Descending motor tracts such as the reticulospinal tract and the rubrospinal tract may contribute to voluntary movement and may assist in recovery of function after damage to the motor cortex or to the corticospinal tract (Fig. 2.4). The reticulospinal tract arises from the pontine and medullary reticular formation and descends in the ventral and ventro-lateral columns of the spinal cord. In man, the reticulospinal tract exists as scattered bundles of fibers that terminate primarily in the ventromedial portion of the spinal gray matter at or above the cervical enlargement (Nathan et al., 1996). Only a small proportion of the fibers continue into the thoracic, lumbar, and sacral levels of the spinal cord (Nathan et al., 1996). The origin of the reticulospinal tract, the reticular formation, receives input from a variety of sources, including bilateral motor cortical areas (Kuypers and Lawrence, 1967), and is heavily interconnected with other brainstem structures such as the vestibular nuclei (Kuypers, 1982). The reticular formation, via the reticulospinal tract, exerts control chiefly over axial and proximal limb musculature to assist with postural control and orientation (Lawrence and Kuypers, 1968b; Kuypers, 1982). In man, the reticulospinal tract has been cited as an anatomical substrate for the recovery of function, along with the ipsilateral corticospinal tract, after partial cordo-tomy (Nathan and Smith, 1973) and after capsular stroke (Fries et al., 1991). If the reticulospinal tract provides compensatory control over spinal motoneu-rons after damage to the motor cortex or the corti-cospinal tract, this compensatory control would be greatest over the axial and proximal upper limb muscles.
The rubrospinal tract arises from the magnocellu-lar red nucleus, crosses directly to the opposite side in the anterior tegmental decussation, and descends in the lateral column of the spinal cord, just anterior to and somewhat intermingled with the corticospinal tract. Axons from the rubrospinal tract terminate in dorsal and lateral parts of the intermediate zone of the spinal cord. In contrast to the bilateral motor cortical input to the reticular formation, the red nucleus receives input from the ipsilateral cerebral cortex (Kuypers and Lawrence, 1967). In non-human primates, the rubrospinal tract exerts control over proximal and distal limb motoneurons and can provide some degree of compensatory control after damage to the corticospinal tract (Lawrence and Kuypers, 1968a, b; Kuypers, 1982; Cheney et al., 1991; Belhaj-Saif and Cheney, 2000). In man, the rubrospinal tract is considerably smaller and only a minimal number of axons likely extend down into the upper cervical cord (Nathan and Smith, 1982). Although the extent and relative importance of the rubrospinal tract in man remains unclear, the possibility exists that the rubrospinal tract could provide some compensatory control of voluntary movement after damage to the motor cortex or corticospinal tract in humans. If the rubrospinal tract were to provide compensatory control of motoneurons in man, then this control would have to be exerted primarily through the most rostral cervical segments.
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