ABRAHAM M. SHANES 



165 



is indicated by its small penetration after complete equilibration as compared 

 to a number of other molecules and ions. The apparent volume occupied by 

 sucrose, compared to other substances, is shown in table 3. The proportion of 

 nerve sodium and chloride contained in the axons has been evaluated by the 

 kinetics of radioisotope exchange in the steady state following preliminary 

 equilibration in 'hot' Ringer's. As may be seen in figure i, radiosodium first 

 emerges rapidly and then more slowly and logarithmically with time; under 

 conditions which raise the sodium content of the fibers, the kinetics of the 

 slow component is unchanged, but the zero time intercept obtained by extra- 

 polating the slow component is increased by a relative amount which can be 

 shown to be consistent with the conclusion that the intercept of the slow com- 



Table 3. Sodium and potassium levels and different spaces* of desheathed toad 



NERVES equilibrated IN RINGER'S 



The means are given with their standard errors and the number of nerves involved. All 

 data are relative to the final wet weight of tissue. 



* 'Space' is here emploj-ed purely as a relative index of the amount of radioisotope taken 

 up; it may be defined as the fraction of a milliliter of nerve water which would contain the 

 radioactivity found in one gram of nerve, after complete equilibration, were the radioactivity 

 per milliliter the same as in the medium. 



ponent of untreated as well as treated nerve is a correct measure of axonal 

 sodium. The same conclusion may be drawn for chloride, for which the general 

 configuration of the emergence curve and the temperature coefficient duplicate 

 that for sodium (fig. 2). Assuming complete ionization, and including the 

 analytical figures for potassium (52), we obtain the concentrations in the axons 

 shown in table 4, where they are compared with those of the medium. Potassium 

 and chloride concentrations differ in direction from that in the medium in 

 accord with equation i or 2, but not to the same extent as demanded by purely 

 passive behavior. ^Moreover, their concentrations do not conform to a resting 

 potential of the order of 70 millivolts which has been estimated for single 

 frog fibers (26). For this potential difference [K]i should be 26 /xM/ml and [Cl]i 

 7 ^M/ml. The actual value of [K]i requires E to be 112 millivolts, but if chloride 



