THE ACTION OF YOHIMBINE ON MEDULLATED NERVE, WITH 

 SPECIAL REFERENCE TO FATIGABILITY. By John Tait 

 and Jas. A. Gunn. (From the Physiology Department, University 

 of Edinburgh.) 



{Received for publication 21.s< February 1908.) 



Experiments carried out in recent years by Gotch and Burch (1), 

 Boycott (2), Boruttau (8), and F. W. Frohlich (4), have shown that when 

 medullated nerve is thrown into activity by an external stimulus a certain 

 short period of time must elapse before it can function again. This period 

 of inexcitability, or refractory period, is normally very short — not more 

 than -002 second for the sciatic nerve of the frog — but can be much pro- 

 longed by subjecting the nerve to special conditions. Thus low temperature 

 (1) (2), anaesthesia (3) (4), and asphyxia (4), all greatly prolong the refractory 

 period. 



The readiest method of demonstrating the existence of this refractory 

 period is to excite the nerve of a nerve-muscle preparation by two successive 

 maximal stimuli separated by a very short interval of time. By the 

 response of the attached muscle it is then possible to tell whether the nerve 

 has conducted two excitations or only one. If the muscle response is a 

 summated one it is evident that both excitations have been transmitted ; if 

 summation does not occur, then the second stimulus must have been in some 

 way ineffective. Absence of summation occurs only when the interval of 

 time between the two maximal stimuli is sufficiently short. In such cases 

 it has been shown by means of the capillar}- electrometer (1) that tlie block 

 to the second excitation is seated in tlie nerve. 



The length of the refractory period would seem to be dependent on the 

 intensity of the preceding excitation. Generally speaking, it has been found 

 that the stronger the stimulus applied to the nerve the longer does the 

 nerve take to recover its functional capacity. By combining powerful 

 stimulation with anaesthesia or asphyxia or cooling of the nerve, one would 

 therefore expect to get a maximal refractory period, and indeed Frohlich 

 was able to prolong it to '1 second (4). 



Besides the method of applying two successive stimuli it is obvious that 

 a series of rapid recurring stimuli might be applied to the nerve ; and if a 

 sufficiently high rate of excitation could be attained, one would expect that 

 at least some of the excitations would be ineffective. 



VOL. 1. — Al'RIL 1908. 13 



