THE EXTRAPYRAMIDAL MOTOR SYSTEM 



913 



ments of the eyes and head, together with alterations 

 of muscle tone and of mimetic innervation related to 

 attentive arousal. These nonspecific effects are ob- 

 served after stimulation of many structures from the 

 cortex and basal ganglia to the brain-stem reticular 

 formation. The many structures involved in the ipsi- 

 and contraversive turning effects are summarized in 

 figure 1 1 . Apart from the above mentioned body- 

 raising lesponses, we cannot assign special components 

 of these motor patterns of attention to special nuclei 

 of the extrapyramidal system. However, one of the 

 more important reticular functions is the regulation 

 of eye movements and nystagmus which are among 

 the motor correlates of attentive behavior. Vestibular 

 as well as optokinetic nystagmus is coordinated in the 

 pontine and mesencephalic reticular formation. Re- 

 cent unit analysis in the medial lower reticular forma- 

 tion by Duensing & Schafer (47, 48) has disclosed 

 two groups of neurons, the first having fixed relations 

 to certain phases of nystagmus and showing recipro- 

 cal action in the slow and rapid phases of nystagmus, 

 the second being loosely coupled with nystagmus and 

 primarily activated by general arousal. 



Stimulation experiments during stereotaxic opera- 

 tions have shown that the motor accompaniments of 

 general arousal can also he obtained from various 

 structures in man. They have been observed after 

 stimulation of the centromedian thalamus (fig. 7) 

 and other intralaminar nuclei [Hassler (85, 86), 

 Jung {135)], as well as from the pallidum, accom- 

 panied in the latter ca.se by an arousal type of EEG 

 as shown in figure 18. Although weak low-frequency 

 caudate stimulation may cause inacti\ation, stronger 

 stimuli or higher frequencies elicit unilaterally 

 directed attention to the contralateral side as a part 

 of motor readiness. Lower brain-stem structures have 

 not yet been investigated in man. 



Arousal, however, can be ob.served after stimulation 

 of various structures outside the nonspecific activa- 

 tion system, especialK when tetanic stimuli at high 

 frequency are used, for example that reported by 

 Buchwald & Er\in (19) after such stimulation of the 

 caudate and pallidum and of the amygdala and other 

 rhinencephalic structures, as well as that obtained by 

 French et al. (62 ) froin several regions of the cerebral 

 cortex. 



Periodic alternation of sleep and wakefulness was 

 present in Camper's mesencephalic anencephalus 

 although no cerebral cortex and practically no dien- 

 cephalic structures were preserved. Thus, e\en in man 

 the mesencephalic and pontine parts of the non- 

 specific activation system alone may be able to induce 



changes similar to sleep and wakefulness behavior in 

 the intact organism. 



Finally, a few general remarks on the reticular 

 formation should be made. This structure seems to be 

 the main lower center for extrapyramidal motor func- 

 tions, and especially for bilateral coordination. Bi- 

 lateral functions are insufficiently brought together in 

 the higher diencephalic and telencephalic centers, 

 there being no commissures and only a few fibers 

 connecting the symmetrical nuclei. In contrast to this 

 the mesencephalic and rhombencephalic reticular 

 nuclei have an abundance of crossing fibers. The 

 neurons of the reticular formation seem also to be 

 specifically suited for longitudinal coordination be- 

 tween caudal and oral centers because their large 

 axons have very many collaterals; one reticular cell 

 may have a ramifying axon reaching from the upper 

 cervical cord caudally to the nonspecific thalamic 

 nuclei cranially, as the Scheibels (225) have shown. 



Caudate and pallidum stimulation produces wide- 

 spread bilateral evoked potentials in the cerebral 

 cortex similar to the recruiting waves obtained from 

 the nonspecific thalamic nuclei and reticular forma- 

 tion [Ajmone-Marsan & Dilworth (2), Shimamoto & 

 \'erzeano (233), Umbach (260), and Hassler (85)]. 

 This is a further argument for the close relation of the 

 higher extrapyramidal centers to the nonspecific ac- 

 tivation system of the brain. Similarly, previous condi- 

 tioning stimulation of the mesencephalic reticular 

 formation facilitated and amplified the eff"ects on the 

 cerebral cortex and other telencephalic structures of 

 test stimuli applied to the caudate [Umbach (262)]. 



One sometimes tends to forget, during the present 

 vogue of brain mythology about consciousness and at- 

 tention, that the reticular formation is mainlv a motor 

 coordinating center, the lower part for respiration, the 

 higher parts for eye movements and body posture. 

 The psychological effects of attention and conscious 

 acts are only secondary specializations, derived from 

 basic reticular functions controlling motor behavior, 

 and preponderant solely from an introspective and 

 anthropocentric viewpoint. 



Relation of Extrapyramidal Functions 

 to Instinctive Be/iar'ior 



As mentioned above, all motor mechanisms of the 

 lower mammals and other vertebrates are "extra- 

 pyramidal' by definition because these animals ha\'e 

 no pyramidal tract. .A full treatment of the results of 

 behavioral research on animals cannot be given here, 

 but certain general trends should be mentioned 



