546 



NERVE. 



motive effects produced in nerve by an electrical current or a series 

 of currents may consist of the algebraic sum of (1) the excitatory 

 electrical response, (2) the similarly directed cathodic extrapolar effects, 

 and (3) the oppositely directed anodic effects. The similarity existing 

 between the excitatory electrical response and the cathodic extrapolar 

 change has led Boruttau to frame the hypothesis that the excitatory 

 response may itself be regarded as an electrolytic change of the kind 

 produced at the cathode of a polarising current ; x the hypothesis will 

 be referred to again later. With regard to the causation of these 

 currents, it is evident that internal 'polarisation (i.e. formation of 



electrolytic products 

 ,£ ^j^zr^ <ff around and within the 



y?^i\ f \ ^2ri\ conducting core) must 



[ \ I /, 1 [ produce extrapolar cur- 



rents during the passage 

 of the polarising cur- 

 rent. The polarisation 

 operates in two ways — 

 first, by setting up new 

 Fig. 283. — Spread of current into extrapolar regions during electromotive Sources 



irL fl 7to°C. a p ° larising cm ' rcnt through the " 10del around and within the 



internal core, in the 

 manner already described ; and, secondly, by bringing into play, through 

 the formation of ions, an increased resistance to the flow of the polar- 

 ising current through the envelope into the core. This increased 

 resistance causes the lines of current flow to spread more and more 

 into the extrapolar region, in order to enter and leave the core respect- 

 ively. 2 The spread is indicated in Fig. 283 ; it obviously will produce 

 extrapolar surface currents of the same direction as those previously 

 referred to. 



In nerve such changes observed during closure in the extrapolar 

 regions constitute the familiar phenomena of electro tonus discovered 

 by du Bois-Beymond in 1843. 



The electromotive changes of electrotonus. — On passing a gal- 

 vanic current through a portion of medullated nerve, the extrapolar 



regions show an 

 alteration of electri- 

 cal state ; the new 

 condition on the side 

 of the anode is 

 termed anelcctro- 

 tonvs, on that of the 

 cathode catclcct.ro- 

 tonus. As in the 

 polarisable model, 

 these changes are such that a current Hows through the nerve towards 

 the anode and away from the cathode ; each part nearer the anode is 

 thus galvanometrically positive to a more remote one, whilst each part J 

 nearer the cathode is galvanometrically negative to a more remote one. 



If the nerve is an excised one with a cross section at each end, then, | 

 as shown in Fig. 284, the anelectrotonic change, being of the same| 



1 Boruttau, Arch. f. d. ges. Physiol, Bonn, 1894, Bd. lviii. S. 29. 



2 Grunhagcu, ibid., 1873, Bd.'viii. S. 419. 



■> c 



Fig. 284. — The thin arrows (1) represent the electrotonic cur- 

 rents present during the flow of a polarising current through 

 a _ nerve from A to C ; the dark arrows (2) represent the 

 direction of the demarcation currents. 



