CARDIAC INHIBITION IN DECAPOD CRUSTACEA 149 



Intrinsic Burst 



Each of the nine neurons of the ganglion appears to fire several times 

 during the intrinsic burst. The sequence of discharge during a burst may 

 remain essentially unaltered for many minutes, even in the isolated ganglion. 

 The silent period between bursts corresponds with a period of depressed 

 excitability. Many, if not all of the ganglion neurons are capable of spon- 

 taneous activity in the absence of synaptic activation. The pattern of dis- 

 charge of such isolated units, however, generally differs from that of the 

 burst. Consequently, we may assume that the normal, co-ordinated burst 

 depends upon some type of interaction among the ganglion neurons. The 

 small posterior cells usually initiate the burst and have consequently been 

 termed the "pacemakers". The larger, anterior motor neurons have been 

 called the "followers"'. The burst may continue, however, in the absence of 

 the usual pacemakers, and there is evidence that electrotonic as well as 

 synaptic influences play a role in maintaining the co-ordinated discharge. 

 Aspects of this problem have been considered in the papers of Maynard 

 (1954, 1955, 1960), Hagiwara and Bullock (1957), Bullock and Terzuolo 

 (1957), Bullock (1958), Otani and Bullock (1959), Watanabe (1958), Hagi- 

 wara, Saito, and Watanabe (1959). 



Although details need not be considered here. Fig. 5, showing intracellular 

 recordings from follower cells in Panulirus, demonstrates the complex 

 electrical phenomena which may occur in each cell during each burst. At 

 least four major types of potential changes may occur in ganglion neurons: 



(1) Electrotonic potentials resulting from slow activity in neighboring units. 



(2) Generator potentials often leading to spontaneous spike discharges, 



(3) Propagated action potentials in axons seen as brief spikes from electrodes 

 in the cell body. (4) Synaptic potentials. According to the nature of the 

 presynaptic terminal, synaptic potentials are of at least three kinds; intrinsic 

 excitatory, accelerator, inhibitor (Table I). 



Extrinsic Impulses 



Impulses arriving in the cardiac ganglion from the dorsal nerve are of 

 three kinds; one large amplitude spike with an estimated conduction velocity 

 of about 1-5-2 m/sec at 25'C, and two of small amplitude with conduction 

 velocities about half that of the larger spike. All three impulses may be 

 initiated by stimulating the dorsal nerve. 



The larger spike is identified as the inhibitor on the following evidence 

 from Honiarus: (1) When the inhibitor nerve (segmental nerve I) is cut near 

 the ventral cord, the large spike disappears from the dorsal nerve and 

 ganglion trunk, and the burst frequency of the ganglion may increase. 

 (2) When the distal stump of the cut inhibitor nerve is stimulated, the large 

 spike appears after each stimulus and inhibition returns. Such stimulation 



