Chapter  4    Disorders of Movement and Posture +++++––––––IndirrectpathwaySubstantianigra parscompactaCortexCortexCaudate/putamen(striatum)VA/VL complex ofthalamusGlobus pallidus,external segmentSubthalamicnucleusGlobus pallidus,internal segmentD1D2A. Normal configuration of basal ganglia connections +++++–––––B. Parkinson disease (hypokinetic)Substantianigra parscompactaCortexCortexDecreasedexcitationMore tonicinhibitionDiminishedCaudate/putamen(striatum)VA/VL complex ofthalamusGlobus pallidus,external segmentSubthalamicnucleusIncreasedIncreasedDegeneratedGlobus pallidus,internal segmentD1D2 +++++––––––C. Huntington disease (hyperkinetic)Substantianigra parscompactaCortexCortexIncreasedexcitationLess tonicinhibitionCaudate/putamen(striatum)VA/VL complex ofthalamusGlobus pallidus,external segmentSubthalamicnucleusIncreasedDiminishedDegeneratedGlobus pallidus,internal segmentD1D2
Figure 4-4. (Continued)
Ataxia and Disorders of Cerebellar Function
The cerebellum is primarily responsible for the coordination of movements, especially skilled voluntary ones, the control of posture and gait, and the regulation of muscular tone. In addition, the cerebellum may play a role in the modulation of the emotional state and some aspects of cognition. The mechanisms by which these functions are accomplished have been the subject of intense investigation by anatomists and physiologists. Their studies have yielded a mass of data, testimony to the complexity of the organization of the cerebellum and its afferent and efferent connections. A coherent picture of cerebellar function has emerged, and it is possible to relate certain of the symptoms and signs of cerebellar disease to discrete anatomic and functional units.
Knowledge of cerebellar function has been derived mainly from the study of natural and experimental ablative lesions and to a lesser extent from stimulation of the cerebellum, which actually produces little in the way of movement or alterations of induced movement. Furthermore, none of the motor activities of the cerebellum reaches conscious kinesthetic perception; its main role, a critical one, is to assist in the modulation of willed movements. The following discussion of cerebellar structure and function has, of necessity, been simplified; a fuller account can be found in the writings of Jansen and Brodal, of Gilman, and of Thach and colleagues.
Early studies of the comparative anatomy and fiber connections of the cerebellum led to its subdivision into three parts (Fig. 5-1 and Table 5-1): (1) The flocculonodular lobe, located inferiorly, which is phylogenetically the oldest portion of the cerebellum and is much the same in all animals (hence its former designation as archicerebellum). It is separated from the main mass of the cerebellum, the cerebellar hemispheres, by the posterior fissure. (2) The anterior lobe, or paleocerebellum, which is the portion rostral to the primary fissure. In lower animals, the anterior lobe constitutes most of the cerebellum, but in humans it is relatively small, consisting of the anterosuperior vermis and the contiguous paravermian cortex. (3) The posterior lobe, or neocerebellum, consisting of the middle divisions of the vermis and their large lateral extensions. The major portion of the cerebellum, the cerebellar hemispheres proper, falls into this, the largest, subdivision.