288 



C. EYZAGUIRRE 



Fig. 2. Schematic drawing of a slow receptor cell and the relation of its endings 

 to the muscle fibers. (From Florey and Florey, /. Gen. Physiol. 39 : 69-85.) 



tions occur, the new branch leaves perpendicular to the region of ending 

 which continues its course and it may bifurcate again in a T fashion at the 

 same muscle fiber. The nerve cells of RM 2 present a somewhat different 

 picture with regard to the dendritic terminals. The dendrites leave the cell 

 body all in about the same direction. There are three to four dendrites, one 

 of them being rather stout. They end in a brush-like manner, their termina- 

 tions being short. They do not seem to be oriented in the direction of the 

 muscle fiber (Figs. 1b and 2). This morphological arrangement could be of 

 importance in understanding some of the basic processes of the phenomena 

 of adaptation (for anatomical details consult Florey and Florey, 1955). 



One feature of these neurons is that the cell body as a whole is completely 

 devoid of terminal nerve endings and the only known efferent connections 

 from the central nervous system terminate around the dendrites forming 

 axodendritic synapses. These synapses, supplied by the "accessory fibers" of 

 Alexandrowicz, have been shown to be of an inhibitory nature (Kuffler and 

 Eyzaguirre, 1955; Burgen and Kuffler, 1957). This particular type of innerva- 

 tion differs from that of many neurons in the central nervous system of 

 vertebrates where the cell body is covered by a number of "boutons termin- 

 aux". Unfortunately, no information is available with regard to the fine 

 structure of the crustacean nerve-cell synapses since no electron microscope 

 studies of these structures have appeared.* 



B. THE EXCITATION PROCESS 



Wiersma et al. (1953) first showed the sensory nature of the organs described 

 by Alexandrowicz; tension applied at both ends of the muscular portion of 

 these structures produced repetitive discharges which could be recorded from 



* See note on page 316. 



