ICHIJI TASAKI AND CONSTANTINE S. SPYROPOULOS 



211 



primarily by the amount of capacitative distortion by the myelin sheath 

 between the node under investigation and the grid electrode and did not 

 reflect the rapidity of the process at the node. It is not possible at present 

 to determine the temperature effect ujion the rising i)hase of the nodal ac- 

 tion potential. 



The end of the absolute refractory period of a node was found to coincide 

 fairly well with the end of the nodal action potential (15, 2). Since this 

 was true at all the temperatures examined, it is clear that the value of Qio 

 for the duration of the absolute refractory period is close to 3.5. 



It should be pointed out in this connection that the classical refractory 

 period, i.e. the least interval between two shocks necessary to produce two 



5 10 15 



SHOCK INTERVAL (msec) 



Fig. 4. Left: Effects of temponvture iiiion the dunition of the action ciinent (d), the 

 amplitude of the action cin-ient (h) and the shock-respon.se interval (c) of a motor 

 nerve fiber of the toad. From J. NeurophysioL, 11: 1948. 



Fig. 5. Right : Recovery of e.xcitability of a single nerve fiber of the toad measured at 

 four different temperatures. The points at which the dotted curves cross the abscissa 

 indicate the absolute refractory period of the node. From Biochim. et biophys. acta 

 3: 1949. 



propagated responses, is slightly longer than the true refractory period of 

 the node (15). In figure 5 the effect of temperature changes upon the ab- 

 solute and relative refractoriness of the toad nerve fiber is illustrated. 



The end of the scjuid action potential is obscured by the strong 'under- 

 shoot' which follows the main spike. Nevertheless, the Qio for the duration 

 of the absolute refractoriness appeared to be similar to that for the spike 

 duration. 



Action Current. Since the action current is a flow of electricity associ- 

 ated with the production of the action potential, its time course is de- 

 termined by the shape of the membrane action potential and the distribu- 

 tion of resistances and capacities inside and outside the fiber. In the frog 

 motor nerve fiber, the rate of potential rise at an active node is very high 

 (approximately 1500 volts per sec.) and the time constant of the myelin 



