ICHIJI TASAKI AND CONSTANTINE S. SPYROPOULOS 215 



node is raised to the critical level by this spreading 'wave', an action jio- 

 tential is initiated. The strong temperature dependence of the strength- 

 latency relation discussed above indicates that the major portion of the 

 temperature effect upon the conduction velocity can be explained in terms 

 of the temperature de])cndence of the resistance of the axis-cylinder and 

 of the nodal membrane. There is at present no direct information about the 

 rate of potential rise at an active node and the relation of this rate to the 

 internodal conduction time. 



In the squid giant axon, there is at present no accurate measurement of 

 the temperature coefficient of the conduction velocity. There is little doubt, 

 however, that the coefficient is close to that of the myelinated nerve fiber. 



EFFECTS OF PRESSURE CHANGES ON NERVE FIBER 



Myelinated Nerve Fiber. The most conspicuous effect of high hydro- 

 static pressure upon the myelinated nerve fiber was found to be an increase 

 in the duration of the nodal action current. This effect can best be demon- 

 strated by using the experimental set-up of figure IC. With this method 

 the increase in the temperature of the nerve fiber resulting from compres- 

 sion of the mineral oil in the pressure chamber was minimized by sur- 

 rounding the fiber with a large volume of Ringer. In figure 7 is presented 

 an example of measurements of the spike duration at different pressures. 

 It can be seen that the duration of nodal activity was increased by a fac- 

 tor of about 4.5 when the pressure was raised from atmospheric pressure 

 uj) to 10,000 psi. This effect of pressure upon the spike duration was re- 

 versible. 



The effect of high hydrostatic pressures upon the amplitude of the ac- 

 tion current was not marked. At pressures up to about 7000 psi, usually the 

 amplitude increased slightly or remained almost unchanged. At pressures 

 above 8000 psi, the amplitude became slightly smaller than at atmospheric 

 pressure. In a few experiments there was a slight (up to 10%) decrease 

 in the amplitude even below 7000 psi. The conduction velocity was slightly 

 retarded by high pressures. At 5000 psi it was 5-15% lower than at atmos- 

 pheric pressure. The rheobasic voltage of the fiber was also slightly affected 

 by pressure; at 5000 psi it was increased by 5-20%. When the high pres- 

 sure was maintained for a long period of time, the rheobasic voltage was 

 found to increase progressively. 



Squid Giant Axon. The effects of high pressure upon the squid action 

 potential were similar to those on the frog nerve fiber. At high pressures 

 both the duration of the rising phase and the amplitude of the action po- 

 tential increased slightly while the duration of the falling phase was 

 increased very markedly. There was a complicating factor, however, in 

 the squid axon. At about 5000 psi the axon started to fire impulses spon- 



