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Chapter 17: Extrapyramidal Motor Systems: Basal Ganglia & Cerebellum

A 46-year-old woman receives a diagnosis of having a movement disorder associated with disturbances of the basal ganglia. In particular, she presents with writhing, uncoordinated movements at rest of her left arm and leg. A magnetic resonance imaging scan reveals the presence of a tumor affecting mainly the right caudate nucleus and putamen. Which of the following best explains why the dysfunction is expressed on the side of the body contralateral to the region of the basal ganglia directly affected by the stroke?

A. Fibers from the basal ganglia to the spinal cord are crossed.

B. Fibers from the basal ganglia project to motor nuclei of the brainstem, whose axons then project to the contralateral spinal cord.

C. Fibers from the basal ganglia project to the ipsilateral motor cortex.

D. Axons from the basal ganglia project to the cerebellum, whose outputs are known to modulate the contralateral side of the body.

E. Fibers from the basal ganglia project directly to the contralateral motor cortex.

C. The basic principle governing how the basal ganglia control motor activity is that they do so by modulating neurons of the motor cortex and premotor areas (of the ipsilateral side) via synaptic connections in the ventrolateral (VL) and ventral anterior (VA) nuclei of the thalamus. One can see from the following circuits that damage to the basal ganglia on 1 side of the brain will affect cortical neurons on the same side:

  • Globus pallidus (medial segment) → VL nucleus → area 4 of cortex

  • Globus pallidus (medial segment) → VA nucleus → area 6 of cortex

This will result in dyskinesia expressed on the contralateral side of the body because the corticospinal tract is crossed. The other possibilities listed in the question are not viable. Projections of the basal ganglia to the brainstem nuclei are minimal. The basal ganglia do not project fibers down to the spinal cord, nor do they project to the cerebellum.

A 43-year-old man who began to display marked involuntary movements at times of rest is seen by a neurologist, who concludes that he is suffering from Huntington disease. Which of the following neurotransmitters is lost or reduced in this individual?

A. Dopamine in the neostriatum

B. Substance P in the substantia nigra

C. Acetylcholine (ACh) and γ-aminobutyric acid (GABA) in intrastriatal and cortical neurons

D. Serotonin in the neostriatum

E. Histamine in subthalamic nucleus

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