FREDERICK CRESCITELLI 193 



example, that the maximum membrane potential was at about 37°C for 

 mammalian A fibers whereas it was at about 25°C for mammalian C fibers. 

 Similar maxima for these fibers was found for the negative after-potential. 

 The idea of temperature-dependent systems varying in this way is at- 

 tractive, of course, because it calls to mind the behavior of enzymes and 

 other proteins in relation to temperature. 



RESPONSES TO SODIUM — ROLE OF THE OSMOTIC SUBSTITUTE 



The low temperature experiments were instructive in demonstrating the 

 importance of sodium concentration on the rate of conduction. This point 

 is brought out for the case of the B fibers in the four experiments (columns 

 A, B, C, D) of figure 10. These action potentials reveal, first of all, the 

 slowing of conduction in the B fibers as low sodium block developed in 

 the test nerve segment (records 1-7, 12-18, 23-27, 34-37). Following com- 

 plete block in each case a solution was added which had the limiting con- 

 centration (11 niM) of NaCl. As already explained, such a solution was able, 

 at the low temperature, to restore much of the potential. Such restoration 

 is displayed in records 8-9, 19-21, 28-32 and 38-42. These records all 

 bring out the extra conduction delay associated with the presence of a 

 solution having a deficiency of sodium. Such a lowering in rate of conduc- 

 tion is expected under conditions of sodium deficiency because, for one 

 thing, the rate of rise of the action potential is lower under such conditions 

 (12). The influence of an increase in sodium concentration on conduction 

 velocity is strikingly demonstrated in records 10, 22 and 33 which were 

 obtained after the addition of Ringer's solution. This involved a change in 

 NaCl concentration from 11 mM to 110 mM. This change, though it caused 

 only little effect in height of B wave in some cases (records 10, 22) , brought 

 about a striking reduction in conduction time. 



Another factor which appears to be involved in these responses to sodium 

 is the nature of the so-called inert osmotic substitute, which is necessary 

 in solutions deficient in NaCl. Both choline and TMA were utilized in 

 separate experiments. Though the responses were qualitatively similar 

 with these two organic ions, the recovery of B activity in the presence of 

 the limiting concentration of NaCl was always greater with TMA. The 

 comparison in behavior to these two ions is summarized in records 33-44 

 (fig. 10). Record 33 is the control with Ringer's solution in contact with 

 the nerve segment. A solution containing choline chloride in place of the 

 NaCl of Ringer's solution was then added to the nerve segment with the 

 result shown in records 34-37. Following complete block of the B fibers 

 a solution was then added which contained 11 mM of NaCl along with 

 99 mM choline chloride and the other constituents of Ringer's fluid. The 

 B activity slowly returned (not presented in the figure) and its maximum 



