20 DAVID P. C. LLOYD 



and difficult to satisfy as knowledge of central nervous activity grew. And 

 grow it did, due importantly to the fact that electronic means had made 

 possible direct observation of central activity itself. This had great advantage 

 over the older necessity of inferring central events from observation of peri- 

 pheral events in the causal sequence. A high point in the development of 

 direct recording means and observation was Lorente de No's (1939) use of 

 inserted needle electrodes to reveal directly the activity of the chains of 

 interneurons. 



The criteria for postulating central inhibition as a direct action as I saw 

 them in 1940 are set forth in the following quotation (Lloyd, 1941): 



"Fibers, or synaptic endings of fibers, having an inhibitory action, have often been 

 invoked to provide a mechanism by which to explain central inhibition. There has 

 been no direct evidence to support the contention that such fibers or endings exist in 

 the mammalian central nervous system. Furthermore, subnormality of central neurons 

 serves as a mechanism to explain central inhibition without employing other than 

 known processes, provided central neurons are activated prior to the appearance of 

 inhibition. Activation of central neurons is almost inevitable if the central latency of 

 inhibition is greater than the minimal synaptic delay of approximately 0-5 to 0-6 msec. 

 Therefore, to postulate an active inhibitory process requires a demonstration of 

 inhibition under circumstances which preclude the possibility of subnormality in 

 accounting for a response deficit. In efi'ect, the paths taken by the inhibitory action and 

 the necessary testing excitatory action must not have elements in common before 

 impinging upon the final common path (the motoneurons); the motoneurons must not 

 have been discharged by the conditioning impulses." 



It was then shown that certain dorsal root afferent volleys inhibited the 

 monosynaptic reflex elicited by other dorsal root afferent volleys in circum- 

 stances that met the criteria for direct action upon the motoneuron (Lloyd, 

 1941). 



SOME TOPICS OF CURRENT INTEREST 



At this point a change in approach in this discussion seems indicated to 

 consider specific topics in the problem of inliibition, having developed what 

 seem to me to have been the main avenues of thought, and having made 

 what seems to me to be a fair statement of the position at the onset of the 

 last two decades. 



Inhibition — Monism or Pluralism 



What is to be included under the heading of inhibition? Obviously the 



answer to this question to different people is different. In the Integrative 



Action whilst discussing MacDougall's neurin hypothesis Sherrington 



wrote (1906): 



"It appears to me unlikely that in their essential nature all forms of inhibition can be 

 anything but one and the same process." 



a passage that was presented again in the 1925 formulation of reflex inhibition. 



