26 DAVID P. C. LLOYD 



The functional importance of monosynaptic connection lies in the fact 

 that by this sort of connection the motoneuron inevitably is influenced by 

 action in the peripheral receptor. There is no stage at which the reaction can 

 be blocked. If mechanical change takes place of a sort to excite muscle 

 spindles in one muscle then the motoneurons of all the members joined to 

 it by monosynaptic interconnection, together constituting a myotatic unit 

 (Lloyd, 1946b, 1960), are influenced positively or negatively according to 

 synergic or reciprocal action. Thus no movement is possible without this 

 system coming into play to confer harmony of action in the movement 

 made. 



The disynaptic system. In all the relationships of muscles examined (Laporte 

 and Lloyd, 1952) those between muscles which possess monosynaptic inter- 

 connection in a given direction, excitatory or inhibitory, proved to contain 

 an element in the opposite sense, inhibitory or excitatory, by means of 

 pathways that possess in their Hnkage an intercalated interneuron. That the 

 function of the interneurons of the disynaptic system is to serve as a valve 

 seems quite clear. Their role has been described as follows (Lloyd, 1958): 



"To illustrate this (the valve-hke action) one returns to consideration of the stretch 

 reflex of the lengthening reaction, with their respective monosynaptic and disynaptic 

 pathways .... Not enough difference exists between the stretch threshold of the 

 muscle spindle, afferent end-organ for the stretch reflex, and that of the tendon organ, 

 afferent for the lengthening reaction, to account for the great difference in reflex 

 threshold of the two reflex arcs in the decerebrate animal. Furthermore, in the spinal 

 animal it is the inhibition of the lengthening reaction rather than the excitation of the 

 stretch reflex that is the presenting feature (Henneman, 1951) . . . 



"That stretch excitation of autochthonous motoneurons is present at all degrees of 

 stretch follows from the fact that monosynapticity of action implies inevitability of 

 action. That the inhibition may dominate in the spinal animal and yet be held in check 

 in the decerebrate means that the internuncial link of the disynaptic path is open in 

 the former and closed in the latter." 



It is, then, by this nieans that an inco-ordinate clash of reflex effect is 

 obviated in integrative action. 



But the disynaptic system has as its field of action one much wider than 

 the myotatic unit and in this wider field of action inhibition only has been 

 found (Laporte and Lloyd, 1952). This external field of action transcends 

 the boundaries of a given region and influences for the most part, but not 

 exclusively, the extensors. The lengthening reaction must be considered as 

 a general collapse of the extensor tonus of the limb. 



Polysynaptic systems. Inhibition in polysynaptic systems is inadequately 

 studied, but this is no measure of its importance. The chains of interneurons 

 fashioned into the patterns described by Lorente de No (1933) here find their 

 expression in all but a very few reactions of the spinal cord. Although their 

 structure is not known, it is economy of hypothesis to suppose that there 

 are two pools, constituting half-centers after the concept of Graham Brown, 

 that (speaking only of inhibition) are inhibitory to the flexor and extensor 



