218 THE BIOPHYSICAL PROBLEM OF NERVE CONDUCTION 



showing that the external surface of the nerve is an electric equipotential 

 surface. 



If, however, the stimulus, an electric discharge across A, is adjusted 

 so as to excite a single nerve impulse (i a ), it signals its arrival at x\ by a 

 recordable impulsive deflection of the electric recorder E. The deflec- 

 tion takes place in such a direction as to indicate that a unidirectional 

 impulsive current i\ is traveling in the external circuit from d to c. After 



o 



OL 



Direction 



of motion 



Time 



Current flows from 

 c to d 



2 4 



Current flows from 

 d toe 



-2 



-4 



Units of time l/lOOO sec 



J | | | L 



Fig. VI-7. 

 record. 



Diphasic action potential reproduced to scale from an oscillograph 



a very short interval of time a second impulsive electric deflection is 

 observed, indicating that another momentary current pulse i 2 has 

 passed in the opposite direction, i.e., from c to d. This reversed electric 

 deflection takes place the instant the pulse of nerve excitation passes the 

 point x 2 . These externally recordable successive impulses of current, 

 with the two phases of change in potential, are called the diphasic action 

 -potential. A scale drawing of this diphasic action is shown in Fig. VI-7. 

 The time interval from crest to crest is proportional to the distance 

 between the contacts c to d. The electrical implication is that the pulse 

 of nerve excitation lowers the normal potential V 2 to V\ on passing under 

 the contact x\ ; hence, a flow of current i\ from d to c through the external 

 circuit takes place. After the destructive activity has passed the point 

 X\, restoration sets in, accompanied by a slower return of the potential 

 to its original value V 2 . 



After a short interval, depending on the distance X\ — x 2 , the nerve 

 pulse reaches x 2 , where in turn the normal surface potential V 2 at d is 

 lowered to V\, accompanied by the reversal of the previous electrical 

 phenomena. 



