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The output of the motor cortex (and the adjacent premotor and supplementary cortices) is shaped by input from the basal ganglia and cerebellum. The basal ganglia and cerebellum participate in corticocortical loops that begin and end in motor regions. The basal ganglia are involved in the initiation and patterning of movements (see Ch. 7), and the cerebellum is involved in the coordination of movements. Both structures are also involved in cognition and eye movements. Lesions of the cerebellum can lead to incoordination of movements (ataxia), imprecision of movements (dysmetria), difficulty with rapid alternating movements (dysdiadochokinesia), truncal and gait instability, and difficulty with articulation of speech (dysarthria). Due to cerebellar involvement in oculomotor and vestibular function, cerebellar lesions can also cause nystagmus, vertigo, nausea, and vomiting.
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ANATOMY AND FUNCTION OF THE CEREBELLUM
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Overall Structure of the Cerebellum
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Like the brain, the cerebellum has two hemispheres consisting of a cortex, deep white matter, and deep gray matter (the bilateral dentate, emboliform, globose, and fastigial nuclei) (Fig. 8–1). At the midline, a cerebellar structure called the vermis lies between the hemispheres. The midline vermis controls coordination of the middle of the body, so pathology of the vermis leads to truncal and gait instability. The laterally placed cerebellar hemispheres control the lateral parts of the body: the limbs. Therefore, lesions of the cerebellar hemisphere can cause limb ataxia. Lesions in the cerebellar hemispheres cause deficits in the arm and/or leg ipsilateral to the affected hemisphere (in contrast to lesions of the cerebral hemispheres which cause deficits in the arm and/or leg contralateral to the affected hemisphere).
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The left and right flocculi and the midline nodulus (together referred to as the flocculonodular lobe) are anterior cerebellar structures involved in vestibular function and eye movements.
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The Cerebellar Peduncles
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In order to coordinate movements, the cerebellum must have access to two types of information (Figs. 8–2, 8–3, and 8–4):
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