THE R(3lE of reflex INHIBITION 605 



activity which executes the following or extension phase of 

 the step. To such an extent is this the case sometimes that 

 the extension or second phase seems self-induced by the in- 

 hibition. The central discharge ensuing on withdrawal of an 

 inhibitory stimulus has been termed rebound discharge, and 

 the contraction by which the muscles evidence it, rebound 

 contraction. How potent it is in reflex stepping is shown by 

 the fact that a stimulus which excites the muscle group figured 

 in A of the annexed figure (fig. 8) is followed on its cessation 

 by a rebound contraction of all the muscles figured in B. So 

 that the flexion phase of the step evoked by the stimulus is 

 succeeded by a complete extension phase without further 

 stimulus at all. This extension phase is at once cut short by 

 inhibition on reapplying the stimulus appropriate for flexion. 

 A series of the complete diphasic acts of stepping can thus be 

 produced by simple repetition of a single stimulus (fig. 9). The 

 same phenomenon is observable in the rhythmic alternating 

 reflex of respiratory movement of the chest (H. Head). 



In these cases the inhibition is central — that is, has its 

 seat in the central nervous organ, e.g. spinal cord or brain ; 

 and the post-inhibitory exaltation is an exaltation of the ex- 

 citability and activity of an intra-spinal (or intra-bulbar) 

 element of the reflex arc. But in cases where the seat of 

 inhibition is peripheral there is some evidence of an analogous 

 rebound, a post-inhibitory exaltation of activity on the part of 

 the inhibited mechanism. Thus inhibition of the heart-beat by 

 stimulation of the vagus nerve going to the heart is often 

 followed on withdrawal of the stimulus by an exaggeration of 

 the heart's action, the activity of the heart-beat becoming greater 

 than that obtaining before the inhibition was provoked. If we 

 consider that at the root of the antagonism between comple- 

 mental colour sensations, such as red and green which neutralise 

 each other so as to result when balanced in a colourless grey, 

 there lies a nervous antagonism comparable with that between 

 excitation and inhibition, it is noteworthy that stimulation by 

 the one is followed on its withdrawal by an exalted sensitivity 

 to stimulation by the other and even to a spontaneous appear- 

 ance of the complemental colour (successive contrast). The 

 increased tendency of a motor centre after inhibition to dis- 

 charge impulses forms obviously a factor making for co-ordina- 

 tion in reflex sequences where alternate phases of inhibitory 



