102 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



The Action Current or Negative Variation. Du Bois-Rey- 

 mond proved that when the excised muscle or nerve is stimulated 

 its demarcation current suffers a diminution or negative variation. 

 If, for instance, the excised nerve gives a demarcation current suf- 

 ficient to cause a deflection in the galvanometer of 50 mms., then 

 if the nerve is stimulated by a series of induction shocks the galva- 

 nometer will show a lessened deflection, say, one of 40 mms. The 

 negative variation in this case is equal to 10 mms., on the scale of 

 the galvanometer used. It has been shown that this negative varia- 

 tion is due to a current in the opposite direction whose strength, in 

 the example given, relative to that of the demarcation current is 

 as 10 to 50. Frequently the phenomenon of the negative varia- 

 tion is known also as the action current. The explanation given 

 for this action current is that the nerve or muscle when excited 

 takes on an electrical condition which is negative as regards any 

 unexcited or less excited portion of the nerve. The effect upon the 

 demarcation current is illustrated in the accompanying diagram. 



The demarcation current in a nerve is led off to a galvanometer 

 by electrodes placed at b and c. When the nerve is stimulated at 

 a the excitation set up passes along the nerve, and wherever it may 

 be that portion of the nerve is thrown into an electronegative condi- 

 tion. When this condition reaches a point at which it can influence 

 the galvanometer that is, when it reaches 6, it will dimmish the 

 difference in potential that exists between b and c, and therefore 



reduce the current 



_4_ flowing from b to c. 



Bernstein* has 

 shown that this neg- 

 ative condition 

 moves in the form of 

 a wave. That is, at 

 any point the nega- 



Fig. 45. Schema to indicate the method of detecting tivity grOWS to a 



the action current in a stimulated excised nerve: 6 and c, maximum and then 



the leading off electrodes, one on the longitudinal, one on . . 



the cut surface; the demarcation current passes through diminishes. More- 



the galvanometer, g, in the direction of the arrows ; a, stimu- . , 



lating electrodes from induction coil ; the stimulus causes a OVCr, it travels at B, 



negative condition, which passes along the nerve; when , fi ., T ^1 ~ : * , 



this reaches b it causes a partial reversal of the demarca- aennite VClOClty 



tion current, giving the negative variation or action cur- w hi c h j s easily 



measured. Accord- 

 ing to his experiments, the velocity of this wave in the frog's 

 motor nerve is from 25 to 28 meters per second, and the length 

 of the wave is about 18 mms. Hermann, on the contrary, be- 

 lieves that, in the excised nerve at least, the length of the wave 

 may be greater, reaching perhaps 140 mms. 



* Bernstein, " Untersuchungen iiber den Erregungsvorgang im Nerven 

 und Muskelsysteme," Heidelberg 1871. 



