THE EXTRAPYRAMIDAL MOTOR SYSTEM 



921 



15). Tremor may be facilitated Ijy impulses arising 

 from the pallidum and the afferent pathways to the 

 motor cortex, and conducted by the corticospinal 

 tract and other extrapyramidal pathways. We regard 

 the essential mechanism of tremor as a release of 

 rhythmic antagonistic mo\ements occurring in lower 

 spinal or buUiar levels. 



Lower Centers of Statokinetic Regulation 

 in the Brain Stem 



The substrate of Hess" direction-specific movements 

 of eyes, head and trunk resides in the lower extra- 

 pyramidal structures of the diencephalon and mesen- 

 cephalon, as shown in figures 8 and 9. Stimulation in 

 the cat of these structures results in rotatory, raising, 

 lowering or turning movements, while destruction 

 results in positions which are the mirror image of 

 those resulting from stimulation (fig. 10). Rotatory 

 movements around the longitudinal axis are obtained 

 from the interstitial nucleus, from its fiber connections 

 and from medial fibers of the brachium conjunctivum. 

 The efferent paths run through the interstitiospinal 

 tract. Raising movements around the biteinporal 

 axis are obtained from the prestitial nucleus. The 

 efferent paths go through the medial longitudinal 

 Ijundle and rubrospinal tract. Lowering movements 

 around the bitemporal axis are obtained from the 

 precommissural nucleus and its descending tract to 

 the tegmentum. Ipsiversive turning movements in the 

 horizontal plane around the vertical axis are obtained 

 from the region of the mesencephalic reticular forma- 

 tion supplied by ipsilateral vestibuloreticulothalamic 

 fibers and by the vestibulothalamocortical system. 

 These parts of the reticular formation seem also to 

 contain the efferent mechanisms of contraversive cor- 

 tical and subcortical adversive systems, after crossing 

 in the mesencephalon. 



These results are obtained only in quadruped ani- 

 mals. The organization of these systems is essentially 

 different in man who has developed a quite different 

 relation between head position, eye movements and 

 trunk axis in his upright posture. The common extra- 

 pyramidal disorder of oblique head turning in human 

 torticollis and torsion dystonia probably represents a 

 regression to lower phylogenetic mechanisms released 

 by asymmetrical disturbances of higher centers regu- 

 lating head and body posture. 



Extrapyramidal Cortical Areas 



Somatomotor movements obtained by stimulation 

 of the cerebral cortex after complete destruction of 



the pyramidal tract were among the first extrapyram- 

 idal motor functions demonstrated experimentally. 

 However, the neurophysiological significance of the 

 so-called "extrapyramidal areas' of the cerebral cortex 

 (areas 6, 8 and 4s) still remains problematic. Further, 

 the existence of a component of the pyramidal tract 

 arising in extrapyramidal areas seems rather certain. 

 On the other hand, adversive movements after stimu- 

 lation of area 6 can be obtained without connections 

 of this area with area 4 and after interruption of the 

 pyramidal tract. The complex integration of motor 

 activity requires close coordination of the extrapyram- 

 idal system with the pyramidal system in the 

 cerebral cortex, the cerebellum and the peripheral 

 receptors, particularly in man. Only in the lower 

 forms does the extrapyramidal motor system, to- 

 gether with the reticular activating system and spinal 

 mechanisms, seem to be sufficient to integrate in- 

 stinctive motor behavior. 



Afferent Mechanisms 



Apparently the proprioceptive influences on the 

 extrapyramidal centers necessary for their function 

 act through the cerebellum. The various loops of 

 neuronal chains illustrated in figure 12 seem to be 

 the main anatomical mechanisms. Besides the cere- 

 bellar contribution, direct proprioceptive influence 

 must also be postulated for the various direction- 

 specific movements of the head and eye. At least cer- 

 vical joint and labyrinthine receptors signaling the 

 position of the head inust operate continuously as 

 afferent regulators. Many other afferent impulses 

 apparently impinge on the neurons of the higher 

 extrapyramidal centers, as recent unit records from 

 the striatum have shown. 



Efferent Meclianisms and Cooperation with 

 Motor Cortex and Cerebellum 



How the efferent pathwass of the extrapyramidal 

 motor system are coordinated with the pyramidal and 

 cerebellar systems can not yet be formulated with con- 

 fidence. A tentative scheine suggested to Hassler in 

 1956 by the anatomical connections is shown in figure 

 1 2. The most important efferent pathways to the 

 spinal motor horn .seem to be the rapidly conducting 

 reticulospinal tracts. The cerebellum contains the 

 principal mechanisms coordinating the different extra- 

 pyramidal centers with each other, with the reticular 

 formation and with the cortical motor system. Special 

 efferent structures of the cerebellum reach the reticu- 



