CONDUCTION OF THE NERVE IMPULSE 1 07 



Fig. 28 



y AMP. 



'MX^^ 





Fig.26 



v-r>. 



FIG. 26. Demonstration of the dependence of ncivous conduction upon the flow of electric current 

 outside the fiber. A. Action current recorded with an amphfier connected between the middle and 

 the distal pools; stimulus given at E. B. Block of conduction caused by replacing the Huid in the 

 middle pool with an 0.2 per cent cocaine-Ringer's solution. C. Restoration of conduction by lifting 

 the middle electrode from the surface of the fluid. Time marks, i msec, apart. [From Tasaki (123).] 



FIG. 27. Demonstration of the effect of a shunting resistance of 20 megohms across the insulated 

 internode upon nervous conduction. AMP represents a high input-impedance preamplifier. Record 

 A was taken with the resistance disconnected; Record B with the resistor connected. [From Tasaki & 

 Frank (128).] 



FIG. 28. Measurement of the safety factor in nervous conduction by narcosis. Top record: Normal 

 binodal action current. Second through Joiirth records: 3, 7, 38 and 38.1 minutes after introduction of a 3 

 per cent urethane-Ringer's solution into the proximal pool. [From Tasaki (124).] 



lifted above the surface of the saline at this moment, 

 there occurs a marked increase in the recorded cur- 

 rent and, at the same time, the tiine course of the 

 current becomes diphasic (record C). In a motor 

 nerve fiber with its innervating; muscle left intact, it 

 is seen that the diphasicity in the recorded current is 

 always associated with propagation of an impulse 

 across the narcotized region in the middle pool. 



The mechanism of restoration of conduction in 

 this experiment is as follows. The portion of the fiber 

 in the middle pool treated with cocaine is inexcitable. 

 The activity of the portion of the fiber in the lateral 

 pool induces a current that spreads along the fiber in 

 the middle pool, but this spreading current is too 



weak to e.xcite the portion of the fiber beyond the 

 middle pool. When the electrode in the middle pool 

 is removed, the leakage of the spreading current 

 through the portion of the fiber in the middle pool 

 is reduced and, consequently, the current that reaches 

 the other side of the middle pool is increased. Thus, 

 the spreading current becomes suprathreshold for 

 the portion of the fiber beyond the middle pool. 



The question has been raised (37, 66, 128, 145) as 

 to whether it is possible to block propagation of a 

 nerve impulse by insulating a nerve fiber between 

 the two neighboring nodes. First, we must discuss a 

 troublesome factor related to the experiment de- 

 signed to answer this question. 



