346 BRAIN MECHANISMS AND LEARNING 



In investigating further the effect of increased use, it was important to 

 design experiments in which central synapses were subjected to an exces- 

 sive discharge from annulospiral endings and in which adequate controls 

 were available. For example, tenotomy or nerve section for all but one 

 muscle of a synergic group was employed in order to place excessive stress 

 on the remaining muscle, and as a consequence it was presumed that there 

 was excess discharge from its stretch receptors. An appropriate dummy 

 operation was performed on the other side. By having the animals walk 

 in a treadmill for 20 to 30 minutes every day, it was ensured that there 

 were ample opportunities for this muscle to be subjected to the excessive 

 stress. After some weeks the monosynaptic reflex evoked by an afferent 

 volley in this muscle nerve was always larger (Fig. 5), the main increase 

 being 50 per cent or more in the fourteen animals so far investigated, than 

 for the corresponding afferent volley on the other side (Eccles and Wester- 

 man, 1959). Control observations were made for reflexes from nerves to 

 synergic groups of muscles with undisturbed innervation on both sides 

 (Fig. 5). Since there was no significant asymmetry for such mono- 

 synaptic reflexes, it can be concluded that tlie excess use had resulted in 

 enhanced function. A slightly smaller relative increase was also observed 

 during post-tetanic potentiation of the reflexes evoked by afferent volleys 

 from the stressed muscle and its symmetrical control. Thus we have here 

 the most convincing demonstration yet provided that excess synaptic use 

 over several weeks results in enhanced synaptic function. Experiments are 

 in progress to test for the effects of excess synaptic use on other synaptic 

 systems, e.g. for the activation of Renshaw cells from the ventral spino- 

 cerebellar tract by impulses from Golgi tendon organs. 



DISCUSSION 



It will be appreciated that these investigations are restricted to simple 

 synaptic systems in the spinal cord. If, as now seems likely, excess activa- 

 tion gives a prolonged increase in synaptic efficacy, experimental investiga- 

 tion could be extended to the more complex polysynaptic reflex pathways 

 and, finally, to pathways in the higher levels of the nervous system. Since 

 investigations of learning and of conditioned reflexes have been carried 

 out almost exclusively with these higher levels of the central nervous 

 system, it has generally been thought that synapses at these levels had 

 properties of 'plasticity' that were not shared by synapses in the spinal cord. 

 Experimental evidence is now against any such qualitative distinction 

 between the synapses of higher and lower levels of the central nervous 



