BRINK AND OTHERS: CHEMICAL EXCITATION OF NERVE 469 



the nerve makes the calcium-deficient region negative to the adjacent 

 parts of the cell. Under these conditions, the associated current flow 

 is inward across the plasma membrane in the hyper-excitable region 

 of the axon. The conducted impulses still occur, but at a reduced 

 frequency, compared with the activity initiated by a solution contain- 

 ing less potassium chloride. The frequency of response is lower, the 



3 



=! -I 



s 



20 30 



MINUTES 



FiGUBE 11. The depression by KCl of the activity produced by topical application of isotonic 

 sodium citrate is related to the associated decrease in membrane polarization. 



Upper curve is potential difference between treated region of nerve and normal part of nerve. 

 At the first arrow, the solution was changed from isotonic sodium citrate to one containing the 

 same amount of citrate, but with 15 mM K in place of some of the sodium. At the second 

 arrow, the ner\'e was returned to a solution containing isotonic sodium citrate without potassium. 

 At the third arrow, the solution containing potassium was again applied to the nerve. 



The lower curve shows change in frequency of response recorded from a single fiber in this 

 nerve. A similar suppression of activity during the first cycle of depolarization was observed, but 

 not recorded. 



higher the concentration of potassium chloride, and, therefore, de- 

 creases, as the degree of depolarization increases. When the calcium- 

 deficient part of the cell is made sufficiently negative, the activity is 

 suppressed, but it begins again as this depolarization is removed by 

 washing out the potassium chloride (figure 11). Conversely, if this 

 region of the nerve is made positive to adjacent parts, as by a solution 

 of sodium thiocyanate, the frequency of the impulses is increased. 



Thus, small differences of potential between a normal and hyper- 

 irritable region of an axon modify the frequency of the impulses dis- 



