The Action of Yohimbine on Medullated Nerve 195 



of the muscle tetanus in the former case is not due to fatigue of the muscle 

 or of the nerve ends in the muscle. In this respect Yohimbine resembles 

 in its action other anaesthetics. 



At a somewhat later stage the abnormality in the muscle response is 

 clearly seen to be of a definite type. To any given series of continuously 

 applied rhythmical excitations the immediate response of the muscle is a 

 summated tetanus which quickly begins to decline in height, and finally 

 becomes feeble and irregular, or ceases altogether. Thus the general form 

 of the tetanus approaches that of the " fatigue tetanus " described by 

 Frohlich. (See fig. 1, tracing (1), and fig. 2.) 



In the case of other angesthetic agents applied to nerve it was found 

 by Wedensky that the muscle response is largely dependent on the 

 intensity of stimulation used. Wedensky showed (9) that at any given 

 stage of anaesthesia, provided the rate of stimulation is kept constant, there 

 is one definite intensity of stimulation (optimum of intensity) which 

 produces a maximal height of tetanus ; intensities either above or below 

 this optimum cause a less height of tetanic response. In other words, when 

 the nerve is anaesthetised, say by ether or cocaine, weak rhythmical 

 stimulation produces a tetanus of submaximal height, stimulation at some 

 moderate intensity causes maximal height of tetanus, while strong 

 stimulation produces again submaximal tetanus. It is found, too, that 

 tetani of the form which Frohlich calls "fatigue tetani" are more readily 

 obtained with strong stimulation. Furthermore, with deep anaesthesia 

 and strong rhythmical stimulation, the muscle response, as already 

 mentioned, is a single twitch of the same height as the twitch evoked by 

 one single maximal excitation (4). The same is the case when nerve is 

 asphyxiated (4). A similar effect has been shown by one of us (Tait) to 

 occur when nerve is cooled. All these facts indicate that under these 

 conditions the refractory period of the nerve corresponding to strong 

 stimulation is longer than that corresponding to weak. 



Yohimbinised nerve does not conform in this regard to nerve subjected 

 to these other influences. At almost all stages of yohimbine anaesthesia in 

 which alterations of the muscle response to rhythmical stimulation can be 

 detected, this response takes the form of a "fatigue tetanus" — i.e. the last 

 part of the tetanus is at least markedly lower than the first. Furthermore, 

 the highest and best sustained muscle response is not produced by 

 stimulation at moderate intensity, but in every case strong stimulation is 

 more effective than any moderate stimulation as regards both the height 

 and duration of the corresponding tetanus. (See fig. 2.) Thus it is 

 evident that the refractor}^ period of yohimbinised nerve does not increase 

 with the intensity of the stimulation. If anything, the contrary would 

 seem to be the case. 



An examination of the tracings in figs. 1 and 2 shows that when series 

 of rhythmical stimulations are applied in closely succeeding sets or groups 

 to the proximal end of yohimbinised nerve, the successive muscular 



