THE EXTRAPYRAMIDAL MOTOR SYSTEM 



88 1 



CENTRUM MEDIANUM. Accordiiis; to current anatomical 

 concepts the centrum medianum receives afferents 

 from the nucleus emboHforniis of the cerebellum 

 (83) and the reticular formation, but not, in contrast 

 to previous opinions, from a direct ipsilateral second- 

 ary trigeminal pathway. According to the physi- 

 ological findings of Starzl et al. (247), it receives 

 impulses from collaterals of all sensory pathways 

 indirectly through the reticular formation. The 

 centrum medianum is common to all higher mam- 

 mals. It is independent of the cortex (271 ) and conveys 

 complex integrated impulses from the cerebellum and 

 reticular formation to both parts of the striatimi 



(83). 



Experimental observations in animals will first be 

 considered. Because of the location of Forel's teg- 

 mental fascicle or the vestibuloreticulothalamic tract, 

 ipsiversive turning movements were often considered 

 to result from stimulation of the centrum medianum. 

 However, the.se moveinents are apparently repre- 

 sented in a pathway which parallels the vestibulo- 

 reticulothalamic tract and ijypasses the centrum 

 medianum ventrally without entering it [Hassler 

 (8g)]. Electrical stimulation of the centrum medianum 

 evokes an arousal reaction and a recruiting response 

 in various cortical areas, especially in the frontal and 

 anterior parietal cortex [Hanbery & Jasper (78)]. 

 Circumscribed lesions in the nucleus ventralis medialis 

 (VM) and ventralis anterior (VA) of the thalamus, 

 if combined with a destruction of the rostral nucleus 

 reticularis thalami, suppress the recruiting response 

 in the frontal cortex. The unspecific projections of 

 the centrum medianum to the parietal corte.x are 

 inactivated only by making an additional lesion in 

 the lateral part of the nucleus ventralis lateralis (VL) 

 thalami [Hanbery et al. (77)]. This lesion also inter- 

 rupts the efferent pathway from the centrum medi- 

 anum to the putamen and caudate nucleus. Thus 

 eflferent impulses from the centrum medianum cannot 

 cross to the nuclei of the extrapyramidal system and 

 re-enter the thalamus before going to the cortex. The 

 lesions in VA and VM and in the rostal part of the 

 nucleus reticularis interrupt the last projections to 

 the premotor cortex and proliably also the fibers 

 entering the pallidum from below. 



The hypnogenic zone of Hess (108) extends to the 

 centrum medianum. Under appropriate environ- 

 mental conditions low-frequency threshold stimulation 

 of this area can produce a beha\'ioral reaction similar 

 to physiological sleep. This has been confirmed by 

 the electrophysiological experiments of Hess, Akert 

 and Koella (5, 98, gg). The most frequent effects 



following electrical stimulation are an inhibition of 

 respiratory acti\'ity both in frequency and amplitude 

 and a decrease of motor excitability [Hess (108)]. 

 No physiological studies of the effect of acute and 

 chronic destruction of the centrum medianum appear 

 to ha\e been made. 



Observations from human pathology will next be 

 presented. In the hereditary form of torsion dystonia 

 tiie small cells of the centrum medianum projecting 

 to the putamen show primary degeneration [Vogt & 

 Vogt (272)]. This would liberate the putamen from 

 cerebellar and brain-stem reticular control. The 

 nerve cells of the centrum inedianum show premature 

 aging and retrograde degeneration following \arious 

 putamen lesions. The physiological significance of 

 this finding is unknown. 



During the successful operations performed by 

 Talairach et al. (252) in patients suffering from 

 thalamic pain, the centrum medianum has been 

 coagulated repeatedly in part or has had its efferent 

 fibers interrupted. No expected deficiencies resulted 

 from these lesions. Tonic movements of the mouth 

 were seen as a result of stimulation of the centrum 

 medianum (Monnier and others). In collaboration 

 with Riechert and Mundinger we only rarely observed 

 such effects following stimulation of the centrum 

 medianum during operations for trigeminal neuralgia. 

 Stimulation of the centrum medianum in man pro- 

 duces a recruiting response which appears in all 

 areas of the ipsilateral cortex and also slightly in the 

 contralateral hemisphere. Stimulation at 8 per sec. 

 or more had a definite arousal effect in man (fig. 7J. 

 In spite of potentiated anesthesia, patients open 

 their eyes, look around during stimulation and relapse 

 into narcosis as soon as stiinulation is discontinued 

 [Hassler (85, 86)]. Partial coagulation of the centrum 

 medianum during operations in the arcuate nucleus 

 for pain relief did not have any apparent effect. 

 However, bilateral lesions of the centrum medianum 

 have not yet been inade. 



The physiological role of the centrum medianum 

 consists of integrating very heterogeneous sensory 

 and cerebellar impulses and elaborating afferent 

 inflow to the striatal system. It controls the overall 

 excitability of the cortex via the caudate nucleus and 

 putamen and seems to trigger — or to inhibit — the 

 motor components of the mechanisms regulating 

 sleep and wakefulness. 



OTHER THALAMIC NUCLEI BELONGING TO THE EXTRA- 

 PYRAMIDAL SYSTEM. The overwhelming majority of 

 the efferent pathways leaving the internal pallidum 



