FREDERICK CRE8C1TELLI 177 



sitivity of most of the B and some of the C fibers. The differential sensitivity 

 to low sodium is pictured in the next section of the experiment (records 

 7-13) . Eight minutes after electrogram 6 was recorded a solution which 

 was deficient in sodium was added to the nerve segment. This solution 

 had the normal amount of potassium but the NaCl concentration was re- 

 duced to one-tenth that of Ringer's solution. Choline chloride was em- 

 ployed to replace the missing NaCl so as to obtain osmotic equivalence. 

 Record 7, obtained 1 minute after this addition, shows the rapid recovery 

 of many A fibers and of some of the C fibers. Two minutes after the addi- 

 tion (record 8) a majority of the A fibers and a few more C fibers had 

 recovered from the potassium block. While A fiber activity was being 

 restored and maintained (records 7-13), the B group underwent a gradual 

 reduction and finally ceased activity (record 13). In other words the solu- 

 tion with 11 mM NaCl was able to support most of the A but none of the 

 B activity. The solution with 11 niM NaCl caused the C wave to change in 

 form and to assume a double-humped configuration (records 10-13). The 

 changes in the C potential were such as to suggest the existence of two 

 types of C fibers: one group relatively sensitive to high potassium but 

 relatively resistant to the low sodium and a second group with the inverse 

 relation to these two cations. The B and the potassium-resistant C fibers 

 were not ecjually resistant to high potassium. Through the use of a care- 

 fully selected concentration of potassium ions it was possible to demon- 

 strate that these C fibers would even outlast the B group. This point is 

 illustrated in records 17-19 (fig. 1). Record 17 is the control. A solution 

 with KCl at 12 times the Ringer concentration was then added and 

 record 18, obtained 20 minutes later, depicts the complete cessation of 

 activity in the A (only shock artifact remains), the B and many of the 

 C fibers. A significant number of C fibers remained in action, however, 

 even after all B activity had ceased. The addition of Ringer's solution 

 caused complete recovery in all the blocked fibers (record 19). 



Another type of experiment (fig. 2) shows not only the differences in 

 responses of the A and B fillers to sodium and to potassium but also some 

 interesting additional features in the actions of these two ions. The experi- 

 ment was designed so that sodium and potassium were added simul- 

 taneously to the blocked nerve segment and could act competitively, the 

 one (sodium) to produce recovery and the other (potassium) to produce 

 secondary block. The experiment consisted of seven different steps all 

 carried out on the same nerve preparation. For each of these steps the 

 nerve segment was first exposed to a solution lacking NaCl and KCl. 

 Tetramethylammonium bromide (TMA) was employed as the osmotic 

 substitute. Total conduction block, due to sodium lack, occurred in each 

 of these seven steps but this is not included in figure 2. The figure presents 

 only the 'recovery' curves which resulted in each of these seven procedures 



