. 

 [From .Altamirano et at. (5).] 



