THOUGHTS ON INHIBITION IN THE SPINAL CORD 27 



final common paths, respectively. Feeding into these would be all the varied 

 influences supra-spinal, spinal and peripheral meeting in convergence on 

 common ground to make of those interneurons a common path to the 

 motoneuron. 



The recurrent inhibitory system. The last of the four systems mentioned is 

 that of the recurrent inhibition. This has received the great burden of atten- 

 tion in recent years. From an operational point of view recurrer.d inhibition 

 is that which takes place in certain motoneuron pools following antidromic 

 stimulation of axons from other motor pools (Renshaw, 1941 ; Lloyd, 1951b; 

 Eccles et al., 1954; Granit et al., 1957; Brooks and Wilson, 1958). The 

 inhibition is associated with facilitation (Renshaw, 1941; Lloyd, 1951b; 

 Wilson et al., 1960). Although the course of recurrent conditioning, within 

 a nucleus, inhibitory and facilitatory, exactly matches the flows of after- 

 currents about the active motoneurons (Lloyd, 1951b), and hence might 

 seem to be causally related, the effect is generally considered to be due to 

 action of recurrent motor collaterals acting through interneurons, the latter 

 rather widely known as "Renshaw cells". 



Opinions vary concerning the functional role of recurrent inhibition. 

 Renshaw (1941) emphasized the local nature of recurrent inhibition, the 

 main influences in his experiments being confined to motoneurons in the 

 same segmental level. 



In Eccles' (1955) view the functional significance of the pathway for recur- 

 rent inhibition is that of a negative feed-back control over motoneuron 

 activity, which, having no specific distribution, cannot fulfill a co-ordinative 

 role. It is considered as having an anticonvulsant function. 



Granit et al. (1957) regard the recurrent system as they designed for 

 stabilization of the sustained output of impulses in the stretch reflex. Brooks' 

 and Wilson's (1958) view is somewhat different as they see in the recurrent 

 system a mechanism that serves to assist in preservation of the localized 

 nature of stretch reflexes. 



Wilson (1959) has devoted attention to the usually neglected recurrent 

 facihtation which is outside the present main theme. But it is an important 

 consideration that Wilson et al. (1960), by a systematic study of the distribu- 

 tion of recurrent facilitation and inhibition, find a pattern that is not unlike 

 that of the inverse myotatic reflex arising from tendon organ, or 1 B, endings. 

 Facilitation is prominently found among flexor groups. Inhibition, of course, 

 is most prominent between synergists in a myotatic unit. 



Having in mind the various pieces of evidence concerning action in the 

 recurrent system it would seem a fair assumption that it plays a trigger 

 role in the reversal that takes place on forcible stretching from stretch reflex 

 to lengthening reaction. 



As has been mentioned, both spindle and tendon receptors must be active 

 even at moderate stretch for their peripheral thresholds are not greatly 



