22 DAVID P. C. LLOYD 



hibitory effect is brief by comparison with evidences of inhibition in the 

 course of integrated activity. At some point everyone, it seems, has found 

 necessary tiie postulation of delay paths (Forbes), or internuncial chains, to 

 account for prolongation of effect. The point is well made by Liddell in his 

 discussion of "Integration, then and now" (1949). He writes in part: 



"For the scratch reflex of the spinal animal Sherrington suggested the co-operation of 

 at least three neurones, the afferent, the proprio-spinal, and the final common path 

 to the muscles. In light of modern knowledge concerning the fleeting states of activity 

 in single neurones, contrasted with the slow tempo of processes in the integrated 

 scratch reflex it is clear that many side-chains of interneiirones must nowadays be 

 postulated." 



We are on safe ground if we say that the single inhibitory act exerted at 

 the motoneurons by proprioceptive afferents is one that increments to 

 maximum in 0-5 msec and dissipates exponentially to \je in 4 msec. 



Having in hand a well defined example of central inhibition as a single 

 effect with a well-defined time course, one should be wary of extension to 

 other systems. The presently known example pertains to a very particular 

 reflex path endowed with a very particular sort of afferent fiber, and a very 

 particular sort of neuron, the motoneuron. Other sorts of endings acting 

 upon other sorts of neurons may, and probably do, exert in the spinal cord 

 a "single" inhibitory effect of different dimension. 



Place Theory 



In many formulations of inhibition location upon the neuron of synaptic 

 knobs, supposedly excitatory or inhibitory, becomes a consideration. Place 

 theory, as it may be called, arises in connection with the hypothesis of 

 Gerard and Gesell already discussed. It appears also in hypothesis con- 

 sidering decremental conduction and in hypothesis of direct inhibition by 

 specific inhibitory action. 



Until recently most thought on place theory has been based on physio- 

 logical induction from anatomical studies on Mauthner's cell (Bartelmez and 

 Hoerr, 1933; Bodian, 1937, 1940). Of more interest, however, for studies on 

 the mammalian spinal cord are those of Sprague (1958) who followed terminal 

 degeneration of various groupings of afferent fibers in regions where moto- 

 neurons are known to be excited or conversely to be inhibited by the action 

 of those afferent fibers. Briefly put, degeneration was found on the cell 

 bodies and some dendrites of motoneurons that would be excited mono- 

 synaptically by the severed dorsal roots, but was confined to dendrites of 

 those motoneurons that would be inhibited. These findings at the very least 

 assure that place theory, whatever the exact framework may be, must be 

 taken seriously. 



One form of hypothesis in which place theory might play a role is an 

 interference hypothesis reconsidered. The recent redemonstration of decre- 



