THOUGHTS ON INHIBITION IN THE SPINAL CORD 25 



number of carefully made observations and considerations (Frank and 

 Sprague, 1959; Lloyd and Wilson, 1959). 



Thus, although a chemical hypothesis is at the present time the more 

 worthy of consideration we have no fully acceptable hypothesis, nor have 

 we sufficient infomiation upon which to build a theory. The search must 

 go on. 



The Role of Inhibition 



The role of inhibition in reciprocal innervation came into prominence 

 through the series of papers written by Sherrington at the turn of the century. 

 In his own words (Sherrington, 1933): 



"This 'reciprocal innervation' was quickly found to be of wide occurrence in reflex 

 actions operating the skeletal musculature, its openness to examination in preparations 

 with 'tonic' background (decerebrate rigidity) made it a welcome and immediate 

 opportunity for the more precise study of inhibition as a central process." 



A little later in his Nobel Lecture Sherrington remarks: 



"I will not dwell upon the features of reciprocal innervation; they are well known. 1 

 would only remark that owing to the wide occurrence of reciprocal innervation it was 

 not unnatural to suppose at first that the entire scope of reflex inhibition lay within 

 the ambit of the taxis of antagonistic muscles and antagonistic movements. Further 

 study of the central nervous action, however, finds central inhibition too extensive and 

 ubiquitous to make it likely that it is confined solely to the taxis of antagonistic 

 muscles." 



Sherrington's well considered view is that which is generally held today 

 with vastly more detailed information to hand. This particularly is true of 

 the disynaptic reflex system (Laporte and Lloyd, 1952) of the spinal cord 

 which has as its receptor origin the Golgi tendon organs and its expression 

 in classical reflex physiology as the lengthening reaction. The monosynaptic 

 reflex system of the myotatic reflex is more clearly confined in its fields of 

 action to the requirements of reciprocal innervation (Lloyd, 1946a, b, 1960). 

 In fact the notion is altogether a worthy one that the monosynaptic system 

 in operation is within itself a major factor in the genesis of reciprocal innerva- 

 tion under the command to movement from higher centers. 



In describing the integrative role of inhibition in the spinal cord there 

 are altogether four systems to be considered. These are (1) the monosynaptic 

 system, (2) the disynaptic system, (3) the polysynaptic system, and (4) the 

 recurrent system. 



The monosynaptic system. The structure of the monosynaptic system is 

 now wefl known (Lloyd, 1946b; Laporte and Lloyd, 1952; R. M. Eccles and 

 Lundberg, 1958). There is agreeinent that the inhibitory action arising from 

 the muscle spindle primary endings of one muscle is distributed to the motor 

 nuclei of other muscles in accordance with the requirements of reciprocal 

 innervation. 



