214 INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



When a rectangular voltage pulse is applied across the air gap in the 

 experimental arrangement of figure lA, the voltage across the nodal mem- 

 brane is expected to rise first slowly, then quickly and finally to approach 

 a steady level (32) . The time required for this sigmoid ascent of the mem- 

 brane potential is determined primarily by the cable property of the 

 myelinated nerve fiber. The capacities of the nerve fiber do not change 

 with temperature, but the resistances increase as temperature drops. For 

 this reason, the time required for the spread of potential increases with a 

 fall in temperature. Even at about 5°C, the process of excitation at the 

 node appears to progress in a period of time much shorter than the time 

 required under these experimental conditions to raise the membrane po- 

 tential to the threshold value (31, 28). A recent observation on the action 

 potential of a single node (29) is in good agreement with the view that 

 the action potential starts approximately at the moment when the mem- 

 brane potential reaches the critical, threshold level. It is reasonable, there- 

 fore, to attribute the change in both the strength-latency and strength- 

 duration relations with temperature mainly to the change in the cable 

 constants of the resting nerve fiber. 



It is well known that a nerve fiber fails to respond if the stimulating 

 voltage is increased at a rate smaller than a certain minimal gradient. 

 This minimal gradient, which was determined with the arrangement of 

 figure ID, decreased with a fall in temperature (9). A strange fact about 

 the temperature dependence of the minimal gradient is that, unlike the 

 spike duration and thresholds for brief shocks, it shows a definite hystere- 

 sis with changing temperatures (9). 



In the squid giant axon, the threshold membrane depolarization was 

 found to decrease slightly as the temperature was lowered. The decrease 

 was about 50% for a drop of temperature from 22° to 6°C. When an axon 

 was stimulated by pulses of constant membrane current, a full-sized ac- 

 tion potential was developed when the membrane potential reached this 

 critical threshold level. In other words, the threshold membrane potential 

 is practically independent of the intensity and of the duration of the cur- 

 rent pulse (in normal axons). 



Conduction Velocity. The value of Qio for the conduction rate in tlie 

 frog myelinated fiber was approximately 1.8 (3, 8). Since a nerve impulse 

 propagates along the fiber as the result of successive electric excitation of 

 the nodes (15), the temperature dependence of the conduction velocity 

 was directly related to the effect of temperature changes upon the latency 

 of the appearance of the action current at the individual nodes (fig. 6). 

 This problem was discussed in the previous section. When a node of a 

 fiber is thrown into action, a 'wave' of a rapid change in membrane poten- 

 tial spreads toward the adjacent node. When the potential at the latter 



