The Action of Yohimbine on Medullated Nerve 201 



in anipHtude of the excitatory processes as they traverse the yohimbinised 

 area. When the conductivity by deepening of the anaesthesia is just 

 about to disappear, these responses become minimal. In this respect the 

 anaesthetic action of yoliimbine corresponds with the action of coohng, of 

 aspliyxia, and of anaesthesia produced by means of the more commonly 

 used anaesthetics. 



Before concluding, we ought to say that the fatigue effects which we 

 have ascribed to the action of yohimbine can be due only to a change in 

 the nerve itself. In a normal nerve-muscle preparation, strong stimulation, 

 especially of the distal end of the nerve, may so alter the nerve-endings in 

 the muscle that subsequent stimulation of the nerve may produce effects 

 which might be erroneously ascribed to fatigue of the nerve. In all the 

 experiments carried out by us which show fatigue phenomena this fallacy 

 is excluded by the fact (1) that the intensity of the stimulation applied to 

 the nerve was at no time more than just maximal ; (2) that the fatigue 

 phenomena in each case appeared before distal stimulation was used. 



1. A two per cent, solution of yohimbine lactate has been 

 applied to the middle portion of the sciatic nerve of a frog's 

 gastrocnemius preparation, and alterations in the conductivity 

 of the nerve observed by means of the muscular response to 

 rhythmical stimulation applied proximally to the alkaloid- 

 affected portion. The rate of stimulation varied between 144 

 and 4 per second. 



2. In its action on nerve, yohimbine resembles in many 

 respects the already known action of other anaesthetics, of low 

 temperature, and of asphyxia. It ultimately abolishes conduc- 

 tivity. The process of abolition of the conductivity is gradual, 

 and is characterised by a progressive diminution in the ampli- 

 tude of excitatory processes which traverse the affected part of 

 the nerve, and by a prolongation of the refractory period of 

 the nerve. By means of it, too, fatigue changes may be shown 

 to occur in the nerve. 



3. On the other hand, the action of yohimbine differs in 

 important respects from that of asphyxia of low temperature, 

 and of anaesthesia with ordinary agents. The tetanic responses 

 of the muscle corresponding to rapid rythmical stimulation of 

 the proximal end of a yohimbinised nerve are always of one type, 

 and resemble the "fatigue tetani " described by F. W. Frohlich. 

 In stages of deep anaesthesia it is not easy to demonstrate the 

 occurrence of initial non-summated maximal twitches as a result 

 of rapid rhythmical stimulation of the nerve. The duration of 

 the refractory period does not seem to vary directly with the 

 strength of the stimulus applied to the nerve, and is more 



