THE CENTRAL NERVOUS SYSTEM 80 1 



As an example of the numerous class of reflexes in which the 

 excitation of certain muscles is accompanied by the inhibition 

 of their antagonists (reciprocal inhibition), we may take the 

 ' flexion reflex/ the flexion at the knee, hip, and ankle of the 

 hind-limb readily elicited in the spinal dog by ' nocuous ' or 

 harmful stimuli (such as a prick, a strong squeeze, chemical 

 agents, or excessive heat) , or by electrical stimuli applied to the 

 skin of the limb or of any afferent nerve of the limb. 



Sherrington has shown that when the legs of the animal are 

 so prepared that only the flexors can act on one knee, and only 

 the extensors on the other, stimulation of symmetrical points on 

 the two sides in the area of skin (receptive field) from which the 

 flexion reflex can be evoked causes contraction (excitation) of 

 the flexors and simultaneous relaxation (inhibition) of the tone 

 of the extensors. The same is true when corresponding afferent 

 nerve-twigs are stimulated on the two sides. From this it is 

 inferred that each of the nerve-fibres from the receptive field of 

 the reflex divides in the cord into two sets of end-branches 

 (e.g., collaterals) a set which produces excitation when it is 

 stimulated, and another set which produces inhibition. 



The difference in action is specific in the sense that no mere 

 change in the kind or intensity of stimulation affects it. Yet 

 there are facts which show that the specificity is not absolutely 

 immutable, and that a change of conditions in the spinal cord 

 may permit excitation of a given group of muscles to be produced 

 by the stimulation of an afferent path which is primarily inhibi- 

 tory for them. One of the most striking illustrations of this 

 possibility is seen in the action of strychnine. Stimulation of 

 the internal saphenous nerve below the knee say in a dog after 

 removal of the cerebrum is known always to produce inhibition 

 of the portion of the quadriceps extensor whose contraction 

 causes the knee-jerk. 



If now the animal be poisoned by a small dose of strychnine, 

 stimulation of the nerve causes no longer reflex relaxation, but 

 reflex contraction of the muscle. This fact indicates that the 

 essential action of strychnine is something different from a mere 

 reduction of the resistance to the spread of impulses in the cord 

 (Sherrington). Tetanus toxin produces a similar effect, though 

 more slowly. 



Not only is the tone of the extensors diminished or abolished 

 during the activity of the flexors, but the contraction of the 

 knee extensors evoked by striking the patellar tendon, which is 

 called the knee-jerk, either fails to appear, or appears but feebly, 

 when the flexion reflex is simultaneously elicited, even when the 

 mechanical antagonism of the flexor contraction has been elim- 

 inated by previously detaching the flexors from the knee. 



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