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Chapter 10: Descending Motor Pathways and the Motor Function of the Spinal Cord

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A patient suddenly developed weakness of his left arm. Damage to which of the following brain regions is the most likely site of injury?

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A. Ventral spinal cord white matter

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B. Lateral spinal cord white matter

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C. Ventral pons

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D. Precentral gyrus

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Comment: The patient has a highly selective lesion because the impairment is restricted to the left arm. White matter lesions in the brain stem and spinal cord are unlikely to produce a single limb motor impairment because axons from all body parts intermingle within very small regions. Among the choices, only the precentral gyrus has a clear somatotopic organization. Moreover, occlusion of a small cortical branch of the middle cerebral artery could selectively damage the arm area of primary motor cortex, in the precentral gyrus.

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Complete the following analogy. Limb control is to trunk control, as

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A. the lateral corticospinal tract is to the ventral corticospinal tract.

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B. the reticulospinal tract is to the rubrospinal tract.

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C. the motor cortex on the medial brain surface is to the motor cortex on the lateral brain surface.

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D. the middle cerebral artery is to the basilar artery.

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Comment: Both the medial and lateral motor cortex represent limb muscles. The territories supplied by the middle cerebral and basilar arteries also contain pathways or nuclei for both limb and trunk control.

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Which of the following statements best describes why unilateral damage to the descending cortical pathway in the basis pedunculi results only in weakness/paralysis of contralateral lower facial, arm, and leg muscles?

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A. Upper facial and trunk muscles are not controlled by the corticospinal and corticobulbar tracts.

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B. Upper facial and trunk muscles are controlled by the ventral corticospinal tract only.

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C. Upper facial and trunk muscles receive bilateral corticospinal and corticobulbar tract control.

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D. Upper facial and trunk muscles are not controlled by the corticospinal and corticobulbar tracts.

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Which of the following statements does not describe a feature of the ventral corticospinal tract?

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A. It descends primarily to the cervical spinal cord only

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B. It controls distal muscles only

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