212 INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



sheath (0.4-0.5 msec, at room temperature) is far longer than the rising 

 phase of the nodal action potential (17). The initial part of the action 

 current of the fiber, therefore, consists of a purely capacitative flow of 

 current through the myelin sheath between the node in action and the 

 recording partition. 



Previously, one of us (8) reported that the spike amplitude of the ac- 

 tion current recorded by the technique of figure lA decreased with a fall 

 in temperature at a rate of about 30% for a change of 10 degrees (see curve 

 h in fig. 4). Recently, Maruhashi (personal communication) found that 

 this temperature dependence of the amplitude of the action current became 

 negligibly small when the distance between the recording partition and the 

 active node was reduced to about 0.1 mm. Since the resistance of the axis- 

 cylinder varied and the capacity of the myelin sheath did not vary with 

 temperature changes, it is not surprising that the temperature coefficient 

 of the amplitude of the action current was dependent upon the length of 

 the axis cylinder between the active node and the recording partition. 



In the squid giant axon, the current I (t) that flows through the axoplasm 

 when an impulse travels along the axon at a uniform velocity v is given 

 simply by 



where V(t) represents the time course of the action potential and R the 

 resistance per unit length of the axoplasm. The temperature dependence 

 of V (t) and R have been discussed above. As to the temperature effect on 

 the impulse velocity v, we could not find any accurate information in the 

 literature. It is clear, however, that the temperature effects on the remain- 

 ing two terms in the above expression, i.e. R and dV(t)/dt, would tend to 

 reduce the current I (t) with falling temperature. 



Membrane Resistance During Activity. In the range of temperature 

 between 3° and 10°C, the time course of the impedance-loss during activity 

 of a nodal membrane was roughly parallel to that of the nodal action 

 potential or action current (30, 31) . The duration of the period of decreased 

 membrane impedance has, therefore, the same temperature dependence as 

 the spike duration. It is not known whether the maximum impedance-loss 

 during activity of a node varies with temperature or not. 



In the squid giant axon, the maximum impedance-loss during activity 

 was found to decrease with a fall in temperature. The duration of de- 

 creased membrane impedance was found to have a temperature coefficient 

 of the same order of magnitude as that of the duration of the falling phase 

 of the action potential. No attempt was made to estimate the absolute 

 value of the membrane resistance at the peak of activity by the A.C. 

 method. By the method of voltage clamp, it was found that at 22°C the 



