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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



cells with depolarizing p.s.p.'s above the critical 

 level, the greater will be the degree of facilitation on 

 repetitive stimulation. If the majority of the cells in 

 the excited zone develop p.s.p.'s only slightly below 

 the level of critical depolarization, a few repetitive 

 stimuli will rapidly evoke a maximal response. This 

 gives rise to the general class of cPemblee reflexes 

 (cf. 44). Augmentation of p.s.p.'s (synaptic facilita- 

 tion) discussed earlier (p. 168) also will favor pro- 

 duction o{ d' em blee refiexes. 



When the p.s.p.'s of the excited zone are small, 

 the responses may recruit very gradually with repeti- 

 tive stimuli. Particularly when the synaptic organi- 

 zation is complex and the synaptic drives are weak 

 will the latency of the response be long. Under these 

 conditions it may also be expected that moer fre- 

 quent stimuli will shorten the latency markedly and 

 increase the rate of growth of the response and per- 

 haps its maximum value. In other words, the more 

 weakly effective synaptic drives, including multi- 

 synaptic pathways, will show a greater frequency- 

 dependence. Since the production of hyperpolarizing 

 p.s.p.'s also involves excitation of synapses, the de- 

 velopment of inhibitory activity will depend similarly 

 upon the stimulus parameters. 



Another effect in which the complexits of the 

 synaptic organization plays a role is that of after- 

 discharge. The involvement of multisynaptic path- 

 ways carries the likelihood that additional side 

 paths will also be brought into activity and thus 

 give rise to a circulating activity (78) or a series of 

 delayed reverberations which may cause discharges 

 of the final common path long after the initial 

 stimulus is ended. This reverberation may take place 

 by one-to-one excitation, but it is likely that another 

 phenomenon plays an even greater role. This is the 

 summation and persistence of p.s.p.'s associated with 

 accumulation of a persistent transmitter agent. As 

 individual Renshaw cells are capable of persistent 

 repetitive discharge by a single stimulus (61), so 

 some of the interneurons mediating excitatory 

 p.s.p.'s can also remain active for a long time. The 

 interplay of excitatory and inhibitory synaptic 

 activity may produce complex patterns of waxing and 

 waning after discharge. In individual cells this pat- 

 terning would be reflected by a greater or lower 

 frequency of discharge. Complex interactions of 

 excitatory and inhibitory types occur even in the 

 relatively simple nuclear structures like that of 

 Clarke's column in the spinal cord (104, 115). The 

 involvement of a widespread network of neuron 

 complexes in after-discharge is indicated also by the 



fact that increasing the strength of the initiating 

 stimulus may lead to no increase in the maximal 

 amplitude of the reflex respon.se but only in the dura- 

 tion of its after-discharge (182). 



Role of Inlnhitioii in Central Nervous System 



The interrelations of depolarizing and hyper- 

 polarizing p.s.p.'s in the.se various manifestations, 

 in.sofar as they are dependent upon the specific 

 organization of synapses, are beyond the scope of 

 this chapter, but some general discussion is ap- 

 propriate (cf. 1 01, 161). As was described earlier, 

 hyperpolarizing p.s.p.'s need attain only relatively 

 small amplitudes to produce inhibition. The effect, 

 a sudden cutting off of conductile activity, may 

 block the synaptic transfer to many systems which 

 would normally participate in an activity. The re- 

 sults of a given excitatory and inhibitory interaction 

 will differ depending upon the site at which an index 

 of the effect is obtained. In a specific example let us 

 assume that a single cell is acted upon by the synaptic 

 interplay. Whether or not it is excited to produce a 

 spike will have important consequences for the ac- 

 tivity of other downstream neurons for which the 

 cell chosen as an example serves as a valve. How- 

 ever, when recording from the interior of the cell, 

 depolarizing and hyperpolarizing activities may be 

 oi)served even in the absence of a spike. Thus, dif- 

 ferent criteria apply to activity in different parts of 

 a complex pathway. The relations between activity 

 in one part and another may even be dimmed or 

 may disappear. 



The activity set into motion by a synaptically 

 complex pathway thus may be undetected in the 

 overt response. For example, a single stimulus to the 

 head of the caudate nucleus in the cat giv-es rise to a 

 relatively simple, brief electrical response in a re- 

 stricted cortical region. Analyses with paired or 

 repetitive stimuli disclose (167) that many excitatory 

 and inhibitory influences are activated, some for 

 long periods of time. It is worth noting that ana- 

 tomical data can rarely give information as to the 

 presence of such intricate synaptic linkages and, of 

 course, cannot distinguish those that are excitatory 

 from the inhibitory. 



It is most likely that in its normal functioning the 

 central nervous system utilizes inhibitory activity as 

 a means for braking excitatory activity which might 

 otherwise be unduly prolonged or inclined to rever- 

 beration. In that sense, therefore, inhiijitory synaptic 

 electrogenesis would aid the precision of ner\ous 



