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In this chapter, the disorders of the automatic, static, postural, and other less-modifiable motor activities of the nervous system are discussed. They 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.

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The activities of the basal ganglia and the cerebellum are blended with and modulate the corticospinal and cortical–brainstem–spinal systems. The static 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. Cerebral lesions that involve the corticospinal tracts result not only in a contralateral paralysis of volitional movements but also in a fixed posture in which the arm is flexed and the leg extended. Similarly, as noted in Chap. 3, interruption of the motor projections by a lesion in the upper pons or midbrain may release a posture in which all four limbs or the arm and leg on one side are extended, and the cervical and thoracolumbar portions of the spine are hyperextended (decerebrate rigidity). In these aberrant motor patterns, one also sees evidence of labyrinthine, tonic neck, and other postural reflexes that are mediated through nonpyramidal bulbospinal and other brainstem motor systems. Observations such as these and the anatomic data presented in Chap. 3 have blurred the classic distinctions between pyramidal and extrapyramidal motor systems. Nevertheless, this division remains a useful, if not an essential, concept in clinical work, because it compels a distinction between several motor syndromes—one that is characterized by a loss of volitional movement accompanied by spasticity—the corticospinal syndrome; a second by akinesia, rigidity, and tremor without loss of voluntary movement—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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Table 4-1 Clinical Differences between Corticospinal and Extrapyramidal Syndromes 
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Much of the criticism of the pyramidal–extrapyramidal concept derives from the terms themselves. The ambiguity related to the term pyramidal was discussed in Chap. 3, in which it was pointed out that pure pyramidal lesions do not cause total paralysis; when total paralysis exists, there is always some degree of involvement of other descending motor pathways ...

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