BRINK AND OTHERS: CHEMICAL EXCITATION OF NERVE 473 



sodium chloride. When the frequency of response had become con- 

 stant, the sodium chloride solution was replaced by one in which some 

 of the sodium chloride had been replaced by sodium citrate. The fre- 

 quency of impulse discharge then increased, as is shown. AVith each 

 further increase in the proportion of sodium citrate, there was a further 

 increase in the average number of impulses per second. When the 

 nerve was subsequently returned to solutions containing successively 

 smaller concentrations of citrate, there was a parallel decrease in the 

 impulse frequency. Finally, in isotonic sodium chloride, the initial 

 low degree of activity was resumed. This decrease in average fre- 

 quency, associated with the return to isotonic sodium chloride, is obvi- 

 ously not due to a restoration of calcium to the nerve. It seems prob- 

 able, therefore, that a nerve made active by removal of calcium is sen- 

 sitive to changes in the concentration of the citrate ion. This contrasts 

 with the previously mentioned lack of effect of citrate upon a nerve in 

 the presence of Ringer's proportion of calcium ions. 



In a similar manner, the stimulating action of sodium thiocyanate 

 is enhanced by first removing some of the calcium from the nerve. 

 Also, tetraethyl ammonium chloride will stimulate frog nerve,^^ and 

 we have found that its effectiveness in initiating impulses is greater, 

 if the axon is sensitized by preliminary removal of some of the calcium. 



Another quaternary ammonium salt of interest in this discussion 

 is acetylcholine. Lorente de No has shown^^ that it does not alter the 

 membrane potential of frog nerve, even in massive concentrations. On 

 the other hand, Nachmansohn argues for the possibility of such an ac- 

 tion, on the grounds that cholinesterase-inhibiting agents, which should 

 permit the accumulation of acetylcholine, do cause depolarization of 

 squid nerve. ^^ In our experience, acetylcholine does not induce a dis- 

 charge of impulses when it is applied to the axons of a peripheral frog 

 nerve trunk, and we have not been able to increase the frequency of 

 chemically excited impulses by adding acetylcholine to the calcium- 

 deficient fluid. Also, we have investigated the effects of this substance 

 on mammalian nerve, by perfusing the stellate ganglion of a cat. In no 

 case have we found any evidence that impulses are thus initiated in the 

 pre-synaptic fibers within the ganglion, even though as much as 500 

 micro-grams of acetylcholine were added to each cc. of perfusion fluid. 

 This was determined by observing that no impulses were discharged 

 over the fibers of the preganghonic trunk (figure 15). Finally, there 

 remains the contrasting and significant observation, that much lower 

 concentrations of acetylcholine do cause the discharge of rhythmically 

 recurring impulses in the post-synaptic neurons. The cell bodies or the 



