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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



central grey via the frontopontine (Arnold) and 

 parietotemporopontine bundles (Tiirck). An afferent 

 control circuit involving the cerebellum becomes 

 effective in regulating voluntary movements by 

 means of these very numerous corticopontine con- 

 nections — especially in primates and in man — the 

 fibers of which are much more numerous e\en in the 

 macaque than are the fibers of the pyramidal tract 

 [Verhaart (263, 264)]. As shown in figure 12 this 

 circuit goes back to the cortex since the nuclei pontis 

 intermedii are closely connected with the neo- 

 cerebeliar hemispheres whicli cooperate intimately 

 with the motor cortex, particularly by excitation of 

 area 47, the principal site of origin of the largest 

 fibers of the pyramidal tract. The neocerebellum 

 projects to area 47 via the brachium conjuncti\um 

 and the nucleus \entro-oralis posterior thalami 

 (X'.o.p.). The existence of this neuronal link has also 

 been confirmed electrophysiologically by W'alker 



(285). 



Thus, by means of the detour o\er the cerebellar 

 hemispheres, the psychomotor cortical areas can 

 stimulate the precentral area responsible for volun- 

 tarv motor activity. Simultaneously there is a pos- 

 sibilitv that niger impulses initiated by the activity 

 of the psychomotor cortical areas may be conveyed 

 to the peripheral motor system. Indeed, the impulses 

 destined for the motoneurons and originating in the 

 cortical regions giving rise to the pyramidal tracts 

 do not travel via the pyramids alone. As Lloyd (165) 

 has shown, the impulses traveling in the pyramidal 

 tract are preceded by impulses conducted by long 

 reticulospinal and propriospinal fibers. These im- 

 pulses activate the interneurons and probably also 

 the motoneurons of the anterior horn. The pyramidal 

 fibers themselves do not usually connect directly 

 with the motoneurons but first induce firing of 

 interneurons which in their turn influence the ex- 

 citability of the motoneurons or of spinal synapses of 

 the peripheral mechanisms controlling muscular 

 tone or length (the external loops of fig. 12). 



Some of the impulses from area 47 go via cortico- 

 pontine or collateral fibers to the oral pontine ganglia 

 from which they may proceed to the lateral part of 

 the cerebellar anterior lobe (lobulus quadrangularis 

 anterior). This structure is responsible for integration 

 of pyramidal impulses with impulses from the muscle 

 and tendon receptors and probably also from other 

 exteroceptive and vestibular sources. The results of 

 this coordinative activity are communicated to the 

 nucleus ruber via the nucleus dentatus magnocellu- 



laris, according to Hassler (83), and eventually reach 

 the motor cortex, so completing a feed-back circuit. 



In its turn, the nucleus ruber regulates the ex- 

 citai;)ility of the interneuronal mechanisms of the 

 anterior horn and of the gamma cells \-ia the rubro- 

 spinal tract and rubrorcticulospinal fibers, its inte- 

 grating cerebellofugal impulses with others originating 

 in the precentral cortex, the pallidum and the systems 

 regulating statokinetic acti\'ity and locomotion. 

 Another feed-back system originates in the nucleus 

 ruber, the central tegmental pathway to the inferior 

 olives. These structures, in which the feed-back 

 impulses are coordinated with impulses from the 

 anterior columns of the spinal cord, project to all 

 areas of the cerebellar cortex, including to the lobulus 

 quadrangularis anterior which, as noted above, is 

 the origin of afferent impulses to the nucleus ruber. 

 In this way a multineuronal feed-back circuit which 

 controls the ruber activity in part Ijy means of its 

 own impulses is closed. 



Not only are the cortical and rubral efferent motor 

 systems provided with such feed-back mechanisms 

 but so is the striatum itself (the cerebelloemiiolo- 

 centrostriatal neuronal chain shown in fig. 12). The 

 intermediate zone of the anterior cerebellar lobe 

 where the spinocerebellar impulses from the muscle 

 receptors are coordinated with those from the motoi 

 cortex is provided with a sort of 'efferent copy' (von 

 Hoist) through the feed-back systems from area 47 

 reaching it via the oral pontine ganglia. These 

 integrated impulses are projected from the inter- 

 mediate zone of the cerebellar anterior lobe via the 

 nucleus emijoliformis and the centromedian nucleus 

 to the putamen and caudate nucleus. 



These extrapyramidal centers convey their im- 

 pulses through the internal and external pallidum 

 and nucleus ventro-oralis anterior of the thalamus 

 (\'.o.a.) to area 6aa, area 6a/J and the supplementary 

 motor area following manifold coordination with 

 impulses from other structures. These cortical areas 

 also communicate the pattern of their efferent im- 

 pulses via the oral pontine ganglia to the lateral zone 

 of the anterior cerebellar lobe and in part also back 

 to putamen, caudate nucleus or pallidum (88). 

 Thus it appears that each efferent motor system is 

 provided with such a self-regulating feed-iiack 

 mechanism. 



Further, impulses from the intermediate zone of 

 the anterior cerebellar lobe proceed via the nuclei 

 globosi to the statokinetic systems of the mesenceph 

 alon: the nuclei interstitialis, prestitialis and pre- 

 commissuralis. These mechanisms controlling posture 



