THE EXTRAPYRAMIDAL MOTOR SYSTEM 



9'9 



CONCLUSIONS 



A synopsis of the many ob.ser\ations and experi- 

 ments collected in the precedina; pages proves rather 

 disappointing. To draw general physiological conclu- 

 sions from the various experimental and clinical facts 

 known about the extrapyramidal system is difficult 

 for three reasons: a) the arbitrary anatomical defini- 

 tion of the extrapyramidal motor system in the basal 

 ganglia limits consideration of functional motor corre- 

 lations; b) the marked divergence between the 

 symptomatology of human extrapyramidal disorders 

 and those experimentally produced in animals; and 

 c) the hazard of making deductions from the results 

 of lesions, stimulations and electrical recordings in 

 limited regions. With these difficulties in mind we 

 may try to draw soine general conclusions. 



Four recent trends of research have contributed im- 

 portantly to our knowledge of the extrapyramidal 

 motor system and of the functions of the basal ganglia : 

 a) the discovery by Hess (100-105, 108) of direction- 

 specific moveinents evoked by diencephalic and 

 mesencephalic stimulation in freely moving cats, and 

 the localization by Hassler (89) and by Hassler & Hess 

 (91) of the nuclei and tracts in the brain stein medi- 

 ating these responses; b) the application of modern 

 neurosurgical techniques, particularly stereotaxic in- 

 strumentation by Spiegel, Wycis, Talairach, Hassler, 

 Riechert, Narabayashi, Leksell, Cooper and others, 

 to problems of extrapyramidal function (92), the 

 results of which have completely changed our con- 

 cepts of the origin of the parkinsonian syndrome; 

 c) the discovery of the motor control of muscle pro- 

 prioceptors by Sommer (236) in man, of the gamma 

 motoneuron system in animals by Leksell (158) and 

 Hunt & Kuffler (120), and of the supraspinal control 

 of this system by Granit & Kaada (72) in which 

 extrapyramidal, cerebellar and pyramidal neuronal 

 circuits play an important role; and d) the develop- 

 ment of microelectrode studies of single neuron units 

 in the basal ganglia, begun by Segundo & Machne 

 (231), through which the rich variety of afferent im- 

 pulses converging on extrapyramidal centers is now 

 being explored. 



Dijferent Organization of Extrapyramidal System 

 in Man and Animals 



Although the gap between animal experiments and 

 neurological disorders in man cannot be bridged, 

 the.se di\'ergences clarify some essential features of the 

 physiological and anatomical organization of the 



human motor system. The different anatomical de- 

 velopment of extrapyramidal structures in man and 

 animals, the development of upright gait, the pre- 

 ponderance of the human pyramidal system and, 

 finally, the different arrangement of the motor mecha- 

 nisms for postural supporting reactions and automatic 

 movements are factors at least partially accounting 

 for the differences observ-ed. 



Characteristic features of human extrapyramidal 

 disorders are the various hyperkinetic syndromes 

 which are unknown or less apparent in animals. This 

 demands a general explanation of hyperkineses on a 

 physiological basis. Such an explanation can now be 

 given. According to Hess, the antagonistic 'tonic' 

 forces of the central motor regulation systems repre- 

 sent latent motion but are normally balanced. Mani- 

 fest movements occur only after this balance has 

 changed by physiological order or pathological dis- 

 order. A disequilibrium due to inhibition or to a loss 

 of one of several balanced forces in the central nervous 

 system may result in unintentional movements in the 

 opposite direction. We assume that the more differ- 

 entiated and delicate balance of human motor func- 

 tions, subserving manual manipulation and regulating 

 upright gait, makes them also more liable to disorders 

 resulting in involuntary movements. 



Mechanism of Extrapyramidal Disorders 



Hyperkinetic symptoms probably similar in mon- 

 keys and man can be produced by lesions of the sub- 

 thalamic nucleus and its surrounding pathways which 

 result in hemiballism and hemichorea. Postural 

 tremor can be evoked in monkeys and less consistently 

 in cats after midbrain tegmental lesions, but parkin- 

 sonian syndromes accompanying disease of the sub- 

 stantia nigra in man cannot be reproduced in 

 animals. Although parkinson-like symptoms are de- 

 scribed in monkeys following large bilateral mesen- 

 cephalic lesions overlying the nigra which were 

 diminished by destruction of the caudal pallidum 

 [Schreiner et al. (229)] as in human parkinsonism, 

 the results of pallidotomy seem to be somewhat differ- 

 ent in man and monkey. 



Decerebration rigidity after midbrain transection 

 or large tegmental lesions in animals is not com- 

 parable to the rigidity of human parkinsonism but is 

 a syndrome revealing lower ijrain-stem motor mecha- 

 nisms released after elimination of postural regula- 

 tion by the higher extrapyramidal and pyramidal 

 centers. Two different physiological mechanisms of 

 decerebrate rigidity depend on hyperactivity of the 



