156 



DONALD M. MAYNARD 



Fig. 10. Inhibitory after eflect (Paniiliiiis). Twenty-two inhibitory stimuh at 

 92/sec were given at varying times before the driving stimulus. The numbers to 

 the left represent the interval in seconds between the last inhibitor impulse in the gan- 

 glion and the driving stimulus. N, normal response. Stimulating inhibitor nerve 

 and posterior ganglion; recording from anterior ganglion. Time signal, 60/sec 



(Maynard, 1954). 



Inhibition of Pacemaker Neurons 



The effects of inhibition on the isolated small or pacemaker neuron must 

 be determined indirectly, for it is impossible to isolate a small unit without 

 severing its inhibitory connections. In certain preparations, however, activity 

 in all but one small neuron may be inhibited (see Fig. 20) leaving a functionally 

 isolated unit. Figure 13 diagrams the activity of such a unit firing in repeated 

 trains, not continuous runs. Further inhibition at higher frequencies leads 

 to a progressive decrease in train frequency, maximum spike frequency in 

 the train, number of impulses per train, and mean spike frequency. Adapta- 

 tion and post-inhibitory rebound occur, so inhibition of the posterior small 

 units is apparently qualitatively similar to inhibition of the anterior followers. 

 There is, however, a quantitative difference; the pacemakers usually require 

 much higher frequencies of inhibitory stimulation for complete inhibition 

 than do the followers. 



