BRINK AND OTHERS: CHEMICAL EXCITATION OF NERVE 461 



these considerations, the action potentials recorded from a bundle of 

 numerous fibers reveal little of what is occurring in the individual units 

 (figure 3). Under these conditions, the investigation of the proc- 

 esses of excitation and response encounters the same difficulties experi- 

 enced in the study of groups of sensory endings or motor nerve cells 

 (Adrian f Adrian and Zotterman ;^ Adrian and Bronk*) . 



The solution here is the same as there : that is, to isolate and measure 

 the activity in a single fiber. Only when this is done can one observe 

 the more or less rhythmic train of impulses discharged from the chem- 

 ically modified region (figure 3). The difficulties inherent in this 



i 



Figure 3. Above: Impulses recorded from branch of sciatic nerve of frog stimulated by topical ap- 

 plication of isotonic sodium citrate. Below: Impulses recorded from single « fiber dissected from 

 this nerve. Time in 1/5 seconds. 



experimental procedure partly explain the relative paucity of our 

 knowledge regarding the nature of chemical excitation. 



It is worthy of emphasis that an axon possesses the capacity (as does 

 a cell body or sensory ending) for transforming the continuous environ- 

 mental action of a physical or chemical agent into a series of recurrine 

 events which are made manifest as nerve impulses. 



It has been said that the frequency of impulses developed varies from 

 fiber to fiber, and depends upon the intrinsic characteristics of each 



