CONDUCTION OF THE NERVE IMPULSE 



FIG. 20. Abolition of the action potential of a single node by pulses of inward membrane current. 

 The lower trace in each record indicates the time course of the voltage applied between No and Ni 

 in the diagram of fig. 16, top. The amplitude of the recorded action potential was approximately 100 

 mv. Time marks in msec. A toad nerve fiber at io°C. [From Tasaki (126).] 



tential necessary to initiate an action potential. The 

 second pulse of inward current is applied during; the 

 falling phase of the action potential and lowers the 

 membrane potential down to various levels. 



When the change in the membrane potential 

 caused by the second pulse is slight (records B, B', C), 

 the potential rises after the end of the pulse back to 

 the level which might have been reached if the sec- 

 ond current pulse had not been applied. When the 

 membrane potential is lowered by the second pulse 

 below a certain critical level (records C, D), the po- 

 tential does not rise after the end of the pulse but falls 

 further to the potential level of the resting membrane. 

 At the critical intensity of the second pulse (record 

 D'), the membrane potential in .some instances rises 

 to the level of the active membrane and in others ''alls 

 to the level of the resting potential. A further increase 

 in the intensity of the second pulse lowers the mem- 

 brane potential below the resting potential (record E, 

 E'); however, after the end of the pulse, the mem- 

 brane potential rises and settles usually at the level of 

 the resting potential. 



Similar records of abolition of action potentials 

 have been taken from a squid giant axon which has 

 been treated with intracellularly injected tetraethyl- 

 ammonium chloride. This chemical when applied ex- 



ternally is known to prolong the duration of the ac- 

 tion potential of the frog nerve and muscle fiber (46, 

 78). Prolonged action potentials of the squid or of 

 the toad motor nerve fiber show a remarkable resem- 

 blance to the action potential of the heart muscle. 

 When the action potential is prolonged as it is in these 

 cases, the time constant of the membrane is far 

 shorter than the duration of the action potential and 

 the demonstration of the phenomenon of abolition is 

 thereby made easy. 



It is seen that the critical potential le\el for aboli- 

 tion gradually rises during activity. Toward the end 

 of the prolonged action potential, the critical level for 

 abolition is close to the level of the ' shoulder' of the 

 action potential at which the membrane potential 

 starts to fall rapidly. 



It is an interesting fact that the action potential 

 which has been abolished in its very early phase 

 leaves behind it no refractory period. This is shown 

 by the superposed record in figure 21. Record A in 

 the figure is an ordinary unabolished action potential 

 of a single node of the toad motor fiber. When this 

 action potential is abolished in its later stages by a 

 pulse of inward current through the node (record B), 

 there is a relati\ely refractory period following this 

 prematurely terminated response; a strong current 



