33^ BRAIN MECHANISMS AND LEARNING 



Post-tetanic potentiation will be specially considered later, when discussing 

 the kind of enduring changes that can occur in synapses. 



EFFECTS OF DISUSE ON SYNAPTIC FUNCTION 



Prolongcci and total disuse of monosynaptic reflex arcs has been secured 

 by severing the dorsal roots just distal to their ganglia, thus retaining 

 functional continuity between parent cell bodies and the central projec- 

 tions within the spinal cord of their silenced fibres (Ecclcs and Mclntyre, 

 1953). Several weeks after this operation, the reflex responses evoked by 

 stimulation of these disused dorsal roots have been compared with the 

 control responses on the other side. Volleys in the disused dorsal roots 

 were always much less effective in evoking monosynaptic reflexes into 

 either flexor or extensor muscles. Post-tetanic potentiation was, however, 

 effective in restoring much of the lost function, but it was still far below 

 the potentiated control response (Fig. i A, B). 



The question at once arises : to what extent can reflex deficiency on the 

 operated side be attributed to changes in the presynaptic pathways other 

 than those due to mere absence of the normal impulse barrage? Histo- 

 logical examination revealed two possible causes, but there was good 

 evidence that only part of the discrepancy could be so explained. For 

 example, there was in some experiments destruction of some dorsal root 

 fibres either at the initial operation,' or subsequently by scarring. Again, 

 there was some shrinkage of the dorsal root fibres (about 10 per cent), 

 which is probably attributable to diminished turgor consequent on the 

 regenerative outgrowth from the ganglion cells into the peripheral stump 

 (Gutmann and Sanders, 1943; Sanders and Whitteridge, 1946; Szenta- 

 gothai and Rajkovits, 1955). It is not possible conclusively to refute the 

 suggestion that the synaptic knobs shrink more than the main axonal 

 shafts from which they spring, the large depression of their reflex excita- 

 tory power being thus explained. 



However, subsequent to a prolonged repetitive stimulation these 

 disused synapses exhibited a behaviour which was not simply explicable 

 by depressed function and which indicated that the disused synapses had 

 acquired special properties. In particular the post-tetanic potentiation of 

 monosynaptic reflexes evoked from the disused roots ran an abnormal 

 time course: maximum potentiation of the reflex occurrec^ later than in the 

 normal control; the decline from this maximum occurred several times 

 more slowly; and, most significantly, the reflexes did not return to the 

 initial size, as with normal post-tetanic potentiation, but exhibited a 

 residual potentiation that persisted for hours (Eccles and Mclntyre, 1953). 



