CARDIAC INHIBITION IN DECAPOD CRUSTACEA 



159 



10 20 30 



STIMULATION FREQUENCY, 



40 50 



PER SECOND 



Fig. 13. Single small cell, effect of inhibitor stimulation frequency (Panii/iriis). 

 Ordinate, parameters of spontaneous activity; abscissa, inhibitor stimulation fre- 

 quency. The pattern of activity in the unit is shown in the diagram, a brief 

 train of high frequency activity followed by a longer recovery period 



(Maynard, 1954). 



The eflfects of inhibitor activity on the various potentials in the ganglion 

 are not completely known. Generator and oscillating potentials are obviously 

 blocked by the hyperpolarizing potentials, but there is no complete descrip- 

 tion of their action on the e.p.s.p. or the generation of propagated spikes. 

 In view of the experiments illustrated in Fig. 8, such action inay be assuined 

 depressant. Depolarizing i.p.s.p.'s apparently summate with e.p.s.p.'s. This 

 is probably not additive, and in two preparations, the figures (Figs. 14, 15) 

 suggest that summed, depolarizing i.p.s.p.'s result in a depression of indivi- 

 dual e.p.s.p.'s. Definite statements on this subject are premature, however. 

 Ahhough I do not feel justified in terming all depolarizing effects of the 

 i.p.s.p. excitatory, there are some instances — usually fimited to the non- 

 spontaneous, first two follower neurons in PanuUrus — in which the de- 

 polarizing i.p.s.p. apparently potentiates the generation of spike potentials 

 (Fig. 15). In this sense the i.p.s.p. may be excitatory. Such spike potentiation 

 does not correlate with potentiation of synaptic potentials; the latter in fact 

 diminish in amplitude. 



