948 



HANDBOOK OF PHVSIOLOG\' 



NEUROPHYSIOLOGY II 



FIG. 27. Simultaneous condi- 

 tioning effects upon the mono- 

 synaptic reflexes of the two sides 

 of the third sacral segment 

 caused by the Group I afferent 

 \olleys employed to elicit those 

 reflexes. Open symhols, mean 

 amplitude of monosynaptic re- 

 flexes on one side. Closed symbols, 

 mean amplitude of monosynap- 

 tic reflexes on the other side. 

 Triangles and circles represent 

 independently made observa- 

 tions by the two authors. Fur- 

 ther description in text. [From 

 Wilson & Lloyd (98).] 



»>>-i— »— 5^-gr A g " o— 100 



V' \ 



/ 



/ 



o^ A 



_1 J I L 



0.6 0.4 0.2 Q2 0.4 0.6 0.6 04 02 0.2 04 0.6 



06 0.4 0.2 02 04 0,6 0.6 0.4 0.2 Q2 0.4 0.6 



Milliseconds Milliseconds 



a way that one of them initially antecedes, then co- 

 incides with and finally trails the other. Figure 27 

 illustrates the result of four such experiments. If the 

 latency for inhibition were longer than that for excita- 

 tion, which it is conceded would necessarily be the 

 case if the inhibitory path contained an interneuron 

 in series, there would be a period on either side of 

 synchrony during which neither reflex would be in- 

 hibited. Making every possible allowance, that period 

 would amount to a minimum of 0.2 to 0.3 msec, on 

 either side of synchrony. From the results in figure 27 



the only possible conclusion is that the inhibitory 

 pathway, like the excitatory pathway, is monosyn- 

 aptic. It is evident, therefore, that the chemical 

 hypothesis in its present form is not adequate to ac- 

 comodate the facts. 



Unfortunately the exact peripheral origin of the 

 afferent fibers concerned in the monosynaptic and 

 disynaptic pathways of the lower sacral and caudal 

 segments is not known. The monosynaptic pathways 

 presumably serve for reciprocal innervation of the 

 lateral tail muscles. 



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