104 



THE PHYSIOLOGY OF MUSCLE AND NERVE. 



effects during the passage of an electrical current from a battery. Ordinarily, 

 however, when it is only desired to stimulate a nerve or mucle, metal (plat- 

 inum) electrodes are employed. 



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

 mend 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 h 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 b, it will diminish the 

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



reduce the current 

 _^ 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- 

 tivity grows to a 

 maximum and then 

 diminishes. More- 

 over, it travels at a 

 definite velocity 

 which is easily 

 measured. Accord- 

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

 motor nerve is from 25 to 2S meters per second, and the length 

 * Bernstein, " Untersuchungen uber den Erregungsvorgang im Nerven 

 und Muskelsysteme," Heidelberg 1S7L 



Fig. 46. — Schema to indicate the method of detecting 

 the action current in a stimulated excised nerve: b and c, 

 the leading off electrodes, one 9n the longitudinal, one on 

 the cut surface; the demarcation current passes through 

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

 lating electrodes from induction coil; the stimulus causes a 

 negative condition, — which passes along the nerve; when 

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

 tion current, giving the negative variation or action cur- 

 rent. 



