THE ROLE of reflex INHIBITION 597 



intensity of muscular action appears to be obtained by pitting 

 inhibition against excitation. In short, here again inhibition is 

 made to grade the intensity of a concomitantly excited motor 

 centre's discharge. 



Certain reflex actions of natural occurrence similarly exhibit 

 in some cases a contemporaneous contraction of antagonists. 

 Thus, stepping consists essentially of alternate extensions and 

 flexions of the limb and its reflex execution appears due to 

 the interaction of two sets of stimuli, one set causing extension 

 of the limb and other flexion of the limb. Two antagonistic 

 stimulations influencing the extensor and flexor centres in 

 opposite directions alternately predominate in competition one 

 with the other. In some forms of stepping — e.g. when the step 

 is full and free — one stimulus for a moment completely sup- 

 presses the other. Complete rest and unimpeded discharge thus 

 alternate in each centre in turn. But in some forms of stepping, 

 as when the step is not free but crouched, etc., the one group 

 of stimuli only partially counteracts the other. Each centre 

 then comes into action alternately but in one of them at least 

 the discharge is at no time wholly quelled. That centre 

 therefore never obtains a complete rest. It is probable that 

 faulty habits of walking, running, etc., frequently have this 

 latter character. The gracefully balanced stepping which proper 

 systems of drill and exercise cultivate owe their excellence to 

 physiological avoidance of this incomplete balance. The proper 

 execution of the act ensures a moment of complete rest to each 

 of the opposed motor centres engaged. Fatigue is in this 

 way minimised. 



It is sometimes assumed that under reciprocal innervation 

 two antagonist muscles necessarily can never be in contraction 

 at the same time, and that the two antagonistic motor centres 

 never exhibit discharge of motor impulses concurrently. That 

 this need not be the case is clear from the above examples. 

 Not unfrequently the antagonists do concurrently contract and 

 discharge. What reciprocal innervation does provide is that, 

 in the execution of a muscular act, augmentation of contraction 

 or motor discharge shall never occur concurrently in protagonist 

 and antagonist, nor conversely decrease of contraction or dis- 

 charge occur concurrently in the two. Reciprocal innervation 

 secures that any increase in the contraction or discharge of 

 the protagonist shall be accompanied by corresponding 



