BRINK AND OTHERS: CHEMICAL EXCITATION OF NERVE 465 



thousands of impulses (figure 7). This initial high frequency dis- 

 charge usually occurs after a previous period of chemical excitation that 

 had been arrested by the restoration of calcium (figure 6). It also 

 occurs during the actual restoration of calcium.^ 



A nerve which has been once modified by the withdrawal of cal- 

 cium continues to give such a response of high initial frequency to a 

 successive activation, even though it has been in unmodified Ringer's 

 fluid for many hours (figure 6). Whether the impulses start at a high 

 frequency that declines, or whether the frequency gradually increases, 

 the final, sustained average frequency is about the same for a given 

 stimulating fluid. This frequency is, to an important degree, deter- 

 mined by the calcium content of the nerve, and our experiments also 

 suggest that it is, in part, dependent upon the rate of removal of cal- 

 cium. This latter factor may be especially important in the determina- 

 tion of the transient changes of frequency. 



Ill 



The resting metabolism of nerve has long been thought of as neces- 

 sary for maintaining the organization of its unstable structure against 

 the tendency of the structure to become disorganized. In accordance 

 with this view, the less stable structure resulting from the withdrawal 

 of calcium should have a higher metabolic requirement for its main- 

 tenance. This increased metabolism has been observed.^" We, too, 

 have made such measurements of the oxygen consumption of nerve 

 from which varying amounts of calcium have been withdrawn, while, 

 at the same time, measuring the excitability and recording any impulses 

 that were initiated." Alterations of calcium content in the nerve, suffi- 

 cient to cause a lowered threshold to electric stimuli, but insufficient to 

 cause the rhythmic discharge of impulses, induce an increased oxygen 

 consumption (figures 8 and 9). As the calcium content is further 

 reduced, the oxygen consumption increases still more. Finally, a level 

 of calcium content may be reached which is sufficiently low to cause 

 the rhythmic discharge of impulses, and associated with this calcium 

 content there is a still higher oxygen consumption (figure 8) . 



This progressive increase- of oxygen consumption of nerve with de- 

 creasing concentrations of calcium, starting at calcium levels too great 

 to permit the development of spontaneous activity, raises the question 

 as to the meaning of the term, "resting oxygen consumption." The 

 oxygen consumption of axons which are resting, in the sense of not 



conducting impulses, may be quite different in different chemical 

 environments. 



