CARDIAC INHIBITION IN DECAPOD CRUSTACEA 151 



before spontaneous inhibitor activity resumes, suggesting tliat the axon 

 carrying spontaneous and "stimulated" inhibitor impulses is the same. 



Since there are but three large (1-5 ju) fibers in the dorsal nerve and since 

 only the large impulse described above can be correlated with inhibitory 

 effects, it seems very probable that only one inhibitory fiber passes through 

 inhibitory and dorsal nerves into the ganglion. 



A few recordings from the dorsal nerve of Panulirus interruptus, in contrast 

 to Homarus, show several spikes when the severed inhibitor trunk is stimu- 

 lated. Simultaneous records from the ganglion, however, show only one 

 impulse, and Terzuolo and Bullock (1958) find but one inhibitory post- 

 synaptic potential upon stimulation of the dorsal nerve. This finding agrees 

 with the observation that certain fibers of the pericardial organ system run 

 in the dorsal nerve to the ligaments and surface of the Panulirus heart while 

 they do not appear to do so in Homarus. It may be presumed that in the 

 spiny lobster there is also one inhibitor fiber in each dorsal nerve. 



Maynard (1953) thought stimulus-response curves in Panulirus argus 

 might indicate several inhibitor fibers. This interpretation appears erroneous, 

 however, for the curves may be explained by the present findings that with 

 high stimulus strengths or durations, the inhibitor fiber responds repetitively, 

 and that with high stimulation frequencies (70/sec), it frequently skips stimuli 

 or fails to respond altogether. 



When the first accelerator nerve is cut near the thoracic ganglia in Homarus, 

 the larger of the two small spikes in the dorsal nerve disappears, but may be 

 elicited again by stimulation of the distal stump. The smaller spike remains 

 after both inhibitor and first accelerator nerve are cut, its frequency un- 

 affected by stimulation of either. A second accelerator like that known to be 

 present in brachyurans (Smith. 1947") and palinurans (Maynard, 1953) may 

 be inferred in Homarus. The second accelerator impulse is abolished by 

 cuts in the lateral musculature posterior to the first accelerator nerve. Activity 

 in either or both of these smaller extrinsic fibers is associated with increased 

 activity in the ganglion, confirming their identification as accelerators. 



Spontaneous Activity in Regulator Neurons 



The inhibitor fiber discharged spontaneously in sixteen out of eighteen 

 bloodless Homarus preparations. In six of the sixteen, spontaneous accelerator 

 impulses also occurred, and in two of these six, both accelerators were active. 

 Frequencies varied from 1 to 16 impulses/sec in the inhibitor and up to 36/sec 

 in the accelerators. At lower frequencies, activity was erratic and uneven; 

 at higher frequencies, rhythmic and continuous. 



INHIBITION IN THE GANGLION 

 If the posterior two-thirds of a Panulirus gangfion trunk is removed often 



