Cerebellar Efferents

The cerebellar nuclei are the principal source of cerebellar efferent fibers. The efferent fibers from the cerebellum are distributed to several parts of the central nervous system (see Fig 17-3 ). Cerebellar output is essentially ipsilateral. Most of the efferents leave the cerebellum via the superior cerebellar peduncle and a few via the inferior cerebellar peduncle. Efferent cerebellar pathways descend to the brain stem and spinal cord, and ascend to the cerebral cortex. Efferents from the flocculonodular lobe project mainly to the vestibular nuclei in the brain stem directly and indirectly via the fastigial nuclei. Efferents from the globose and emboliform nuclei descend in the contralateral superior cerebellar peduncle and ipsilateral inferior peduncle. These fibers end mainly in the reticular formation and the vestibular nuclei in the brain stem. Output from the dentate nuclei ascends in the ipsilateral superior peduncle. These fibers end in the contralateral thalamus (ventral lateral thalamic nucleus), the contralateral red nucleus, and some nuclei in the brain stem. The main efferent output from the red nucleus goes to the spinal cord (rubrospinal tract). The main projection from the thalamus goes to motor and premotor cortices. In unilateral cerebellar lesions, symptoms of limb ataxia occur

Figure 17-3 The efferent connections of the cerebellum. 1, Premotor cortex. 2, Motor cortex. 3, Pyramidal tract. 4, Ventral lateral thalamic nucleus. 5, Ventral anterior thalamic nucleus. 6, Griseum centralis mesencephali. 7, Red nucleus (parvocellular part). 8, Red nucleus (magnocellular part). 9, Uncinate cerebellar fasciculus (ascending ramus). 10, Central tegmental tract. 11, Rubrospinal tract. 12, Superior cerebellar peduncle. 13, Vermis. 14, Intermediate part of cerebellar hemisphere. 15, Cerebellar hemisphere. 16, Uncinate cerebellar fasciculus. 17, Fastigial nucleus. 18, Interposed nucleus. 19, Dentate nucleus. 20, Nodulus (flocculonodular lobe). 21, Flocculus (flocculonodular lobe). 22, Vestibular nuclei. 23, Reticular formation. 24, Medial longitudinal fasciculus. (From Nieuwenhuys R, Voogd J, van Huijzen C: The Human Central Nervous System. A Synopsis and Atlas. Berlin, Springer, 1988, p 166.)

ipsilaterally because the major parts of both the corticospinal and rubrospinal tract cross within the brain stem (double-crossing). In addition, the dentate nucleus sends projections to cognitive areas of the contralateral frontal cortex. y

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