CHAPTER XII 



POLAR EFFECTS OF A GALVANIC CURRENT ; ELECTROTONUS 



A POLARISING current produces changes of excitability not only at its poles but 

 also in the adjacent parts of the nerve, and even some distance away from 

 them. This is due to the fact that owing to spread of current in the extra 

 polar regions as well as at and between the poles, changes of potential are 

 manifested in those regions during the passage of the current, these being accom- 

 panied by physiological changes, viz. increased excitability near the kathode, 

 and diminished excitability near the anode. Such a condition is known as 

 electrotonus ; that obtaining at and near the kathode being termed katelectro- 

 tonus ; that obtaining at and near the anode, anelectrotonus. 



The spread of the electrical changes beyond the poles is illustrated by the 

 following experiment. 



Paradoxical contraction. Dissect out the sciatic nerve of a frog, cutting all 

 the branches save that to the calf muscles, but leaving the cut branch to the 

 peronei muscles as long as possible. Place the cut end of this branch upon 

 non-polarisable electrodes connected with a battery and rheochord and have 

 a mercury key in the circuit. On making or breaking the circuit the gastro- 



PIG. 48. PARADOXICAL CONTRACTION SHOWN BY INDUCED CURRENTS. 



cnemius will contract. Owing to the electrotonic spread of the current in the 

 fibres of the peroneus nerve the fibres to the gastrocnemius in the trunk of 

 the sciatic become stimulated ; in an excitable preparation this will occur even 

 with very weak currents. The experiment can also be performed with ordinary 

 metallic electrodes connected with the secondary coil of an inductorium (fig. 

 48). On stimulating the peroneus nerve by closing the merciiry key the 

 gastrocnemius is made to contract. If the peroneus branch is tied or crushed 

 near its junction with the sciatic the effect can only be got with strong currents. 

 The experiment should be repeated by placing a thread, wetted with Ringer, 

 along the sciatic nerve, and laying its free end on the electrodes. In this case 

 also the excitation caused by spread of current will only show itself with strong 



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