CARDIAC INHIBITION IN DECAPOD CRUSTACEA 



155 



,0 20 



SECONDS 



Fig. 9. Relation of latent pause to inhibitory intensity and duration {Homants). 

 Ordinate, latent pause or the interval between the end of inhibition and the first 

 post-inhibitory activity; abscissa (.v), interval preceding burst, prolonged by 

 inhibition; abscissa iz), inhibitor stimulation frequency. The vertical plane at 

 0-45 sec on the abscissa (.v) represents the normal burst frequency. Points on 

 the ordinate base line — latent pause, — represent first activity during inhibition, 

 and the corresponding value on abscissa (.v) gives the duration of initial inhibition 



(Maynard, 1954). 



The relations between inhibition and the post-inhibitory events are 

 especially clear in these preparations (Fig. 12). An increase in duration of 

 inhibition is followed by an increase in the maximum driving frequency — 

 assumed to ineasure post-inhibitory excitation — and a decrease in the latent 

 pause. These observations, together with various elaborations, suggest that 

 the probability of discharge of a ganglion neuron within a given period of 

 time after the termination of inhibition is related to at least three distinct 

 phenomena: (1) The depressed excitability directly caused by inhibitor 

 action. (2) The process initiated by such depression which effectively increases 

 excitability and which is presumably responsible for post-inhibitory rebound. 

 (3) The sequence of excitability variations which follow normal intrinsic 

 activity of the ganghon neurons. 



