;66 



PROOF OF ELECTKOTONUS IN MOTOR NERVES. 



I Proof of Electrotonus in Motor Nerves. To test the laws of electrotonus, take a frog's 

 nerve-muscle preparation (fig. 401). A constant current (p. 542) is applied to a limited part 

 of the nerve by means of non-polarisable electrode?. A stimulus, electrical, chemical (saturated 

 solution of common salt), or mechanical is applied either in the region of the anode or cathode ; 

 and we observe whether the contraction which results is greater when the polarising current is 

 opened or closed. We shall consider the following cases (fig. 409). 



(a) Descending extrapolar anelectrotonus. With a descending current we have to test 

 the excitability of the extrapolar region at the anode. If the stimulus (common salt) applied 

 nt R (while the circuit was open) causes in this case (A) moderately strong contractions in the 



Fig. 408. 

 Scheme of the electrotonic excitability, 

 limb, then these at once become weaker, or disajipear as soon as the constant current is trans- 

 mitted through the nerve. After the circuit is opened, the contractions produced by the salt 

 again occur of the original strength. 



(b) Descending extrapolar cathelectrotonous (A). The stimulus (salt) is at R v and the 

 contractions thereby produced are at once increased after closing the polarising current. On 

 opening it they are again weakened. 



(c) Ascending extrapolar anelectrotonus (B). The salt lies 

 at r x ; the moderately strong contractions excited by the salt 

 before the current is made, become feebler after the current is 

 made. 



(d) Ascending extrapolar cathelectrotonus (B). The salt lies 

 at r. In this case we must distinguish according to the strength 

 of the polarising current : (1) When the current is very weak, 

 which can be obtained with the aid of the rheocord (fig. 379), on 

 closing the polarising current, there is an increase of the con- 

 traction produced by salt. (2) If, however, the current is stronger, 

 the contractions become either smaller or cease. This is due to 

 the fact that with strong currents the conductivity of the nodes 

 is diminished or even abolished (p. 565). Although the salt 

 acts on the excitable nerve, there is no contraction of the muscle, 



/ V / as tbe con( luction f an impulse is prevented by the resistance in 



I I l M the nerve - 



The law of electrotonus may also be demonstrated on a com- 



\m \m pletely isolated nerve. The end of the nerve is properly disposed 



W W upon electrodes connected with a galvanometer, so as to obtain 



W a strong current. If the nerve, when the constant current is 



' closed, is stimulated in the anelectrotonic area, e.g., by an in- 



jv ,qq duction shock, then the negative variation is weaker than when 



\iii <* * li. the polarising circuit was open. Conversely, it is stronger when 



Method ot testing the excita- it is stimulated in the cathelectrotonic area. The currents from 



Diiity in electrotonus. R, tne extrapolar areas of a nerve in a condition of electrotonus, 



r "i> *",, where the com- exhibit the negative variation when the nerve is stimulated 



mon salt (stimulus) is ap- {Bernstein). 



y [Tigerstedt, instead of employing an electrical or chemical 



stimulus to excite the electrotonic nerve, used an apparatus like Heidenhain's tetanometer, 

 whereby the nerve was beaten gently with a small ivory hammer. He fully confirms Pfliiger's 

 results.] 



Proof in Man. In performing this experiment it is important to remember the distribution 

 of the current in the body. If both electrodes, for example, be placed over the course of the 

 ulnar nerve (fig. 410), the currents entering the nerve at the anode (+ a a) must diminish the 



