CARDIAC INHIBITION IN DECAPOD CRUSTACEA 157 



Fig. II. Paradoxical driving, relation of latent pause to inhibitory duration 

 (Panulirus). a through E, increasing duration of inhibitory stimulation: a, no 

 response; b, two-spiked response, long latent pause; c-e, one spike response, 

 decreasing latent pause. Inhibitor stimulation frequency, 100/sec; inhibitor 

 trains repeated every 2-4 sec. Stimulating inhibitor nerve; recording from 

 electrode pair in midcardiac ganglion. Time signal, 60/sec; time line, 0- 1 sec 



(Maynard, 1954). 



From the description of inhibitor action on ganghon neuron activity given 

 above, the analysis logically splits in two directions. One proceeds to an 

 examination of the cellular mechanisms which underly the events described, 

 particularly in so far as membrane conductance and potential changes are 

 involved. The other direction takes the events as described and asks how 

 they are altered when the ganglion neuron is placed in a dynamic system and 

 how they in turn affect the total output of the system. It is useful to dis- 

 tinguish between the two aspects, for I think it will become apparent that 

 the terminology and concepts applicable to one, although related, do not 

 necessarily prove the most useful for the other. For example, in so far as 

 the heart beat is concerned, the probability of propagated impulse initiation 

 in motor neurons is the measure of inhibitor activity. In so far as the neuron 

 membrane is concerned, however, one may argue that the appropriate 

 measure of inhibitor activity is the change in relative membrane conductance 

 to specific ions, and that effects on spike probability are secondary. 



