21 8 BRAIN MECHANISMS AND LEARNING 



animal lifts its corresponding limb in response to the conditioned stimulus 

 signalling pain stimulation, the excitations are rapidly redistributed along 

 the axial musculature of its body. Owing to this rapid process, the animal 

 assumes a new posture with new relations between the points of support 

 and the centre of gravity, enabling it to raise the corresponding limb 

 (Shumilina, 1939). 



An electromyographic analysis of the antagonistic muscles in the 

 different hmbs of animals and man made it possible to show that this 

 excitation, which we have named 'positional excitation', very rapidly 

 reaches the extensors of the limbs, and the entire body shifts to three 

 points of support within several fractions of a second before the limb 

 previously stimulated by electric current is raised (Shumilma, 1949; 

 Kasyanov, 1950). 



hi this interesting phenomenon we thought it possible to analyse the 

 composition of the effector complex of excitations in so commonplace a 

 conditioned motor act as the jerking away of the hind limb. 



For the physiologist interested in higher nervous activity and animal 

 behaviour this phenomenon of the dual nature of excitations in the motor 

 conditioned reflex is important because both the tirst and second stages 

 arc dearly of a couditiivwd reflex character despite their different central localiza- 

 tion. As a matter of fact, the rapid redistribution of the tonic tensions in 

 the axial musculature of the body and the proximal parts of the limbs is 

 not 'diffuse' in the ordinary physiological sense. This positional excitation 

 is distributed over very dcfniite motor neurones which shape the animal's 

 posture precisely corresponding to the future raising of one oi the limbs. 

 From the biomechanical point of view we know very well that, it it had 

 not been for this preliminary phase of shift of the centre of gravity in 

 accordance with the basic points oi support, the dog would fall, in 

 attempting to raise the hind limb, as a bronze statuette falls when one of 

 its legs is broken. 



A conditioned motor reflex may be elaborated tor any of the dog's 

 four limbs by reinforcing it with electric current, and in each individual 

 case the positional excitation will be distributed over the body segments 

 anew, i.e. depending on the limb which will be lifted within several 

 fractions of a second after the application ot the conditioned stimulus. 

 This fact is the best proof that both stages in the development of motor 

 excitations are ot a conditioned reflex nature. 



At the same time we know very well that the central localization of 

 both excitations is not the same. Whereas the positional excitation forms 

 at the level of subcortical centres, especially in the neural elements of the 



