174 



DONALD M. MAYNARD 



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Fig. 27. Effect of GABA on burst pattern, applied to followers only {Homanis). 

 A. Normal burst, b. Burst after concentrated GABA solution applied to anterior 

 follower neurons, c. Burst after washing. Upper trace, p, from posterior portion 

 of ganglion; lower trace, «, from anterior portion of ganglion. (') indicates 

 impulse originating in posterior cell which is unaffected by GABA; (•) indicates 

 impulse originating in cell between anterior and posterior electrodes, probably 

 large cell 4, which is relatively unaffected by GABA. Note complete inhibition 

 of spikes originating more anteriorly. Diagram shows isolated ganglion laid across 

 five separate pools of perfusion fluid with position of recording electrodes. GABA 

 applied in shaded pool. Time line, 0- 1 sec. 



regarding the consequences of differential inhibition of the pacemakers and 

 followers. 



CARDIO-ACCELERATION 



The two cardio-accelerator fibers may be stimulated separately or together. 

 Their effects summate temporally and spatially, and generally increase the 

 frequency and amphtude of the heartbeat (Fig. 29). Rhythmic beats are 

 usually restored to irregular or inactive hearts, but in rare instances, accelera- 

 tor activity may induce alternating weak and strong contractions in an 

 initially regular beat (Maynard, 1953). In the Homarus ganglion, accelerator 

 activity increases spike number and frequency during the burst as well as 

 increasing the burst frequency (Fig. 30). Spike initiation during interburst 



