192 INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



NaCl (llniM) was added to the blocked nerve segment. This caused a 

 typical selective response with the majority of the A fibers restored to 

 activity while the B fibers remained in block until restored by the addi- 

 tion of Ringer's solution (at R). The same experiment performed with the 

 nerve at 3.2°C (lA, IB) gave entirely different results. In the first place 

 the rate of block of the A and B groups was nearly the same. (In some 

 experiments the A fibers actually blocked at a somewhat greater rate 

 than did the B fibers.) In the second place, though the addition of the 

 limiting concentration of NaCl resulted in a rate of recovery of A fibers 

 slightly lower than the rate at normal temperature, it produced in the 

 B fibers a considerable restoration of activity. As a matter of fact, the B 

 group returned to action at low temperature at a rate faster than the A 

 fibers. In other words, cold actually reversed the differential action of 

 sodium in the restoration of these two groups of fibers. This behavior is 

 singularly unlike the selective responses to excess potassium which, it will 

 be recalled, were unaffected by reducing the temperature. 



The action of cold in increasing the rate of low sodium block of the A 

 fibers relative to the B group might have been the result of a leak and ac- 

 cumulation of potassium. Under these circumstances the A fibers, as al- 

 ready pointed out, being relatively more sensitive to potassium would 

 tend to block faster, relative to the B fibers. It should be added, however, 

 that this effect of cold was obtained under conditions where the treated 

 nerve segment was immersed in fluid and underwent several washings 

 during the addition of the test solution. The accumulation of potassium ions 

 was therefore minimized. In any case this explanation cannot account for 

 the recovery in the cold of many blocked B fibers following the addition 

 of the limiting concentration of NaCl. It appears necessary to invoke some 

 other action of low temperature in order to interpret the shift in differential 

 action produced by cold. One obvious interpretation is to assume that the 

 A and B fiber groups possess temperature-dependent systems in so far as 

 the effectiveness of sodium ions is concerned. If the system for each of these 

 two fiber groups is then assumed to have a temperature optimum with a 

 fall-off on either side of this optimum as for a bell-shaped curve, then it is 

 only necessary to make the additional assumption that the optima for the 

 A and B groups are displaced along the temperature axis with the B opti- 

 mum shifted toward lower temperatures. This scheme could account for 

 the observed behavior to cold, for above the point of intersection of the 

 two bell-shaped curves the A fibers would l)e relatively more sensitive 

 to sodium ions whereas below the intersection the reverse would be true. 

 This interpretation is interesting from one point of view, i.e. that it is 

 so similar to the suggestion of Lundberg (18) who arrived at his conclu- 

 sion from somewhat different types of experiments. He concluded, for 



