THE GENERAL PRINCIPLES OF MOTOR INTEGRATION 



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inhibitory) effects for total bodily responses of both 

 somatic and autonomic kinds can be recognized. 



Righting Reflexes and the Midbrain Animal 



Although the decerebrate animal exhibits the 

 tonic neck and labyrinthine reflexes and may be 

 able to stand for a time if appropriately propped 

 upright on its limbs, the attitude is a caricature of 

 that of the normal animal. There is no response to 

 correct some inequality of posture, and once thrown 

 off balance the animal topples over in a passive man- 

 ner with complete absence of righting. The reactions 

 are greatly different if the level of section is ce- 

 phalad to the midcollicular level where sequential 

 reactions begin to make their appearance. The 

 difference is most apparent in the thalamic or de- 

 corticate preparation (6, 68). Whereas the decere- 

 brate and spinal mammalian animals exhibit static 

 reactions, the thalamic one shows integrated kinetic 

 behavior. It can right itself, raise itself to standing 

 position, adjust its posture to surface and gravity, 

 and exhibit progression. Magnus & de Kleyn (68) 

 first analyzed the body-on-body, neck-on-neck, 

 neck-on-head and labyrinthine components of the 

 righting reflexes. Their anatomical mechanism is 

 still in some dispute. The simple structure of the red 

 nucleus in the higher mammals, and the correspond- 

 ingly small size of the rubrospinal tract, make it 

 unlikely that this structure alone is responsible for 

 the remarkable variety of activity that Rademaker 

 (77) claimed depended on its integrity. Lorente de 

 No (66) found that section of the most cephalic part 

 of the raphe of the pontine reticulum could release 

 decerebrate rigidity. Carpenter (14) has accurately 

 destroyed the red nucleus bilaterally in the monkey 

 with no result other than hypokinesia. The scattered 

 reticular system of the midbrain and its complexity 

 of interlacing fibers is less easily identified by physio- 

 logical experiment and is more likely to serve the 

 critical adaptation of function whose removal results 

 in decerebrate rigidity. The complex fiber system 

 that enters the capsule of the red nucleus and the 

 interlacing network of the field of Forel that lies 

 immediately cephalad constitute a main traffic inter- 

 section in the extrapyramidal motor system of the 

 higher mammals. 



The dorsal part of the midbrain tegmentum where 

 the dorsal mesencephalic nucleus lies medial to the 

 oculomotor nuclei is the most cephalic part of the 

 system for coordination of the eyes with head posture 

 (51, 77). The main portion of this complex extends 



down as far as the upper pontine reticular areas and 

 there relates head and eye movement to the rest of 

 the body. It interrelates the vestibular, mesence- 

 phalic fifth and various oculomotor nuclei. Damage 

 to any part of it, but particularly at the pontine level, 

 results in tonic deviation of the head and eyes to the 

 same side. It appears to be responsible for body-on- 

 head righting reflexes and compensatory movements 

 of the eyes with movement of the head. The more 

 intense rigidity, without release of the smail-fibered 

 gamma motor system, produced by the anemic 

 method of decerebration (33, 43), with persisting 

 effects of cerebellar stimulation still present, indicates 

 that, with slightly more cephalic parts of the ventral 

 mesencephalic reticular nuclei intact, an additional 

 adjuvant factor acting directly on extensor motor 

 neurons is added to the intercollicular type of rigidity. 

 With transection just above the superior coUiculus 

 this more intense but more variable type of rigidity 

 is often seen. It tends either to alternate with running 

 or clawing movements of the forelimbs, or intense 

 flexion of the forelimbs (54, 55, 76). The superior 

 collicular level is evidently critical in the sense that 

 small anatomical differences in section result in 

 profound changes in posture (55). The conclusions 

 of Magnus (68) and Rademaker (77) that the right- 

 ing reflexes are present in the midbrain animal and 

 absent after intercollicular section are misleading. 

 Elements of the kinetic righting reactions begin to 

 appear in the ventral mesencephalic tegmentum but 

 chiefly in terms of very tonic and disequilibrated 

 body-on-body and head-on-body responses. This 

 indicates that modulation of proprioceptive responses 

 by the effects of cutaneous and subcutaneous pressure 

 must begin to become effective at this level. The 

 descending pathway must be independent of that of 

 decerebrate rigidity, for .section of the vestibulospinal 

 tract releases contact supporting reactions when 

 these can still he abolished h\ intercollicular section 



(61). 



It is characteristic of the reactions of the midbrain 

 animal that they represent in high degree the tend- 

 ency to swing violently to the opposite response, a 

 feature that is seen in spinal reflexes as 'successive 

 induction" (8g) and 'rebound' (92), and interested 

 Sherrington as one of the factors that determined the 

 turning point in spinal stepping. Rebound is pe- 

 culiar to types of reflex (e.g. ipsilateral extension) 

 where a conflict of central effects is combined with a 

 type of response that tends to lessen the natural 

 stimulus (negati\'e feed back). This in the spinal 

 animal leads to galloping movements following an 



