12 



INVERTEBRATE PHYSIOLOGY 

 ant. 



LNS 



Fig. 4. Diagram of the cardiac ganglion 

 of the spiny lobster, Panulirus intcrruptus. 

 Not to scale ; the ganglion is about 12 mm. 

 long, the largest cells about SOju,. Neuro- 

 piles simplified. LNS, large neuron soma; 

 i, the single inhibitor fiber ; a, the two 

 accelerator fibers from the central nervous 

 system ; np, neuropile ; DA, dendritic ar- 

 borization, possibly sensory; SNS, small 

 neuron soma. 



post. 



mechanism outlined earlier, elicit a short train of up to five impulses. In 

 a normal burst it is not so simple, however. 



The pacer fires repetitively. It seems probable that the pacer fires the 

 other small cells directly and that some or all of them drive the large cells. 

 This means there could result a simple "open chain" determination of 

 pattern, as Maynard calls the type of system proposed by Rijlant for the 

 Liniulus heart, all the cells being controlled directly or through inter- 

 neurons by the pacemaker, in direct line or chain of cominand (Fig. 5). 



But it seems more likely that in the normal burst there is interaction. 

 The frequency /time curves strongly suggest that two additional factors 

 are operating : ( 1 ) great individual dififerences in the time course of ex- 

 citability, responsiveness, or both (we have seen that these are separate 

 properties of a cell) to the same input signal, and (2) feedback, at least 

 positive and possibly also negative. 



This feedback is probably critically responsible for normal burst main- 

 tenance. Once a pacemaker starts activity, followers build it up greatly 



