ICHIJI TASAKI AND CONSTANTINE S. SPYROPOULOS 



213 



membrane resistance at the peak of activity was about 10 ohm-cm^. At 

 about 10°C, the membrane resistance at the peak of activity was approxi- 

 mately 60% higher than the vahie at room temperature. 



Thresholds. The word 'threshold' does not mean much unless one speci- 

 fies a) the manner in which the stimulus was delivered to the nerve fiber 

 and also b) the time course of the stimulating current or voltage. With 

 the experimental set-up of figure lA and using rectangular voltage pulses 

 across the two pools as stimuli, it was found that with a fall in temperature 

 the rheobasic voltage (threshold for pulses of 10 msec, duration) remained 

 almost unchanged (or sometimes was slightly decreased) while the thres- 

 hold for short pulses increased considerably. With extremely brief shocks 

 (for which the constant quantity law holds) the threshold doubled for a 

 decrease in temperature of 13 degrees. 



Fig. 6. Effect of temperature 

 upon the strength-latency rela- 

 tion of the node of Ranvier. V 

 indicates strength of stimulus in 

 mv. and t, the time from the 

 beginning of the stimulus to the 

 appearance of the response. 

 From /. Neurophysiol. 11: 1948. 



The temperature dependence of the threshold membrane potential at 

 the node (29) has not been accurately measured. But, since both the axo- 

 plasmic resistance and the resistance of the nodal membrane increased as 

 temperature was lowered, the temperature independence of the rheobasic 

 voltage mentioned above indicates that the threshold membrane potential 

 did not vary appreciably with temperature. When only a part of an inter- 

 node of a fiber was cooled, the rheobasic voltage measured at this cooled 

 internode was appreciably increased (27). 



The strength-duration relation of a fiber (i.e. the relation between the 

 threshold strength and the stimulus duration) resembled very closely the 

 strength-latency relation obtained under the same experimental condi- 

 tions. In figure 6 is shown the relationship between the latency of action 

 current, t, and the voltage, v, of a long rectangular pulse measured by the 

 method of figure lA at two different temperatures (8). It is clear that the 

 latency prolonged markedly as temperature was lowered. This effect was 

 completely reversible unless the nerve fiber was maintained at extreme 

 temperatures for a long period of time. Our present interpretation of this 

 temperature effect upon the latency is as follows. 



