THOUGHTS ON INHIBITION IN THE SPINAL CORD 19 



Advocates of chemical transmission had peripheral models in that media- 

 tors could have excitatory or inhibitory action at different effector loci 

 dependent upon the target organ, but no one, regardless of position held in 

 the great debate, could point to specifically inhibitory synapses, or col- 

 laterals, or fibers, in the central nervous system. Furthermore, there was an 

 air of "sleight of hand" in carrying observations on neuro-effector systems, 

 not unchallenged at that, over to consideration of junctions between neurons. 

 It is here that the observations of Kibjakov (1932), Feldberg and Gaddum 

 (1934) and of Feldberg and Vartianin (1934) on sympathetic ganglia played 

 an important role. These observations concerned excitation, however. The 

 only inhibition observed in sympathetic ganglia at that time was of the post- 

 reactional type (Eccles, 1935; Bronk, 1939; Lloyd, 1939) although Marrazzi 

 had ascribed an inhibitory action to adrenaline (1939). 



It was during this period that hypothesis of indirect inhibition and hypo- 

 thesis of electrical transmission became identified in people's minds, pre- 

 sumably because so many felt at the time that any current idea of necessity 

 had to be thrust into one or the other hopper. But whatever one's private or 

 pubhc opinions on the mode of transmission happened to be at the time, 

 the indirect hypothesis of inhibition by operation of the subnormal period 

 was foremost for consideration. Wherever one looked, be it in ganglia or in 

 spinal reflexes, the subnormal period was seen giving rise to decreased 

 response. And signs of antecedent excitation or facilitation were usually 

 present. Eccles and Sherrington's study of the flexor reflex was the most 

 modern and detailed study of the spinal reflex mechanism at work, and the 

 theory of inhibition by subnormality developed by Gasser (1937) was capable 

 of accounting for the evidences of inhibition presented by Eccles and Sher- 

 rington (1931). 



The pioneering studies of Lorente de No on the control of oculomotor 

 neurons belongs to this era. Emphasis was placed upon the action of chains 

 of interneurons and to the qualities their action conferred upon transmission 

 through the reflex center. Lorente de No (1936) found the subnormal process 

 a "satisfactory explanation for the delayed block of the circulating impulses 

 and cessation of the activity of the chain". His position is well stated in the 

 following quotation (1936): 



"Since refractoriness of internuncial axons affords a satisfactory explanation of the 

 inhibition of the motoneurons, it does not seem necessary to make the additional 

 assumptions necessary for the establishment of the first hypothesis namely that inhibi- 

 tion is due to an active process taking place in the motoneurons themselves. Of course 

 the possibility of the existence of such a process cannot be ruled out yet, but on the 

 other hand no evidence of it has ever been found in spite of continuous search along 

 different lines." 



One will appreciate the fact that the criteria for establishment of inhibition 

 as a direct action in the central nervous system were becoming more rigid 



