EXCITATORY AND INHIBITORY PROCESSES 



301 



axon. Since the axon has a briefer refractory period it may have recovered 

 sufficiently to conduct an impulse. In this manner if several dendrites are 

 invaded at different times, each dendrite impulse could add its distinct effect 

 to the partially recovered soma and thereby excite the more fully recovered 

 axon. This interpretation has been questioned by Bullock (1957) on the basis 

 that similar phenomena occur in the giant fiber of the earthworm. In fact, 

 when a spike, travehng some distance along the giant fiber, arrives at a locus 

 of partial or complete anodal block it hesitates before proceeding or it may 

 die out. After several milliseconds a burst of impulses at high frequency is 

 originated at that point or near to it (Bullock and Turner, 1950). Bullock's 

 objection indicates that this phenomenon does not depend on a particular 

 anatomical arrangement such as that present in the crayfish stretch receptor 



Fig. 12. Tracings of orthodromic impulses set up by antidromic impulse 



recorded at various indicated points on the same nerve cell. Time intervals, 



01 msec; scale 0-5 mV. (From Edwards and Ottoson, /. Phvsiol. {London) 



143 : 138-148, 1958.) 



cell. It does not dispose, however, of the possibility that partial blocks in 

 some portions of the crayfish dendrites inight occur. 



Edwards and Ottoson (1958) have offered a different interpretation with 

 regard to the origin of the grouped discharges which occur either spon- 

 taneously or provoked by antidromic stimuli. These authors have shown that 

 these discharges originate in the axon at some distance from the soma (Fig, 

 12), It is their contention that grouped discharges start nearer to the soma 

 than those occurring normally. Once the grouped discharges are originated 

 the impulse travels both in an orthodromic direction and backwards into the 

 cell soma. Whether the findings reported by Eyzaguirre and Kuffler (1955b) 

 and by Edwards and Ottoson (1958) can be interpreted only according to the 



