192 Tait and Gunn 



Experiments with a rapid rate of stimulation have been carried out 

 on numerous occasions. Neglecting in the meantime those in which the 

 enormously high rates afforded by the discharge of Leyden jars, etc., have 

 been used, where the interval between the individual stimuli is of a 

 different order of magnitude from the refractory period, we shall mention 

 experiments where the rate of stimulation has been over 400 a second and 

 yet not greater than tens of thousands. 



Bernstein (5), using a rate of 500 a second, obtained only a single 

 initial contraction of the muscle. Roth (6), with a rate of 1000-5000, 

 obtained tetanus. Langdon and Schenck (7), with a rate of 1800-2000 

 per second, also got tetanus. Kronecker (8), using a special device whereby 

 he claimed to attain a rate of 20,000 per second, found tetanus, which on 

 repetition became an initial twitch and subsequently failed. From the want 

 of uniformity in these results it was for a time difficult to draw any general 

 conclusion. 



Of late, however, thanks to the work of Wedensky (9) and of 

 Frohlich (4), it has become possible to reach a definite generalisation. 

 Wedensky, who happened to combine the method of rapid stimulation 

 with anaesthesia of the nerve, found that strong excitations at a rate of about 

 100 per second applied to the proximal end of a nerve whose middle portion 

 is deeply ansesthetised, produces simply a single twitch of the muscle. 

 Frohlich pointed out that this twitch is of the same height as the twitch 

 evoked by one single maximal excitation, and that the result occurs only 

 when the successive stimuli are separated by an interval less than the 

 corresponding refractory period of the angesthetised nerve. When, on the 

 other hand, the stimuli succeed each other at an interval greater than the 

 refractory period, then tetanus of the muscle occurs. 



The fact that one initial maximal twitch follows upon repeated stimula- 

 tion of the nerve shows that only the first excitation of the series has taken 

 effect ; this excitation prevents the second from being effective, the second 

 prevents the third, and so on : consequently when once the excitations are 

 applied at a sufficiently rapid rate the nerve refuses to conduct any more 

 than the first excitatory process. 



Such an effect, though at first sight suggestive of fatigue of nerve, is not 

 necessarily fatigue. It is conceivable that a conducting mechanism built on 

 simple physical principles might give the same result. Nevertheless, by a 

 closely analogous method Frohlich succeeded in showing that the nerve 

 does actually become fatigued when subjected to rapid stimulation. When 

 during anaesthesia of the nerve he selected a rate of stimulation which just 

 about coincided with the definite refractory period corresponding to the inten- 

 sity of stimulation used and to the given degree of anaesthesia, he obtained, 

 not a single twitch, but a tetanus of peculiar form. This tetanus, instead of 

 gradually climbing, after the normal fashion of a muscle tetanus, attained 

 its maximum almost immediately, and then rapidly fell off in height until 

 in the space of a second or so the muscle ceased to contract altogether. 



