EXTENSOR REFLEXES AND INHIBITION 



121 



the thread from the myograph to the distal end of the stump of the left leg by 

 twisting the thread round the needle transfixing the pretibial muscles and skin. 

 Carry the thread under the pulley below the stump, and then horizontally to 

 the vertical arm of the bell-crank myograph-lever (PL IX, fig. 8). Arrange 

 the operation table in regard to the experiment table so that the thread is just 

 taut. The pull of the rubber drawing down the myograph writing-lever 

 should be quite slight.] 



Faradize the right sciatic. Note that extension of the left knee ensues. 

 [Bring the myograph-lever to the recording-drum surface.] Faradize right 

 sciatic and about 3" later faradize left sciatic for 5" while right is being 

 faradized. Continue stimulation of right sciatic for about 3" after cessation 

 of stimulus to left sciatic. Note the effect (text- fig. 41). 



Vary the combination of the right and left sciatic stimuli in respect of 

 sequence, time, and strength. Note the results. 



ANNOTATION 



This exercise illastrates postural activity 

 of skeletal muscle and the central inhibition 

 which plays a large part in the co-ordination of 

 muscular acts. In the decerebrate prepara- 

 tion all those muscles which are commonly 

 employed in the maintenance of the erect 

 posture are in a state of exaggerated reflex 

 tonus. The exercise illustrates this on the 

 main extensor muscle of the knee and isolates 

 it by nerve section of all the nerves of the 

 limb except the vastocrureus nerve itself, 

 the bony and fascial attachments of the 

 muscle not being disturbed. 



The exercise shows — 



(1) that this muscle retains its tonic postu- 

 ral action so long as its own nerve (afferent 

 and efferent) remains intact. Its postural 

 action is reflex, and the afferent nerve-fibres 

 on which it depends are those in its own 

 nerve-trunk coming up from distribution in 

 its own tendinous, fascial, and muscular 

 structures, i.e. its postural tonus is a, pro- 

 prioceptive reflex. 



(2) that this proprioceptive reflex is in- 

 hibited when other afferent nerves of the 

 limb e.g. central stump of ipsilateral sciatic. 



are stimulated. 



(3) that, conversely, stimulation of the 

 afferent nerves of the crossed fellow-limb, 

 e. g. contralateral sciatic, increases the postural 

 tonus of the extensor muscle, and also excites 

 a reflex contraction of it, a movement of active 

 further extension of the knee. 



(4) that this reflex contraction and increased 

 reflex tonus of the extensor can, like the 

 proprioceptive reflex, be inhibited by stimu- 

 lation of the ipsilateral limb-afferents, e.g. 

 in this exercise central stumps of left sciatic 

 nerve. In all these cases the seat of the 

 inhibition lies not in the muscle but in the 

 centres in the spinal cord and bulb. 



(5) that a strong and not too long-continued 

 reflex inhibition is followed, on withdrawal 

 of the inhibitory stimulus, by an increase of 

 the tonus contraction ; this post-inhibitory 

 increase of postural activity is commonly 

 termed ' rebound '. 



For subject-matter of this exercise and for 

 the relevant literature see Sherrington, Quart 

 Jnl. of Experim. Physiol, vol. vi, p. 251, 

 1918 ; Brain, vol. xxxviii, p. 197, 1915. 



K 



