CONDUCTION OF THE NERVE IMPULSE 



83 



^ I// h'^ ^ 



FIG. 6. Action potentials of a squid giant axon elicited by 

 stimulating shocks at the two ends, A and B, of the axon. The 

 recording micropipctte was pushed into the axoplasm through 

 the axon membrane. Demonstration of two-way conduction 

 {top'), refractoriness (jniddli) and collision of impulses {hol- 

 torri). Temperature, '2o°C. (Discussion in text.) 



rent developed by fiber I through the surface mem- 

 brane of fiber II. They also demonstrated that the 

 velocity of an impulse in fiber II is afTected by the 

 impulse in fiber I when the amount of the fluid is 

 reduced and when the two impulses are not spatially 

 far apart. 



Arvanitaki (9) and Tasaki (124) showed that, 

 under special experimental conditions, it is possible 

 to make an impulse jump from one fiber to another 

 by leading; the action current of one fiber through the 

 other. 



CABLE PROPERTIES OF THE INVERTEBR.\TE .^XON 



It is easy to introduce a small glass pipette or a set 

 of metal wires longitudinally into a squid giant axon. 

 By using such internal electrodes, electric properties 

 of the giant axon have been extensively investigated 



by a number of physiologists. We shall discuss in this 

 section some of the basic observations which serve to 

 clarify electric properties of the resting giant axon 

 (fig. 8). 



When a glass pipette electrode of about 100 \l in 

 diameter is inserted longitudinally into a giant axon, 

 it is found that the potential of this electrode (relative 

 to the large ground electrode in the surrounding sea 

 water) goes down gradually as the pipette electrode 

 is advanced along the axis of the axon. The potential 

 inside the axon is negative to (i.e. lower than) that 

 of the surrounding fluid medium. When the electrode 

 is advanced more than about 10 mm from the point 

 of insertion on the surface membrane, the potential 

 level of the axoplasm is practically independent of 

 the position of the tip of the pipette. In other words, 

 the space occupied by the axoplasm is practically 

 equipotential. The potential difference between the 

 surrounding fluid medium and the axoplasm deter- 

 mined by this or other .similar methods is called the 

 'resting memljrane potential'. 



I2(H 



Exciubility change in fibre [I 



T 



T 



FIG. 7. Top: Electrode arrangement used for demonstration 

 of excitability changes in a single nerve fiber of the crab caused 

 by the passage of an impulse in the adjacent fiber. A, B, leads 

 for stimulation of fiber I; C, D, leads for stimulation of fiber II; 

 D, E, recording leads connecting with amplifier and cathode 

 ray oscillograph. Bollom: Excitability changes in fiber II during 

 the passage of an impulse in fiber I. Abscissae: time in msec. 

 Ordinates: threshold intensity of fiber II in percentage of its 

 resting threshold. [From Katz & .Schmitt (73).] 



