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In this chapter, disorders of the automatic, static, postural, and other less voluntary motor activities of the nervous system are discussed. Many of them are an expression of what has come to be called the extrapyramidal motor system, meaning—according to S. A. K. Wilson, who introduced this term—the motor structures of the basal ganglia and certain related thalamic and brainstem nuclei. However, others such as myoclonus and various tremors have obscure or multiple causes. These are discussed together because they are often combined and because of their inclusion in the neurological subspecialty of movement disorders.
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THE BASAL GANGLIA (STRIATOPALLIDONIGRAL SYSTEM)
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The activities of the basal ganglia and the cerebellum are blended with and modulate the corticospinal system and the postural influence of the extrapyramidal system is indispensable to voluntary corticospinal movements. This close association of the basal ganglia and corticospinal systems becomes evident in the course of many forms of neurologic disease. In many aberrant motor patterns, one sees evidence not only of the activity of the basal ganglia but also of labyrinthine, tonic neck, and other postural reflexes that are mediated through parapyramidal brainstem motor systems, including the vestibulospinal, rubrospinal, and reticulospinal tracts. Observations such as these have blurred the original distinctions between pyramidal and extrapyramidal motor systems. Nevertheless, this division remains a useful concept in clinical work because it informs a distinction among several motor syndromes—one that is characterized by a loss of volitional movement accompanied by spasticity—the corticospinal syndrome; a second by bradykinesia, rigidity, and tremor without loss of paralysis—the hypokinetic basal ganglionic syndrome; a third by involuntary movements (choreoathetosis and dystonia)—the hyperkinetic basal ganglionic syndrome; and yet another by incoordination (ataxia)—the cerebellar syndrome. Table 4–1 summarizes the main clinical differences between corticospinal and extrapyramidal syndromes.
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Anatomic Considerations
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As an anatomic entity, the basal ganglia have no precise definition. Principally they include the caudate nucleus and the lentiform (lenticular, from its lens-like shape) nucleus with its two subdivisions—the putamen and globus pallidus. Insofar as the caudate nucleus and putamen are really a continuous structure (separated only incompletely by fibers of the internal capsule) and are cytologically and functionally distinct from the pallidum, it is more meaningful to divide these nuclear masses into the striatum (or neostriatum), comprising the caudate nucleus and putamen, ...