55 



NERVE. 



Hermann has extended his inquiry to nerve, and has obtained results 

 which lead to the conclusion that the rate of propagation is so rapid as 

 to imply that electrotonus is instantaneously produced in all parts of the 

 extrapolar regions ; the only differences between distal and proximal 

 parts being in the amount, not in the time relations of the change. 1 



The influence of physiological alterations upon electrotonic 

 currents. The great similarity between the cathodic electrotonic 

 currents and the currents of action has already been alluded to. 

 The striking experiments of Boruttau with the model bring this 

 similarity prominently forward. He found that when induced currents 

 alternating in direction were passed through a portion of the model, 

 the resulting extrapolar effects on both sides were those of catelectro- 

 tonus, i.e., negativity of the part nearest the polarising contacts. This 

 result was not changed when the polarising currents were reversed, and 

 was present when, by means of sinusoidal induced currents, the alternat- 

 ing polarising now was equal in the two directions. 2 It admitted of 

 only one explanation : both the anodic and cathodic electromotive 

 disturbances are present in both extrapolar regions of the model, but, 

 owing to their different intensities of development, the algebraic sum of 

 the two opposing effects is one which shows that catelectrotonus 

 overpowers anelectrotonus. It must be remembered that the essential 

 conditions for the production of the result are the short duration of 

 each individual polarising current, the alternation of these currents, 

 and their frequent repetition. An effect can thus be produced in an 

 artificial model closely resembling the negative variation of nerve. In 

 view of this similarity, it is a matter of great theoretical importance to 

 ascertain the extent to which the physiological conditions of a nerve 

 affect polarisation and electrotonus. 



Influence of excitation upon electrotonus, and of electrotonus upon 

 the electrical response. An excited nerve shows a diminution of the 



electrotonic currents, and if 

 these are the extrapolar pheno- 

 mena of polarisation, then dur- 

 ing excitation polarisation is 

 apparently diminished. 3 The 

 diminution has been deduced 

 from the following experiment : 

 The sciatic nerve (frog) is 

 placed in connection with three 

 independent circuits, the middle 

 set of contacts being that of 

 - the galvanometric circuit, whilst 

 the other two are those of the 

 polarising and exciting circuits 

 respectively. As the galvano- 

 metric contacts are both upon the longitudinal surface of the nerve, 

 the resting difference and the negative variation due to excitation 

 will be extremely small. The polarising circuit is now closed, so 

 as to obtain an anodic extrapolar effect in the region of the galvano- 

 meter contacts. The anodic electrotonic current is thus present in 



1 Hermann and Weiss, Arch. f. d. ges. Physiol., Bonn, 1898, Bd. Ixxi. S. 237-295. 



2 Boruttau, ibid., 1895, Bd. lix. S. 47. 



3 Bernstein, Arch. f. Anat., Physiol. u. vnssensch. Med., 1866, S. 614. 



y 



Anootai 



Y 



Cdt/toclai 

 FIG. 287. 



