INHIBITORY PATHWAYS TO MOTONEURONS 



51 



L6DR^_|SE^ 



Fig. 3. The experimental arrangements are shown diagrammatically in h. In a-f 

 a monosynaptic reflex spike response (monophasically recorded from S\ VR {upper 

 traces)) was generated by an aflerenl volley from BST, which also produced a 

 diphasic spike in the records from the dorsum of the cord (lead SEi in h) at the 

 upper level (lower traces). This SSr volley reached the cord at various times after 

 a maximum group 1 afferent volley from quadriceps, which is at a fixed position 

 in A-F, its arrival time at the upper La level being given by the left perpendicular 

 broken line. The superimposed traces for each of the testing intervals a-f were 

 formed by firstly photographing eight traces for the BST volley alone at 7-sec 

 interval to give the control reflex spike, and then a further eight traces with 

 quadriceps volley in addition. The second vertical broken line at 1-59 msec from 

 the first passes through the onset of the inhibition as signalled by the reflex spikes 

 for all but record A. In g there is an i.p.s.p. produced by the Q volley and intra- 

 cellularly recorded from a fi5r motoneuron at the 5i segmental level a little later 

 in this same experiment and at the same sweep speed. The manner of production 

 of a record such as c or d is illustrated in the construction, i-l. In I there are 

 schematic reflex spikes in ten fibres dispersed over 0-5 msec, as in a normal 

 monosynaptic reflex, which is derived as in j by summation. Inhibition beginning 

 at the dotted line in k, delays the onset of the sixth to eighth spikes as shown 

 and suppresses the ninth and tenth. As a consequence the summed reflex potential 

 (the continuous line in l) deviates from the control (the broken line in i ) at a point 

 just later than the dotted line (Araki et al., 1960). 



