ICHIJI TASAKI AND CONSTANTINE S. SPYROPOULOS 207 



stimulating circuit was smaller than 1000 ohms. The time constant of 

 voltage rise was, therefore, determined by the product of the capacity of 

 the variable condenser and the 10-kilohm resistor in the figure. The final 

 value of the exponentially rising voltage was given by the voltage divider 

 consisting of the fixed 50-kilohm resistor and the variable resistor. A 

 linearly rising stimulating voltage was produced by adjusting the duration 

 of the square voltage pulse to a value far shorter than the time constant 

 of the exponentially rising potential. By starting the second pulse of the 

 opposite polarity at various intervals after the onset of the first pulse, 

 triangular voltage pulses were produced. 



METHODS USED IN MEASUREMENTS ON SQUID AXONS 



Measurement of Resting and Action Potential. The well-known 

 method of introducing a saline-filled glass pipette longitudinally into an 

 isolated axon (19, 20) was employed. Both the axon and the intra-cellular 

 glass pipette were fixed to a glass slide so that the preparation could be 

 transferred from one vessel of sea water to another. Stimulating shocks 

 were applied to the axon near its end through a pair of metal (usually 

 steel) wire electrodes. 



Measurement of Membrane Resistance and Capacity. A pair of in- 

 tra-cellular metal wire electrodes were used, one wire for passing a cur- 

 rent pulse through the axonal membrane and the other for measuring the 

 change in membrane jiotential caused by the current. The current electrode 

 was usually made from a fine enameled silver wire. The enamel coat at the 

 tip of the current electrode was scraped off for a length of about 10 mm. 

 This electrode was inserted longitudinally into an isolated axon. The axon 

 was about 40 mm in length. The exposed portion (about 2 mm in length) 

 of the potential electrode was located in the middle of the scraped portion 

 of the current electrode. A precaution was taken to avoid direct metallic 

 contact between the current and potential electrodes. 



Measurements were made in three ways. In one case, a strong short 

 l)ulse of inward current was applied to the membrane to charge the ca- 

 pacity and the exponential decay of the membrane potential was followed ; 

 the initial rise in potential was a measure of the capacity and the rate of 

 the exponential decay was a measure of the time constant of the mem- 

 brane. The second method of measuring the membrane resistance and 

 capacity was to pass a w'eak constant current through the membrane and 

 to follow the change in membrane potential ; the initial rate of potential 

 change was a measure of the membrane capacity and the final level of the 

 potential, a measure of the membrane resistance. The third method em- 

 ployed was the 'method of voltage clamj)' used extensively by Hodgkin 

 and Huxley (21). The membrane potential was suddenly clamped at a 

 new level, and the intensity of the membrane current necessary to main- 



