18 DAVID P. C. LLOYD 



brain stem reticular formation and substantia gelatinosa Roiandi in the spinal cord. 

 It was thought that the electrotonic action took place by spreading of evoked currents 

 through intercellular fluid to the nearest nerve elements or nerve circuits."" 



It should be noted that Beritoff is quite emphatic that he did not have a 

 syncytium or reticulum in mind when he, earlier, employed the term 

 "neuropil". 



Another expression of this general idea is to be found in the study of 

 "intermittent conduction" by Barron and Matthews (1935). In their formula- 

 tion fluctuating activity in the grey matter would give rise to a fluctuating 

 potential gradient in collaterals that dip into the grey matter from passing 

 tract fibers and hence to a fluctuating block at the collateral junctions. A 

 significant quotation from their paper is: 



"By means of this intermittence the state of the activity of the grey matter as well as 

 the sensory discharge from the periphery may be signalled to the higher centers. The 

 intermittent discharge depends for its frequency on the intensity of the stimulus to 

 the peripheral sense organ, while its interruption depends upon factors influencing the 

 grey matter. Hence such an intermittent stream of impulse signals upward not only 

 information of the stimulus but also the state of the grey matter in some region. So 

 whatever the mechanism of intermittence may be, it seems clear that this inhibition of 

 conduction in a continuous fiber must be a possible mechanism of nervous integration 

 within the central nervous system which does not involve a synapse."" 



For a brief time it was thought (Obrador and Odoriz, 1936) that a similar 

 action occurred in sympathetic gangha, but this proved not to be the case 

 (Lloyd, 1938). 



The other extrasynaptic hypothesis considers evidences of impulse block 

 in presynaptic fibers by action of impulses in other immediately adjacent 

 presynaptic fibers. Such presynaptic interaction was considered by Renshaw 

 (1946) and subsequently in more specific form by McCoUoch et al. (1950). 



This, then, is the background for discussion, and these the thoughts on 

 inhibition that constitute that background. 



THE SITUATION 20 YEARS AGO 



It is worthwhile at this juncture to turn our minds back 20 years to see 

 what was the situation in 1940, just before the proof of the existence in the 

 spinal cord of direct inhibitory actions changed public opinion in some 

 degree and overcame the reluctance of many to accept specifically inhibitory 

 action in the central nervous system, whatever might be the mechanism, 

 electrical or chemical, at the synapses involved. 



The decade prior to 1940 had witnessed the great debate between pro- 

 tagonists of chemical and electrical transmission at nerve terminals. Dis- 

 cussion centered almost exclusively about excitation. Inhibition was an 

 awkward and, therefore, largely neglected action. 



