216 THE BIOPHYSICAL PROBLEM OF NERVE CONDUCTION 



galvanometers; if necessary, the potential may be amplified by means 

 of a vacuum-tube voltage amplifier. The difference in potential main- 

 tains itself as long as the " injury " as such does not undergo a change 

 in its physicochemical nature due to degeneration of the tissue. The 

 injury does not produce the difference in potential; it merely allows the 

 difference of potential to manifest itself as an active source of electro- 

 motive force. 



The Action Potential 



Our definite knowledge of the response of living tissue to electrical 

 excitation began with the celebrated investigations of du Bois-Reymond, 

 extending from 1846 to 1860 and published under the title " Unter- 

 suchungen uber thierische Elektrizitat." By 1871 Bernstein had 



Stimulating 

 electrodes 



HI 



+ 

 I 



5 mV 



Direction of 

 activity 



H I I I I I M ill 



-*-i a 



Localized bioelectric current 



-■*• — . . — ■*■ 



a it 

 G 



Fig. VI-5. Monophasic action current pulse is shown traveling from the stimu- 

 lating electrode G. Shading simulates degree of depolarization. Injury potential 

 between A and B neutralized by counter emf from potentiometer P. If E is an 

 oscillograph, the fluorescent screen will show the above monophasic action potential 

 pulse as it passes the contact B. 



demonstrated that when an excised muscle or nerve was stimulated at 

 any point along the tissue a wave of activity ran along the nerve. This 

 wave was identified as a negative pulse of an electrical activity rising to 

 a potential of about 5 millivolts. The existence of a traveling pulse of 

 activity may be demonstrated as illustrated in Fig. VI-5. An excised 

 nerve with its active end A placed in contact with a small metal electrode 

 is connected through a potentiometer P and an electrometer E to a 

 duplicate electrode B attached to the passive surface of the nerve sheath. 

 The normal demarcation current is neutralized by a counter electro- 

 motive force from the potentiometer until the electrometer deflection is 

 zero. 



At G, several centimeters to the left of contact B, is placed a pair of 

 stimulating electrodes. The two platinum points are connected to the 



