166 



DONALD M. MAYNARD 



10 20 30 40 50 



inhibitory impulses per second 



Fig. 20. Inhibition of spontaneous ganglion activity, diflFerential effect on pace- 

 maker and follower units {Pamdinis). Increasing inhibitor stimulation frequency 

 (abscissa) leads to progressive reduction in number of follower impulses per 



burst (-0-) before appreciable effect on either mean frequency ( ) or 



duration ( ) of small unit pacemaker burst. Rectangles give range of values 



found for duration and frequency of burst at each stimulus frequency. 



train of Unit 1. When Unit 2 does become active, its latency is less than 

 usual and more impulses occur at higher frequencies than in the normal 

 burst. This suggests that Unit 2 is hyperexcitable at the time of bursts 5, 7, 

 9, 11, but hypoexcitable at the time of the intervening bursts. In the later 

 phases of inhibition Unit 2 fires at each burst, but the latency is longer and 

 the frequency of spikes is less than normal. The initial hyperexcitabihty 

 during odd bursts 5-1 1 must consequently result from recovery during the 

 increased interburst period rather than a direct excitation by inhibition. As 

 might be expected, as inhibitory adaptation proceeds, and Unit 2 becomes 

 able to follow each Unit 1 train, the large oscillations in excitability are 

 replaced by a more stabilized discharge at subnormal frequencies during 

 each burst. Similar stabilization of alternating beats has been observed upon 

 slight increases in excitability due to accelerator activity (Maynard, 1953). 



Unit 3 and the large motor units follow Unit 2 activity only when the 

 latter's discharge frequency during a burst reaches or surpasses normal 

 levels. The heartbeat in this preparation would consequently be irregular 



