344 BRAIN MECHANISMS AND LEARNING 



No significant change has been observed in the reflexes from such 

 relatively disused pathways, there being sometimes a small depression 

 relative to the control, but more often an increase, as has been mdepcnd- 

 ently reported by Beranck and Hnik (1959); furthermore, no significant 

 changes were observed in the post-tetanically potentiated reflexes. A 

 possible interpretation of these negative results is that the various pro- 

 cedures were ineffective in silencing the discharges from annulospiral 

 endings, there being sufficient residual activity to prevent the depression 

 of function that occurs with the complete disuse of severed pathways. 

 Alternatively, it could be maintained that disuse does not cause depression 

 of synaptic function. The depression observed in the synaptic action of 

 severed pathways (Eccles and Mclntyre, 1953; Eccles ct al., 1959) would 

 then be attributed entirely to the shrinkage of the primary afferent fibres 

 as a consequence of the chromatolytic changes of the dorsal root ganglion 

 cells. This dilemma can be resolved only if complete inactivity of mono- 

 synaptic synapses can be secured without severing the primary afferent 

 fibres. An attempt to do this by a prolonged cold block of peripheral 

 nerve has so far failed on account of technical difficulties. This indecisive 

 situation for the disuse experiments makes the investigations of excess use 

 of crucial significance. However, it must be remembered that, on post- 

 tetanic potentiation, the synapses belonging to severed afferent pathways 

 exhibited a slower recovery and a residual potentiation, which has sug- 

 gested that the depression arose specifically from disuse, rather than from 

 a mere shrinkage due to chromatolysis (cf Eccles and Mclntyre, 1953; 

 Eccles et al., 1959). 



EFFECTS OF EXCESSIVE USE ON SYNAPTIC FUNCTION 



If, as seems likely, prolonged disuse has a deleterious effect on the 

 potency of synapses, it follows that normal use has a sustaining function ; 

 and it is further likely that excess use leads to an enduring enhancement of 

 synaptic function above the normal level, which provides of course the 

 synaptic basis for the suggested explanation of learning. There is, however, 

 in the literature very httle experimental data on the effect of excess use. 

 The most significant findings were the increased monosynaptic reflexes, 

 relative to the control side, in the segment immediately rostral to those 

 with severed dorsal roots (Eccles and Mclntyre, 1953)- This asymmetry 

 may reasonably be attributed to the development of some compensatory 

 reaction to the operative disability, particularly in extensor muscles that 

 had been partlv deafferented. The simplest explanation of this compensa- 



