CARDIAC INHIBITION IN DECAPOD CRUSTACEA 



163 



INHIBITION AND PATTERNING 



1 should now like to consider the effects of inhibition on the pattern of 

 firing of individual units in the total burst. In contrast to the isolated unit, 

 the depression of the excitabiHty of component units in an integrating 

 system may not necessarily lead to a depression of all parameters of activity. 



The integrated discharge of the normal, undisturbed ganglion is perhaps 

 best measured by mechanical records of the heartbeat (Fig. 17). Although 

 inhibition often produces a progressive decline in frequency and amplitude 

 and very occasionally may stabilize an irregular beat, in many instances 

 irregular beats of varying amplitude occur during inhibition, or the usual 

 initial inhibition, adaptation, rebound sequence is distorted. The latter 

 instances are perhaps the more interesting, for they imply some deviation 

 from the usual effect of inhibition on the single unit. That they occur in the 

 intact, in situ heart indicates that not all such irregular activity can be ascribed 

 to preparation of the semi-isolated ganglion, and consequently it may be 

 expected in normal systems. 



The smoothly inhibited heartbeat would presumably be accompanied by 

 ganglion activity such as shown in Fig. 18. Burst duration, burst frequency, 

 and impulses per burst decline together, and the usual sequence of primary 

 inhibition, adaptation, plateau inhibition, and rebound is present as with the 

 single unit. Even in such a preparation, however, all parameters of single 

 unit discharge may not be depressed. Figure 19 diagrams the frequency pattern 

 of a single anterior follower unit in a Panulirus ganglion during inhibition. 



Fig. 17. Inhibition of total heart beat (Cambarus clarkii). The numbers below the 

 records show the number of beats per 15 sec; the duration of inhibition is 15 sec, 

 indicated by upper trace. Contraction of heart gives a downward stroke. Fre- 

 quency of inhibitor stimulation given by numbers to left of records. (Courtesy, 

 Wiersma and Novitski, 7. Expti. Biol. 19 : 255-265, 1942). 



