THE ROLE OF REFLEX INHIBITION 603 



tempo of the rhythm is found to depend on a reflex co-ordination 

 of this kind. So with the respiratory movements of the chest. 

 Expiration follows on inspiration. When the vagus nerves have 

 been severed the rhythmic alternation of these two opposed 

 movements becomes much slowed. With the vagi intact each 

 remission of the activity of the inspiratory centre is due not 

 to exhaustion of the discharge of the inspiratory centre but 

 to its inhibition by a reflex mechanically generated by the 

 expansion of the lung which inspiration itself produces. The 

 secondary reflex acts upon the inspiratory centre through 

 the afferent vagus. And the vagus at the same time as it 

 quells the discharge of the inspiratory centre excites the activity 

 of the expiratory centre. The inspiratory movement is thus 

 cut short by a reflex inhibition generated by itself; and this 

 inhibition r\ins> pari passu with the initiation of a new movement 

 of opposite sense to the old one. 



6. Post-inhibitory Rebound 



As in the above so also in a number of other important rhythmic 

 reflex actions the actions are alternating in the sense that the 

 part moved is first moved in one direction and then moved in 

 the direction exactly opposite to the former movement. Stepping, 

 cited in the last section, is an instance of this. The limb is flexed 

 and extended alternately. In the flexing of the limb the spinal 

 centres of the flexor muscles are excited and those of the 

 extensors are inhibited. When the stimulus which produces 

 this phase of the step ceases or falls into abeyance the opposite 

 phase, that of extension, sets in ; the excitation of the flexor 

 centres then subsides and with it the state of inhibition 

 of the extensor centres. This subsidence of the inhibitory 

 state appears to be followed by a rebound of excitability, 

 not merely to that degree obtaining in the centre before it 

 was inhibited but beyond that. A post-inhibitory exaltation 

 of activity occurs in the centre on its escape from the inhibition 

 (fig. 7). On withdrawal of the inhibitory stimulus a seemingly 

 spontaneous discharge of nervous impulses ensues from it via 

 its motor fibres, causing its muscles to contract. In this way 

 the application of a stimulus which excites flexion of the limb 

 is followed on cessation of that stimulus by an active extension 

 of the limb. The inhibition of the extensor centres during the 

 flexion phase of the step favours and predisposes them for their 



