THOUGHTS ON INHIBITION IN THE SPINAL CORD 21 



Thus Sherrington held, and as far as I know always held, a monistic view of 

 inhibition. 



On the other hand Gasser (1937) certainly adopted a pluralistic view in 

 writing: 



"Clearly the term "inhibition" must apply to very different mechanisms. Inhibition 

 of the heart is attributable to the intervention of a humoral substance, acetyl-choline. 

 The cardiac tissue is altered. Inhibition of the flexor reflex is caused by the high thresh- 

 hold of the internuncial neurons. The so-called 'inhibited' motor neuron is unaltered; 

 it simply has failed to be excited. It is improper, therefore, to speak in terms implying 

 that there is a general explanation of inhibition. One can only describe mechanisms 

 which would be included in that category."" 



While not adopting a specific view Bremer raised an important question as 

 to what should be included in the category (1947): 



"A fact which no doubt contributes to this confusion is that the term inhibition is 

 rather loosely applied to categories of central depression which are not, or may not be. 

 homologous. For instance it often designates the post-reactional depression of a 

 reflex .... This generalization may prove ultimately to be justified by the demonstra- 

 tion of the fundamental identity of mechanisms of these post-reactional refractor! 

 nesses with the depression of central activity resulting from afferent volleys not evoking 

 themselves any discharge of the same motoneurons, or even any visible discharge 

 But provisionally, and for the sake of clearness, the term inhibition should in the 

 reviewer's opinion, be reserved to central depressions which cannot be attributed to 

 the post-reactional refractoriness of motoneurons. Even with such limitations, there 

 are still great incertitudes as to the phenomenal homogeneity of central inhibitions . . ." 



The discovery in 1940 of an undeniable example of direct inhibition seems 

 to have emphasized the monistic view, somewhat along the lines proposed 

 by Bremer. Certainly most of the recent and current research endeavor is 

 the direct offshoot of that discovery. However, it will be my duty in part to 

 "keep the door open" to mechanisms other than those which are discussed 

 in relation to the direct inhibitory process. 



The Time Course of Inhibition 



Let us state at the outset that inhibition can be brief in duration or pro- 

 longed, even when caused by a single afferent volley. In the Ught of present- 

 day knowledge the least enduring inhibition is that directly imposed upon 

 the motoneurons by a single afferent volley into the myotatic reflex pathways. 

 This has a detectable duration of something less than 15 msec (Lloyd, 1946). 

 Who is to say how long the most enduring would last ? A classical example, 

 however, of enduring inhibition in response to a single volley is that of 

 Ballif et al. (1925) which lasted for a second. 



Much has been said in the past concerning the duration of the single 

 inhibiting effect. Beritoflf (1914, 1924) argued for extreme brevity, the indi- 

 vidual process not exceeding 4 msec. On the other hand Eccles and Sherring- 

 ton (1931) concluded that the inhibition produced by any particular iinpulse 

 persists for as long as 50 msec. In any event it is agreed that the single in- 



