THE PHENOMENA OF CONDUCTION IN NERVE 



137 



f^ 1 B 



When stimulated and made to conduct, nerve tissue invariably 

 exhibits a current of action, the region of the impulse being galvano- 

 metrically negative to the resting portion of the nerve. This may be 

 proved by first deviating the needle of the galvanometer by a current 

 of injury (Fig. 79) and then stimulating its distant end with an induc- 

 tion shock (S). As the wave of negativity reaches the plus pole (A) 

 of the current of injury, it reduces its potential and causes a partial 

 reversal of the current of injury. The needle of the galvanometer then 

 swings toward and beyond zero. Immediately, thereafter, the needle 

 assumes its former position, namely at a time when the wave of nega- 

 tivity has arrived at the negative injured cross-section of the nerve (B). 

 Consequently, the current of action in nerve is diphasic. 



The diphasic character of the action current may be shown most advanta- 

 geously by placing both leads of the galvanometer upon the longitudinal surface 

 of the nerve (Fig. 80). This system is 

 isoelectric, because both uninjured points 

 A and B have the same potential. If the 

 nerve is now excited at S with a single in- 

 duction shock, the wave of negativity re- 

 sulting therefrom, will cause a deflection 

 of the needle when it reaches A, because B 

 is still positive. A moment, thereafter, a 

 reversal will take place, B now being 

 negative and A positive. 



In harmony with the results obtained 

 with the help of the rheoscopic frog pre- 

 paration, the action current of nerve may 

 also be employed as a stimulus for a neigh- 

 boring nerve. If a short segment of a 

 nerve (A) is placed next to the nerve of a 

 nerve-muscle preparation (B), the stimu- F 10 - 81. SCHEMA TO SHOW How A 

 lation of A invariably gives rise to a con- NERVE-MUSCLE PREPARATION (B) MAY 

 traction of the muscle. In explanation of S^M^LATED BY AN ACTION CUBKENT 

 this phenomenon it must be mentioned 



that the contraction of muscle B is effected in an indirect maner, i.e., the stimu- 

 lation of nerve A gives rise to an action current which serves as a stimulus for 

 nerve B. The impulse set up in nerve B then descends to the muscle and causes 

 it to contract. It is to be noted, therefore, that the impulse in nerve B is not 

 continuous with the first, but is developed in a manner similar to that of an induced 

 current in the secondary coil of an inductorium. The impulse (action current) 

 traversing nerve A, induces an impulse in nerve B. 



Action currents may also be detected in peripheral nerves if the corresponding 

 area of the cerebral cortex is stimulated. This result is also obtained if the corre- 

 sponding anterior root of the spinal cord is used instead. Sensory nerves are to 

 be preferred for experiments of this kind, because the stimulation by means of the 

 electrical current may then be dispensed with. Thus, Kiihne and Steiner 1 have 

 detected negative variations in the optic nerve whenever the retina was exposed to 

 light, while Steinach 2 has noted similar fluctuations in the sciatic nerve of the 

 frog on stimulation of the tactile receptors of the foot. In the sensory nerves of 

 the lateral organ of fishes these currents have been observed by Fuchs. 3 Records 



1 Untersuchungen aus dem physiol. 



2 Pfluger's Archiv, Ixiii, 1896, 495. 



3 Ibid., Ix, 1895, 173. 



Inst. zu Heidelberg, iv, 1881, 64. 



