THE EXTRAPYRAMIDAL MOTOR SYSTEM 



899 



posite side. Removal of area 8 is followed by a 

 transitory loss of attention and an impainnent of 

 response to all stimuli from the opposite side, a state 

 resembling the hemianopic impairment of attention 

 in human disorders called 'unilateral neglect' by 

 Welch & Stuteville (292J. There is, furthermore, a 

 paralysis of conjugate horizontal movement of the 

 eyes to the side of the lesion and occasionally forced 

 circling of the animals in the same direction. Bilateral 

 resection of area 8 produces a bilateral restriction of 

 the visual fields and increased motor activity with 

 almost continuous locomotor mo\ements [Kennard 

 etal. (147)]. 



Localized stimulation of the postcentral gyrus 

 (areas 3, i and 2) is followed by reflex twitching of 

 those parts of the body having their sensory repre- 

 sentation in the stimulated area. The motor efTect is 

 due to propagation of the stimulus and excitation of 

 the motor representation located approxiinately at 

 the same level in the precentral gyrus. Lesions of 

 these areas 3, i and 2 do not cause any marked motor 

 impairment; a permanent hypotonia of the muscles 

 appears but voluntary motor activity is fully restored. 

 Simultaneously the contralateral proprioceptive re- 

 flexes are slightly enhanced. The hypotonic central 

 paralysis rapidly produces a contralateral muscular 

 atrophy which almost never occurs following precen- 

 tral lesions in spite of stronger impairment of vol- 

 untary motor activity. 



In man the responses to stimulation of the extra- 

 pyramidal cortex are still problematical, but Foerster 

 (58) found responses of the different cortical areas to 

 be very similar to those described by \'ogt & Vogt 

 in the monkey. Penfield and co-workers later studied 

 the differentiation of the precentral cortex in greater 

 detail; they however failed to distinguish between 

 pyramidal and extrapyramidal fields. The delimita- 

 tion of the motor area toward the frontal pole has 

 not been clearly established. The speech arrest and 



endings of the muscle spindles. They send la afferents to 

 the posterior root and send reflex collaterals directly to the 

 alpha motoneurons (a). This regulating circuit can be shifted 

 to higher sensitivity by gamma innervation of the intrafusal 

 muscle fibers of the muscle spindle. When no central innerva- 

 tion by gamma neurons is available, this servomechanism 

 regulating length is interrupted. 



Lower lejl. Spinal servomechanism regulating muscle ten- 

 sion. This mechanism has facilitatory receptors in the flower- 

 spray endings and an inhibitory receptor in the Golgi 

 tendon organs. Their centripetal fibers, the Ih and // aff'erents 

 in the posterior roots, influence the motoneurons by chains of 

 interneurons. This tension circuit can also be shifted to higher 

 sensitivity by gamma innervation. [Redrawn from Haissier (88).] 



\ocalization fields and, in particular, the motor 

 supplementary field located on the medial side of 

 the heinisphere with its somatotopic organization 

 were added to the picture. \'ogt's map of cortical 

 stimulation designates the latter field as part of area 

 6a/3. In the newest maps of cortical stimulation pre- 

 pared by Penfield & Jasper (208) there is no com- 

 ment whatsoever on the problem of the extrapy- 

 ramidal cortical areas and their functions. 



Responses to stimulation during subcortical opera- 

 tions on the thalamic nuclei projecting to area 4, 6 

 and 8 closely resemble the responses of the correspond- 

 ing cortical areas as would be expected on the basis 

 of the very close functional and anatomical inter- 

 relationship between these structures. 



The resection of parts of area 6 by Horsley was the 

 beginning of the modern surgical therapy of hyper- 

 kinesia. The physiological significance of this ob- 

 servation has been discussed above (p. 877). 



Extrajnramidal Efferent Alechanisms and 

 Their Interrelations with Cortical 

 and Cerebellar Systems 



The parts of the extrapyramidal motor system least 

 well known are the efferent mechanisms. Their 

 functional role is understandable only on the basis 

 of an overall analysis of motor mechanisms in relation 

 to the cerebral cortex and the cerebellum. 



Within the organization of the motor system there 

 is a highest level above that of the precentral motor 

 cortex which makes possible cortical teleokinetic 

 activity (see p. 903). This "psychomotor" level is 

 located in primates in the so-called association areas 

 of the frontal and temporal lobes and possibly also 

 in the integration areas of the parieto-occipital cortex. 

 Impulses for extrapyramidal associated movements 

 and for readiness of the peripiieral motor system — 

 probably mediated by the gamma neurons of the 

 anterior horn — can be initiated over connections 

 between these prefrontal and parietotemporo-occipital 

 areas and the nucleus niger (via the tractus pre- 

 frontonigralis and temporoparietonigralis). As we 

 know, the intention of moving or its iinagination 

 produces an increase of muscular tone under normal 

 conditions and may even evoke involuntary move- 

 ments or spasms in patients with hyperkinetic dis- 

 orders and serious pyramidal lesions. A pathway 

 from the nucleus niger to the internal pallidum 

 simultaneously changes the excitability of the pre- 

 motor cortex (areas 6aa, '4s' and 6a|8). These pre- 

 frontal and parieto-occipital association areas have 

 particularly important connections with the pontine 



