302 C. EYZAGUIRRE 



latter's explanation is doubtful. Many factors could play a role in the onset 

 of grouped discharges, especially since this phenomenon seems to appear in 

 abnormally functioning cells. Consequently, cell geometry, changes of 

 excitability of the cells, or sUght injury produced during manipulation of the 

 receptors might determine the appearance of an abnormal "pacemaker". It 

 is possible that this new "pacemaker" might coincide with the normally 

 occurring one, it might be close to it, or it might occur at some distance. 

 Also, it might shift from one site to another during prolonged experimenta- 

 tion. In conclusion, the experiments of Edwards and Ottoson (1958) seem to 

 offer a better explanation of grouped discharges than that offered by Eyza- 

 guirre and Kuffler (1955b). But it is felt that more work is needed in order 

 to take into account a number of variables which may occur in different 

 preparations. 



C. THE INHIBITORY PROCESS 



Synaptic inhibition has been studied both in slowly adapting and in rapidly 

 adapting sensory neuron of crayfish and lobster. Kuffler and Eyzaguirre( 1955) 

 were able to stimulate in the crayfish the "accessory" nerve described by 

 Florey and Florey (1955). Stimulation of this nerve was capable of impeding 

 the onset of sensory discharges or stopping discharges already occurring. 

 Burgen and Kuffler (1957) studied these effects in lobster stretch receptors 

 and were able to find the presence of two inhibitory nerves which corresponded 

 to the "thick" and "thin" accessory nerves described by Alexandrowicz (1951, 

 1952). The effects produced by independent stimulation of these nerves are 

 similar. In general, the thick nerve has a more pronounced inhibitory effect 

 on the sensory neurons than that obtained by stimulation of the thin accessory 

 nerve. Also, the latter has to be stimulated at higher frequency to obtain the 

 desired results. 



At present there is little information concerning the role of the thin 

 accessory nerve on postsynaptic inhibition, and since results obtained by 

 stipiulation of both types of inhibitory fibers in the lobster seem to differ only 

 quantitatively we shall describe only those effects produced by stimulation of 

 inhibitory axons in the crayfish. As said before, the inhibitory synapses 

 supplied by the accessory nerves are mainly located in the dendrites of the 

 receptor cells forming, therefore, an axo-dendritic synapse. 



Inhibitory phenomena occurring in the crayfish stretch receptor cells are 

 not necessarily identical to well known inhibitory processes occurring in 

 other structures. However, inhibition in crustacean stretch receptors resembles 

 not only similar processes occurring in neurons of the central nervous system 

 of vertebrates (cf. Eccles, 1957), but also inhibition obtained at the crustacean 

 neuromuscular junction (Fatt and Katz, 1953) and in the heart of frogs and 

 turtles (Hutter and Trautwein, 1956). 



