Medial and Lateral Descending Brainstem Motor Pathways Respectively Control Proximal Muscles of Posture and More Distal Muscles of Skilled Movement
The descending brainstem motor pathways to the spinal cord can be divided into a medial group and a lateral pathway. The vestibulospinal, reticulospinal, and tectospinal tracts constitute the medial brainstem motor pathways, whereas the rubrospinal tract represents the lateral brainstem motor pathway (see Figure 10-3).
The groupings are generally based on the relative position of these tracts within the spinal cord white matter. The axons of the tracts that represent the medial brainstem motor pathways (vestibulospinal, reticulospinal, tectospinal) will travel in more medial regions of the spinal cord white matter (e.g., ventral column) and will synapse within more medial regions of the spinal cord gray matter. Those medial regions of the spinal gray matter contain medial premotor neurons and medial α motor neurons that control the axial and proximal extensor musculature primarily involved in involuntary maintenance and adjustment of posture. Axons of the lateral brainstem motor pathway (rubrospinal) run in a more lateral region of the spinal white matter (lateral column) and synapse in the more lateral spinal gray matter. The premotor and α motor neurons of this region principally control the distal flexor musculature involved in voluntary skilled movement.Thus, medial brainstem motor pathways project to medial regions of the spinal cord gray matter whose neurons control the more medially located (axial and proximal) extensor muscles of posture, whereas the lateral brainstem motor pathway projects to lateral regions of the spinal gray matter whose neurons control the more laterally located (distal) flexor muscles of skilled movement.
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