THE GENERAL PRINCIPLES OF MOTOR INTEGRATION 



793 



More intense rivalrv of the same reactions, with 

 rhythmic ahernating rebound, results in the parkin- 

 sonian type of tremor (21), following lesions at the 

 subthalamic le\el. The only conclusion in terms of 

 positive function of these structures appears to be 

 that they normally interrelate at their various levels 

 the several conflicting types of cortical and subtha- 

 lamic mechanisms of movement so as to give priority 

 to one or another tvpe of behavior and thus integrate 

 the total reaction of the organism to the environment. 

 The inhibition relayed from the cortex to the caudate 

 nucleus to the thalamus and back to the cortex re- 

 ported by McCulloch (31) and Mettler et al. (ji) 

 appears important in regulating various cortical ef- 

 fects. The poverty of active response to electrical 

 stimulation of the basal ganglia is consistent with 

 such a mechanism. If our view is correct, the mecha- 

 nism of natural activation and function of these 

 structures will require a complex background of 

 activity for its demonstration. 



CONCLUSIONS 



We have attempted to describe the general mecha- 

 nisms of motor beha\ior in simple terms. Two impor- 

 tant general principles emerge. First, every motor 

 reaction has an adequate stimulus, immediate or 

 remote; and second, all that is known of motor func- 

 tion indicates that the nervous system as a whole 

 contributes to each motor act. It is not possible to 

 indicate separate mechanisms for posture and move- 

 ment. Postural reactions are fundamental in neural 

 organization, and mo\ement in its most elementary 

 form is seen as modifications 01 postural responses. 

 There is a similar difficulty in defining "function" 

 which can be used only in a general sense equivalent 

 to activity. The operative physiological term is 

 'performance" which from spinal to cortical levels 

 can be traced in various grades of refinement and in 

 more appropriate relation to the whole organism 

 and finally the whole environment. A segmental 

 reflex may be perfectly performed in terms of the 

 segmental structures it serves, but for its integration 

 into behavior we have to examine higher and higher 

 levels of neural activitv. 



In the organization of integration, several arbi- 

 trary levels of performance can be discerned. At the 

 segmental level an anatomically fixed synaptic co- 

 operation of agonists and antagonists is preserved. 

 At the pontine level the neck and labyrinthine re- 

 ceptors, through their related portions of the inter- 



nuncial reticulum, secure a domination, chiefly in 

 terms of additi\e wider postural reactions, of the seg- 

 mental reflex pattern. At the midbrain and subtha- 

 lamic level this in turn is modified by tactile and con- 

 tactual responses from the body surface, the effect 

 of which has been conspicuously absent or fragmen- 

 tary in the spinal or decerebrate preparation. The 

 cerebellum contributes importantly at this level. 

 Autonomic mechanisms, like .somatic ones, are also 

 seen in fragmentary form at the segmental level and 

 in more complex combinations in the brain stem. 

 The hypothalamus adds the homeostasis of the milieu 

 interne and with it the elementary drive that results 

 in total response in coarsest form. With an intact 

 hypothalamus the decorticate animal also exhibits 

 coarse withdrawal or avoiding behavior with ap- 

 propriate fear reaction and rapid swing to aggression. 

 With .such expression are associated total coordina- 

 tions of autonomic response, already grouped by 

 lower brain-stem mechanisms. 



It is the cerebral cortex, with its projections of the 

 exteroceptors, that dominates the whole, able to 

 select items of behavior and modify them in terms of 

 projected reactions. There are only very slender 

 clues as to the mechanism of cortical domination. 

 Our own studies of the defect in movement of the 

 hand as a result of progressing frontal lobe lesions, 

 and the reverse process in recovery (21), convinces 

 us that contactual sensation has an essential part in 

 the full integration of movement in the higher mam- 

 mals. Though desensitization of one part has little 

 effect on its use, deafferentation of a whole limb in 

 the monkey (75) results in paralysis closely resembling 

 the effect of cortical ablation. Free exploratory move- 

 ment in space cannot be performed without the abil- 

 itv to make contactual palpatory exploration. This 

 in turn is based on the grasp reflex which is a 'trig- 

 gered' response. The appropriate contactual stimulus 

 releases an otherwise latent proprioceptive reaction. 

 With loss of the triggering mechanism, the proprio- 

 ceptive response is continuously overactive to every 

 stretch (spasticity). In this sense the proprioceptive 

 system is restrained by the subthalamic system and 

 harnessed to a series of specific contactual triggers 

 or signals in the pattern of the body-on-body righting 

 reflexes. Both the subthalamic mechanism and seg- 

 mental reflex are restrained by the Rolandic cortex 

 where the specific activation is further differentiated 

 into the delicate contactual patterns of the instinctive 

 tactile grasp reaction and the instinctive placing 

 reaction. From this point of view 'release' is dedif- 

 ferentiation or loss of restriction to specific attributes 



