478 ANNALS NEW YORK ACADEMY OF SCIENCES 



charged from the gangUon cells can be increased by decreasing the cal- 

 cium concentration. It must be said that a further reduction of cal- 

 cium in the perfusing solution may cause a complete block of ganglion 

 cell excitation by impuls3s in the pre-synaptic fibers. 



VI 



Soms evidence regarding the nature of the cellular events which 

 cause the more or less rhythmic discharge of impulses from a chemically 

 excited region of nerve can be derived from a consideration of the 

 temporal distribution of the impulses. Such a study suggests that there 

 is a rhythmic excitatory process in nerve, of a fairly constant frequency, 

 which may or may not produce an impulse each cycle. Definite evi- 

 dence from several sources is now available for the existence of such a 

 process. The role it plays in the regulation of the frequency of con- 

 ducted impulses will be discussed in the following pages. 



The earliest work on the discharge of impulses in single neurons re- 

 vealed a temporal distribution of impulses that was more or less regu- 

 lar, but not quite periodic. Thus, one of us in 1928, when commenting 

 on the failure of the discharge from a fatigued muscle tension receptor, 

 remarked that "one or more impulses drop out of an otherwise fairly 

 regular series, the impulses becoming more and more scattered. "^^ The 

 longer intervals were observed to be approximately equal multiples of 

 the shortest time interval between successive impulses. Adrian observed 

 a similar phenomenon in the discharge of injured mammalian nerve 

 fibers, ^^ and such irregular intermittence appears in Pumphrey's"^ rec- 

 ords of impulses from taste receptors. More recently, this occasional 

 omission of impulses from an otherwise regular series was observed dur- 

 ing the repetitive discharge caused by super-threshold direct current 

 excitation (Erlanger and Blair," Fessard^^). 



This same irregularity in the temporal distribution of impulses is a 

 prominent characteristic in our experiments upon chemically excited 

 axons. This will have been evident in some of the preceding records, 

 but, for the more precise analysis of this phenomenon, additional ex- 

 periments will be presented. The fibers were excited by removing Ca""* 

 from a short length of nerve, by means of sodium citrate, as previously 

 described. The measurements were made on records taken when the 

 nerve was producing impulses at a constant average frequency. Under 

 these circumstances, the temporal distribution may be regular or ir- 

 regular. 



The magnitudes of the time intervals between successive impulses in 

 ^ certain series obtained in the above manner are plotted in figure 19, 



