THE SYNAPTIC MECHANISM FOR POSTSYNAPTIC INHIBITION 77 

 A 



Fig. 5. Reproductions of inhibitory curves in which the inhibitory action of a 

 quadriceps la afferent volley is tested by the size of a monosynaptic reflex spike 

 discharged into the ventral root from BST motoneurons. The ordinates show 

 the percentage sizes of the reflex spikes, the abscissae the testing volley intervals. 

 The approximate time courses of the components of inhibition attributable 

 directly to the hyperpolarization of the i.p.s.p.'s are shown by the broken lines. 

 In B the quadriceps afferent volley and the i.p.s.p. are shown on the same time 

 scale as in Fig. 5c. a is from Bradley ei a/. (1953) and b, c from Araki et al. (1960). 



shorter testing intervals the inhibitory action would be due to superposition 

 of the effects due to the inhibitory current and the hyperpolarization; as is 

 illustrated in Fig. 5a, c, where the broken lines approximately separate the 

 two modes of inhibitory action. 



Testing for inhibition by the depression of reflex discharge has the dis- 

 advantage that the ordinates represent merely the relative population of the 

 discharging motoneurons, and so do not give a direct measure of the intensity 

 of inhibitory action. The most direct measurement is provided by testing 

 excitability by a brief rectangular pulse (about 1 msec in duration) in the 

 depolarizing direction through the intracellular electrode. At each testing 

 interval during the inhibition the intensity of the pulse is adjusted so that it 

 evokes a spike potential in approximately one-half of the trials. When the 

 time course of the inliibitory action is obtained by plotting the reciprocals 

 of current intensities against the testing intervals, it has a time course that 

 displays the same double composition as with testing by reflex inhibition 



