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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



(173) on body posture summarizes the pioneer work 

 done by himself and his school together with the old 

 literature. The tonic reflexes of the head and trunk 

 induced by proprioceptive afferents from the neck 

 and the otolithic apparatus of the labyrinth were 

 found to be represented in the rhombencephalon 

 and upper cervical cord. The righting reflexes {Stell- 

 rejlexe) were regulated by pontine and mesencephalic 

 centers. However, Magnus and Rademaker's concep- 

 tion that normal body posture is the result of an 

 equilibrium of reflexes is certainly too narrow and its 

 experimental basis obtained by brain-stem transec- 

 tions at different levels in rabbits seems insecure. It 

 must be supplemented by Hess' experiments on 

 richlungsspt'zifische motor responses after diencephalic 

 and mesencephalic stimulation in cats, by observa- 

 tions of behavior of different animal species and by 

 clinical observations of extrapyramidal motor mech- 

 anisms in man. Magnus overrates the reflex concep- 

 tion of peripheral regulation mechanisms from 

 labyrinthine and muscular receptors and neglects the 

 spontaneous innate coordinative activity of the cen- 

 tral nervous system, both of which work together to 

 assure body posture. 



The equilibrium of difTerent 'forces' that assures 

 normal body posture is not an equilibrium of reflexes 

 as Magnus conceived it but a central order of co- 

 ordinated self-regulation of several brain-stem centers 

 that may be modulated by receptor and reflex 

 mechanisms with positive and negative feedback. 

 Magnus arrived at his conclusion mainly from experi- 

 ments in rabbits, a species in which labyrinthine 

 afferents are more important for body posture than 

 in other species. Therefore, Rademaker preferred 

 dogs for his experiments on cerebellar functions and 

 Hess used cats for his stimulation experiments. The 

 extrapyramidal functions of man and of these differ- 

 ent species cannot be homologized. But in spite of the 

 marked species difl"erences .some general conclusions 

 on the regulation of body posture can be drawn. 

 Although Rademaker's claim that the labyrinthine 

 and body-righting reflexes were localized in the red 

 nucleus was not confirmed by the localized rubral 

 lesions produced by Mussen (198), localization of 

 these mechanisms above the pontine level is generally 

 accepted. Several structures in the mesencephalic 

 tegmentum and tectum and in diencephalic sub- 

 thalamic nuclei are the main regulators of body 

 posture. Their identification with certain nuclei and 

 tracts of the brain stem was made possible by Hess' 

 studies involving stimulation and coagulation in the 



mesodiencephalon (described in the preceding section 

 on statokinetic structures in the brain stem). 



Optovestibular regulations are basic to orientation 

 in space and arc the essential senscry basis of postural 

 mechanisms. Among the sense organs concerned, the 

 eye contains the main exteroceptive and the laby- 

 rinth the main proprioceptive receptor organs for 

 brain-stem posture regulations. Special adjustments 

 of posture are mediated partly at the spinal level by 

 receptors from muscles, joints and skin. The central 

 substratum of optovestibular regulations is the lower 

 part of the extrapyramidal motor system which 

 coordinates afferent impulses from the eye and the 

 labyrinth. The main integrating apparatus is the 

 reticular formation of the pons and mesencephalon 

 [Lorente de No (166, 167), Jung (134)] and some 

 mesodiencephalic nuclei [Hess & Weisschedel (112), 

 Hassler & Hess (91)]. The red nucleus is a specialized 

 part of this reticular coordinating system differentiated 

 for receiving cerebellar impulses. 



The motor effects of the optovestibular system can 

 be seen in all striated muscles of the body, but those 

 on the ocular muscles are the easiest to record and to 

 measure quantitatively and are also less complicated 

 by the peripheral regulation of limij posture. The 

 vestibular apparatus with its three semicircular 

 canals represents the three planes of space; corre- 

 spondingly, the eye is moved by three pairs of muscles 

 in these planes. Between the labyrinthine receptors 

 and eye muscle effectors a complicated coordinating 

 apparatus is interposed. This integrating apparatus 

 of the reticular formation modulates the impulses 

 from the labyrinth and coordinates them with 

 messages coming from the retina and other sense 

 organs. The optovestibular functions resulting in eye 

 movements and nystagmus have been described more 

 fully elsewhere [Jung (134)]. Not only the stability 

 of posture but also the effectiveness of perception 

 during movements of the eye and the body {Konstanz 

 der Sehdinge) is the result of fine regulation of the opto- 

 vestibular svstem. This system uses a servomechanism 

 coordinating afferent and reafferent messages with 

 intracentral traces of efferent impulses, as shown by 

 von Hoist (280). The action of vestibular receptors 

 on eye muscle innervation was recently summarized 

 by Szentagothai (251). 



HESS' EXPERIMENTS ON ST.'KTOKINETIC REGUL.ATION .\ND 

 THEIR DYNAMIC INTERPRETATION IN TERMS OF TELEO- 



KiNETic AND EREiSMATic MOTILITY. The anatomical 

 correlates of the brain-stem mechanisms for direction- 



