245 



Break-shock Reflexes and "Supramaximal" Contraction-response 

 of Mammalian Nerve-muscle to Single Shock Stimuli. 

 By C. S. Shereington, M.D., Pres.E.S. 



(Received May 26, 1921.) 



The following observations continue some of those reported previously to the 

 Society by Mr. K B. Dreyer and myself (1) and by Dr. K. Sassa and myself (2). 

 But in the present instance the technique has been considerably modified in 

 the hope of securing finer discrimination between the contraction forms 

 obtained. The speed and intensity of the reactions of mammalian muscle with 

 its blood supply intact rendered desirable greater lightness in the moving parts 

 of the myograph. The free vibration rate of the isometric myograph now 

 used has been more than 900 a second; its damping such that when suddenly 

 released from a torsional deviation, approximately that obtaining in the 

 muscle observations, the vibrations ceased to be visible under tenfold 

 ■enlargement in about 0"03 sec. The registration of the myograph movement 

 has been by optical projection on a travelling photographic plate, time being 

 recorded on the plate by a rotary shadow-marker of the pattern devised by 

 Mr. Bull, of the Institut Marey, Paris. 



The reflex preparation has been tibialis anticus muscle in the spinal cat, 

 •decerebrated and free from drugs. The stimulus used has been a single 

 break-shock given by an automatic key and applied by platinum electrodes 

 5 mm. apart to the central stump of the cut and isolated afferent nerve 

 (popliteal, internal saphenous or digital branch of musculocutaneous) or, 

 for the motor control, to the peripheral stump of the cut motor nerve 

 (peroneal), kathode proximally for the former, kathode distally for the latter. 

 The magnification of the muscle movement has been sixty times : that is, for 

 a myogram of 30 mm. height recording a tension of 915 grm., the muscle, e.g., 

 of 12 cm. length, shortened 1/240 of its length; the myograms are thus 

 practically isometric. 



The trend of the results obtained can be conveniently indicated by 

 describing briefly the changes observable (Fig. l)in the reflex myogram under 

 progressive step-by-step increments of strength of the break-shock delivered 

 as stimulus to the afferent nerve, e.g., popliteal. Starting with a break-shock 

 of little above threshold value for reflex stimulation, e.g., the secondary coil at 

 60 cm. from primary on the inductorium scale, the reflex myogram then 

 obtained is of lower crest-height and less steep ascent than is the maximal- 

 twitch myogram (subsequently obtained by break-shock stimulation of 



