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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



not that of lesions in adjacent neuronal relays (such 

 as V.o.a. or Hi). The efferent pathway from the 

 internal pallidum terminates in the nucleus ventralis 

 oralis anterior of the thalamus (\'.o.a.). This nucleus 

 has no descending connections iaut projects to the 

 precentral cortex in the neighborhood of area 6aa. 

 Accordingly, pallidal impulses also have a facilitatory 

 influence on the interneuronal mechanism of the 

 spinal cord via pallidothalamoprecentral fibers and 

 the corticospinal or pyramidal tract. Suppression of 

 this pyramidal facilitation also produces a decrease 

 in the myotatic reflexes and a decrease in the im- 

 pulses for resting rigor. There is no full explanation, 

 however, for the fact that interruption of the py- 

 ramidal tract is much less effective in reducing 

 rigidity than is the interruption of a certain number 

 of its afferents. This may be related to inhibitory 

 and excitatory impulses in the pyramidal tract itself, 

 which are evoked fjy other afferents. 



Suppression of rigor in parkinsonism also improves 

 posture and the control of motor activity which may 

 even become normal. It was surprising that even the 

 hypokinesia or akinesia, the so-called rigor freie 

 Starre of Bostroem, normally considered as an 

 independent sign of parkinsonism, was improved or 

 suppressed after the operation and that even pre- 

 viously lost associated movements reappeared. 

 Hence, it may be concluded that their mechanism 

 seems to be closely related to that of rigor. 



In trying to define the functional role of the 

 nucleus niger one must emphasize rigidity and 

 akinesia as the most important symptoms of its 

 destruction. A positive function of the nucleus niger 

 in the author's opinion might be supranuclear con- 

 trol of the gamma neurons of the anterior horn and 

 an inhibition of myotatic reflexes. As described above, 

 the gamma innervation often starts earlier than the 

 alpha innervation; thus the 'starter function' of the 

 nucleus niger in \oluntary motor activity facilitates 

 phasic muscle innervation, the rapid onset of a move- 

 ment, associated movements and many automatic 

 movements. The role of this 'starter function' in 

 'ereismatic" or supportative aspects of voluntary 

 movement will be considered later in this chapter. 



In this connection we must emphasize the striking 

 melanin pigmentation of the niger neurons. It is 

 most conspicuous in man but is also found in monkeys 

 and in old horses. Melanin is an end product of 

 metabolic processes involving dihydroxyphenyl de- 

 rivates, such as epinephrine and norepinephrine. 

 Since melanin depots are found in the adrenal 

 medulla, it may be suggested that the nucleus niger 



also may produce epinephrine as a transmitter sub- 

 stance. As Marrazzi & Marrazzi (177) showed, 

 epinephrine may have an inhibitory effect on the 

 anterior horn cells. Therefore, one might expect that 

 the nucleus niger exerts its inhibitory influence on 

 tonic muscular innervation and myotatic reflexes in 

 part by means of an adrenergic transmitter mecha- 

 nism. But the facilitation of tremor by epinephrine 

 and the beneficial effect in man of parasympathicoly- 

 tic agents on symptoms of nucleus niger lesions makes 

 this speculation unlikely. 



NUCLEUS RUBER. The effccts of stimulation and 

 destruction of the nucleus ruber are very contra- 

 dictory. For this there arc three main reasons, a) 

 Anatomically the nucleus ruber is not a homogeneous 

 structure; its various components are hardly com- 

 parable in different animal species, h) It is crossed by 

 the brachium conjunctivum; however, only a few 

 fibers of this important tract terminate in the nucleus 

 ruber, c) It is surrounded by a great number of 

 structures witli motor functions unrelated to the 

 nucleus ruber. In order to decrease these difficulties, 

 we shall distinguish between experiments in rodents 

 and carnivores, on the one hand, and findings in 

 primates, including man, on the other. In rodents 

 and carnivores the major part of the nucleus ruber is 

 constituted of large cells; its efferent pathway is the 

 crossed rubrospinal tract. In primates and in man, 

 on the contrary, small cells predominate in the nucleus 

 ruber and its efferent pathway passes through the 

 central tegmental tract to the lower olives and the 

 reticular formation. An accurate interpretation of 

 the work of Mettler and his co-workers, especially 

 of Carpenter's findings (30), is not possible as long 

 as the fundamental fact is not taken into account 

 that, in primates, the nucleus ruber neurons pass 

 through the central tegmental tract. Walberg's 

 (284) findings also are not in conflict with this fact. 

 Both in dogs and cats and in monkeys and chim- 

 panzees stereotaxic stimulation of the nucleus ruber 

 area in the mesencephalon produces a pattern of 

 motor activity characterized by a concave bending 

 of the trunk and the neck toward the stimulated side, 

 a phenomenon known as the tegmental reaction. 

 This is a partial manifestation of the turning move- 

 ment to the ipsilateral side performed by the animal 

 when it is tied up or when its movements are re- 

 strained, as shown by Ingram, Ranson and Hannett 

 in cats. Such results also have been obtained by Kure 

 et al. (154) in dogs, by Mussen (198) in cats, and by 

 Mettler rt al. (184I in cats and monkeys. However, 



