CARDIAC INHIBITION IN DECAPOD CRUSTACEA 



165 



SINGLE UNIT 



PATTERN of ACTIVITY 

 DURING BURST 



INHIBITORY 

 STIMULATION at 40/sec 



Fig. 19. Inhibition of spontaneous ganglion activity, single unit analysis {Panu- 

 linis). Ordinate, unit activity; abscissa {x), time during burst; abscissa (r), time 

 between bursts. The graph portrays a series of vertical planes, each representing 

 the frequency-time curve of one unit during a burst. The distance between these 

 planes represents the time between bursts. Numbers at the top of each plane give 

 the number of impulses of this unit in the respective burst. Inhibited planes are 

 shaded; on and o# indicated beginning and end of inhibition (Maynard, 1954). 



neurons. In any event, whether some units are directly excited or not, the 

 total effect of inhibition on such ganglia is a general decrease in the rnean 

 number of discharges per unit without drastically altering the co-ordination 

 of the burst. 



In contrast to the above, the large follower neurons are often inhibited 

 before pacemaker activity is appreciably disturbed (Fig. 20). The heart con- 

 traction may consequently disappear before any gross alteration in burst 

 frequency occurs, or in less extreme instances (Fig. 21) it may occur at 

 irregular intervals which are mukiples of pacemaker burst intervals. 



Further insight into such activity is provided by an analysis of the frequency 

 patterns of three small units in a PanuUrus preparation (Fig. 22). The primary 

 unit, the presumed pacemaker, is relatively unaffected by inhibitory stimula- 

 tion. Burst frequency is only sliglitly depressed and is not indicated in the 

 figure. During inhibition the second and third units do not fire during every 

 train of the pacemaker. Large units fire only after Unit 3 discharge. During 

 the initial phases of inhibition (bursts 4-11) Unit 2 discharges at every other 



