BRINK AND OTHERS: CHEMICAL EXCITATION OF NERVE 463 



charge of impulses referred to above begins slowly. If the calcium 

 ion concentration is reduced, by adding a calcium-binding agent such 

 as sodium citrate, the discharge begins more quickly and continues 

 longer. Because of this, we have used for many of our experiments a 

 stimulating fluid which contained sodium citrate in place of some of 

 the sodium chloride. When the calcium ion concentration is thus re- 

 duced below about 0.4 mM, impulses are initiated. This is the concen- 



FiGUHE 5. The threshold of an axon decreases with time after topical application (upper arrow) 

 of a solution which lowers the calcium ion concentration. Repetitive activity begins at 14 min. 

 after rheobase has decreased below five per cent of its initial value. Threshold measured with 

 cathode on calcium-deficient region of nerve. 



tration level for activation, whether the calcium be removed by the 

 action of citrate or by the simple process of diffusion. This, and other 

 evidence, suggest that a principal factor in citrate excitation is the low- 

 ered calcium ion concentration. 



We have already stated that the frequency of impulses initiated by 

 the removal of calcium is, at any time, dependent upon the duration 

 of the previous action of the stimulating fluid. The time-course of 

 development of the impulse discharge is also largely influenced by the 

 previous chemical treatment of the nerve. Usually, the impulses be- 

 gin to occur, at random intervals, when the rheobase has fallen to about 

 5 per cent of its initial value. Thereafter, the impulses are discharged 

 in groups, which gradually merge into a more or less regular train when 

 the frequency reaches about 150 per second. Such a gradual increase 

 in the frequency of impulses is shown in one of the curves of figure 6. 



The development of activity is not always so gradual. Sometimes, 

 the initial frequency may be high, and then decline to a lower level 

 that is sustained for some hours, with the development of hundreds of 



