206 INFLUENCE OF TEMPERATURE OX BIOLOGICAL SYSTEMS 



ersed first by an outward current and then by a weak inward current. This 

 membrane current was considered to consist of a capacitative component 

 (flowing through C in the figure) and an ohmic component (flowing 

 through R). The nodal action potential (a rounded triangle with a peak 

 value of 100-110 mv) was uniform and the variation among different 

 fibers was found to be small (10). It was possible therefore to determine 

 the values of the capacity and the resistance by separating the membrane 

 current, I(t) in the figure, into its capacitative and ohmic components 

 (for more details of the principle of this method, cf. 17). The principle 

 of the method for determining the capacity and the resistance of the nodal 

 membrane was similar to that for the myelin sheath with the added com- 

 plication that the membrane potential at the inexcitable node in the mid- 

 dle pool was appreciably smaller than the action potential of a normal 

 node. In the frog myelinated fiber, the change in the membrane resistance 

 with increasing membrane potential was far smaller than the change 

 shown by Cole (18) in the squid axon. 



Measurement of Membrane Impedance During Activity. The change 

 in the membrane impedence during activity was measured by an A.C. 

 Wheatstone bridge. The single fiber preparation was mounted on a bridge- 

 insulator as in figure lA. One arm of the A.C. bridge consisted of the 

 preparation and the electrodes in the pools. The distal node, No , was 

 made inexcitable with a narcotic. The change in the membrane impedance 

 associated with the action current of the node Ni was detected as a change 

 in balance of the bridge. The frequency of the bridge A.C. was between 

 2 and 6 kc. This type of impedance measurement was successful only in 

 the range of temperatures below about 10°C, that is, only when the period 

 of the bridge A.C. was far shorter than the duration of the nodal activity. 



Measurement of Threshold. The threshold voltage (rheobase) for 

 the fiber was measured by the use of long rectangular voltage pulses ap- 

 plied across the 1000-ohm resistor in figure lA. With this arrangement the 

 resistance of the internodal segment between Ni and No was connected in 

 series with the source of the applied stimulus. Under ordinary experi- 

 mental conditions, a change in the threshold voltage was a measure of a 

 change in the threshold intensity of current through the nodal membrane. 



Thresholds for short voltage pulses were determined by using the same 

 arrangement (fig. lA). Under the conditions of this experiment, the rise in 

 threshold with decreasing stimulus duration was determined primarily by 

 the capacity of the myelin sheath and the nodal membrane near the stimu- 

 lating air gap. The classical chronaxie is a measure of the time required 

 to charge the nodal membrane up to a certain level which is practically 

 independent of the stimulus duration. 



Thresholds for exponentially or linearly rising voltage pulses were 

 measured by the arrangement of figure ID. The variable resistance in the 



