THE EXTRAPYRAMIDAL MOTOR SYSTEM 



901 



and locomotor activity, in addition to their efferent 

 impulses to the peripheral motor system, give rise to a 

 feed-back circuit via the medial tegmental tract of 

 Ogawa to the medial accessory olives and thence 

 back to the pars intermedia of the anterior cerebellar 

 lolie and to the vermis anterior. The latter is a col- 

 lecting area for information which is coordinated 

 and then conveyed via the nucleus fastigii to the 

 vestibular nuclei and to the reticular formation. The 

 vestibular nuclei again are pro\ided with a self- 

 regulating mechanism over vestibulocereisellar fibers 

 projecting especially to the \ermis. The mesen- 

 cephalic reticular formation is moreo\er the common 

 control organ for horizontal turning movements 

 to the ipsilateral side and is also a collecting area 

 for impulses of very heterogeneous origins. 



Feedback for regulation of intrinsic activity and 

 that of other motor systems seeins to be a general 

 principle of all motor systems. Each .system is capable 

 of taking into account, for its later activity, modifica- 

 tions in the peripheral motor situation induced by its 

 proper influence and also of coordinating these 

 modifications w'ith information concerning the in- 

 nervation and resistance in the periphery and with 

 impulses from other systems. 



However, this intrinsic and coordinati\e regulatory 

 activity within tlic central motor systems does not 

 seem aljle to cope with all the possible perturbations 

 of peripheral motor activity. Another coordinative 

 process occurs at the spinal segmental level. Here 

 the direct corticospinal or other long fiber systems in 

 general have no direct connections with the motoneu- 

 rons. Reticulospinal and propriospinal fibers are 

 interposed between the extrapyramidal centers and 

 the anterior horn system, while Lloyd (165) was 

 able to demonstrate electrophysiologically the lack of 

 effect of the direct bulbospinal fibers on motor 

 mechanisms. The large number of afferents ending 

 on the anterior horn interneurons and the 

 motoneurons definitely suggests that the question 

 whether the central impulses can produce their 

 destined effect within the peripheral motor system at a 

 certain moment is once more raised and checked in 

 the anterior horn itself. The interneuronal system 

 of the anterior horn is also responsilile for the first 

 control and checking of the afferent impulses from 

 the muscles which ordinarily tend to elicit motor 

 impulses. Furthermore, the firing of both the alpha 

 and gamma motoneurons is constantly controlled 

 peripherally by the mechanisms regulating muscle 

 tension and length, in close correlation with pro- 



prioceptive and possibly also with other as yet 

 unknown peripheral control mechanisms. 



The various pyramidal and extrapyramidal im- 

 pulses integrated in the self-regulating circuits 

 described abo\e are conveyed to the anterior horn 

 by the following pathways: tractus reticulospinalis 

 lateralis and ventralis, tractus rubrospinalis, tractus 

 vestibulospinalis, tractus interstitiospinalis, tractus 

 prestitiospinalis and descending nigrospinal fibers, 

 occasionally interrupted in certain areasof the reticular 

 formation. 



PHYSIOLOGICAL CORRELATIONS .AND 

 FUNCTIONAL SIGNIFICANCE 



In the foregoing portion of this chapter the experi- 

 mental and clinical findings relative to the various 

 extrapyramidal structures have ijcen reviewed in an 

 effort to identify specific functions for each nucleus. 



However, only in the case of the directional mesodi- 

 encephalic responses described by Hess in the cat 

 does the localization of the inorphological substrates 

 and efferent pathways seem certain enough to pro- 

 vide a useful model for study of extrapyramidal motor 

 performances. No other functions can yet be defined 

 in physiological and morphological terms. It seems 

 to be certain only that the extrapyramidal motor 

 system contains many neuronal chains and self- 

 regulating mechanisms with positive and negative 

 feedback, and that close coordination with the corti- 

 cospinal system of the motor cortex and with the 

 cerebellum is the main condition for normal extra- 

 pyramidal function, this relation becoming increas- 

 ingly important in the higher forms, especially in 

 monkeys and man. 



Leaving now an essentially anatomical approach, 

 we will in the following discuss from a physiological 

 viewpoint the activities of the extrapyramidal system 

 in relation to posture and locomotion, spinal reflexes, 

 the thalamoreticular system, instinctive behavior and 

 some aspects of the electrical activit\- of the brain. 



Rale in Posture and Locomotion 



An important function of the extrapyrainidal motor 

 system is the regulation of posture. The mechanisms 

 of postural regulation are localized mostly in the 

 lower brain stem as they involve close coordination 

 witii the cerebellum and vestibular apparatus. 



POSTURAL MECHANISMS AS CONCEIVED BY MAGNUS 



school; optovestibular regulation. Magnus' book 



