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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 1 



close the p.s.p.'s. These, hke the excitatory p.s.p.'s, 

 should vary in amplitude in the population of post- 

 junctional cells. An overt manifestation of the in- 

 hibitory p.s.p.'s would occur if the cells are at the 

 same time producing excitatory p.s.p.'s and spikes or 

 causing reflex activity in muscles. More or less selec- 

 tive afferent activation of inhibition, and its role in 

 spinal reflex activity, was demonstrated by Sherring- 

 ton (44, 182; cf. 140). Relatively specific descending 

 pathways were found by Sechenov (180) in the frog 

 and by Magoun and his colleagues (cf. 147) in the 

 mammal. 



c) FACILITATION. Study of spinal cord reflexes also 

 demonstrated the existence of the excited zone by 

 the effects of temporal and spatial facilitation, the 

 excited cells being then termed the subliminal fringe. 

 Both types of facilitation depend essentially upon 

 the properties of summation and sustained response 



of p.s.p.'s described above. However, subsidiary 

 effects also participate which will be discussed later. 

 The unitary p.s.p.'s are relatively long lasting, in 

 the cat central nervous system having a duration of 

 about 15 msec. (figs. 12, 27). For that time at least, 

 therefore, an excited cell is somewhat depolarized, at 

 first to a large degree, but not to that of the critical 

 level for discharge, and then to a smaller amount, 

 decreasing with time. 



The presence of the e.xcited cells can be tested by 

 applying a second volley either through the pathway 

 which delivered the first condidoning stimulus 

 (homosynaptic testing) or through another inner\-at- 

 ing path (hcterosynaptic testing). 



d) HOMOSYNAPTIC FACILITATION. In this casc, there 

 will be no second response, neither an electrical 

 activity nor a reflex contraction of muscles, if the 

 stimulus interval is verv short. Because of absolute 



B 



msec 



50 mV 



10 msec 



^ — ^^^ 



FIG. 27. Temporal facilitation and shortening of synaptic delays in neurons. .-1, B: From a cat 

 motoneuron at high and low amplification. Two orthodromic stimuli, neither capable of discharging 

 the cell, can evoke a spike by summation of the p.s.p.'s produced by each stimulus. Since the spike 

 occurs only when the critical level of depolarization is attained, the summation interval may be 

 sharply delineated as shown in this example. [From Brock el al. (24).] C: From a rabbit cervical 

 sympathetic neuron. Progressively stronger stimuli to the preganglionic nerve increased the p.s.p. 

 of the neuron and evoked its spike earlier as the critical firing level (shown by arrows) was attained 

 earlier. [From Eccles (64).] 



