IRRITABILITY OF NERVE AND MUSCLE IN ELECTROTONUS. 66l 



extrapolar anelectrotonus is greater with the feeblest currents than 

 that of extrapolar katelectrotonus. In the case of strong currents, this 

 relation is reversed. 



Fig. 232 exhibits diagrammatically the relations of irritability of a nerve 

 (.V ri) that is traversed by a constant current in the direction of the arrow. The 

 curves are so constructed that the degrees of increased irritability in the vicinity 

 of the kathode (K) are represented as elevations above the line representing the 

 nerve, and those of lowered irritability at the anode (A) as depressions. The 

 curve m o i i, p r represents the irritability of the nerve with strong currents, 

 the curve e f i t h k that with currents of moderate strength and finally a b i c d 

 that with feeble currents. 



The electrotonic effects increase with the length of nerve traversed. The 

 alteration of irritability in katelectrotonus appears at the moment of closure of 

 the circuit. Anelectrotonus develops and extends slowly. Electrotonus is 

 diminished by cold, and by heat up to 40. At 30 katelectrotonus is increased 

 and anelectrotonus diminished. Also with induced currents the anode diminishes 

 the irritability. 



If the polarizing current is opened there is at first a reversal of the 

 conditions of irritability. There then follows a transition to the normal 



d 1c 



K 



n 



FIG. 232. Diagrammatic Representation of the Electrotonic Relations of Irritability. 



state of irritability of the resting nerve. At the initial moment of closure 

 Wundt observed that the irritability of the entire nerve was augmented. 



Testing Electrotonus in Motor Nerves. In order to demonstrate the laws of 

 electrotonus in motor nerves the frog nerve-muscle preparation (Fig. 233), con- 

 sisting of the leg and the sciatic nerve, is employed. By means of unpolarizable 

 electrodes (Fig. 225, IV) the current of a constant circuit is conveyed to the nerve 

 throughout a limited distance. An irritant, such as an electrical shock, or chemi- 

 cal irritation by the application of sodium chlorid, or mechanical irritation, is 

 now applied to the nerve at either the anode or the kathode, and note is made 

 whether the contractions following upon the irritation vary in size when the 

 polarizing circuit is opened or when it is closed. The contractions themselves 

 may be recorded from the gastrocnemius muscle with the aid of the myograph. 

 The following examples may be considered: (a) Descending extrapolar anelectro- 

 tonus, that is with a descending current the irritability at the anode within the 

 extrapolar section is to be tested. If in such a case (A) the irritant, sodium 

 chlorid, which is applied at R while the circuit is still open, gives rise to moder- 

 ately large contractions in the leg, these become feebler, or are abolished, as 

 soon as the constant current is passed through the nerve. After opening the 

 current, the contractions appear again in their original strength, (b} Descending 

 extrapolar katelectrotonus (A). The irritating salt is placed at R,. The contrac- 

 tions induced increase immediately on closure of the polarizing circuit. On open- 

 ing the circuit the contractions' resume their previous activity, (c) Ascending 

 extrapolar anelectrotonus (B). The salt is applied at r^ The contractions of 

 moderate intensity present before closure of the circuit become feebler after 



