642 BRAIN MECHANISMS AND LEARNING 



instinctive response, there are at every instant internal stimuli which have a reflexo- 

 genic power. The distinction between inborn patterns of X factors and actual 

 internal stimuli X is not so easy to make and there are many controversies about 

 this point. I don't mean, however, that we should not make some distinction be- 

 tween reflex and instinct. 



Anokhin. I agree with Dr Thorpe: the complexity of the inborn nervous 

 structures is the basis for inborn behaviour. We have devoted our research pre- 

 cisely to the mechanisms whereby this complexity emerges. A study ot the develop- 

 ing structures of the nervous system of the embryo has shown that they mature and 

 establish between themselves synaptic connections in a highly selective way and 

 fully in conformity with the type of behavioural act which is essential to the 

 newborn of a given species, and at the time when it is necessary. 



Thus, tor instance, the descending tract of fibres, arising from the tegmentum of 

 the human foetus, when there are not yet any other descending tracts, reaches 

 the 8th cervical segment and establishes there synaptic connections with precisely 

 the motor neurones of the anterior horns, which innervate the deep flexors of the 

 fingers of the hand. In turn, the nerves originating from the cervical cord of the 

 shoulder, innervating the flexors of the fingers, are m^•elinized earlier than 

 the other nerves of the same level. All these phenomena contribute to the rapid 

 emergence of the grasping reflex in the human foetus which is the first definite 

 behavioural act in the 4th month ot its prenatal development. Such heterochrony 

 is even more clearly observed in the growth ot structures and m the emergence of 

 functions in birds of ditfercnt oecologies. 



Whereas in the chick hatching out of the egg there is an accelerated differentia- 

 tion of the motor elements of the lumbar region of the spinal chord, in the rook 

 this accelerated differentiation of the neurones affects first the segments of the 

 shoulder cervical region. Thus the accelerated maturation of the nervous structures 

 progresses in exact correspondence with the functions which are the first ones to be 

 essential in relation to the characteristics of the oecology of that animal. This can 

 also be seen, for instance, in the fact that in the organs of Corti of the foetus of the 

 rook there develop precisely those receptor elements which correspond to the cry 

 of the adult bird: 'Kr ... r ... r ... a ... ' (this can be observed through a special 

 acoustic analysis). This accelerated, selective, oecology-conditioned development 

 of the nervous structures, has been named by us systcnio-oeiicsis since specific 

 functional systems of the organism develop progressively and in a selective way. 



It is obvious that all these reactions are truly inborn since, admittedly, the new- 

 born has not yet had time for any learning. 



EiBL. RoEDER (1955) pointed out that there is no basic difference between instinc- 

 tive and reflex behaviour as far as the neural elements underlying such behaviour 

 are concerned. Different thresholds of neurones seem to make up the main differ- 

 ence, which is a gradual one. In the stable neurones the excitability remains at a 

 constant resting threshold. It needs a stimulus to bring the excitability to the level 

 of discharge. After discharge the excitability falls to zero, then rises gradually again 



