The corticospinal tract of the marsupial phalanger (Trichosurus vulpecula)

The origin, course and distribution of corticospinal fibers were determined in the marsupial phalanger ( Trichosurus vulpecula ) by the use of the Nauta‐Gygax technique. Findings in 15 cases of cortical lesions indicated that corticospinal fibers arose mainly within the immediate postorbital and midcortical (parietal) regions and descended throughout the ipsilateral pyramidal bundle. The great majority of the fibers cross in the caudal medullary pyramidal decussation. At spinal cord levels, such fibers compose a large dorsal corticospinal tract located ventrally within the dorsal funiculus and a smaller lateral bundle descending in the medial part of the lateral funiculus. Even in cases of complete unilateral decortication, degenerating corticospinal fibers could be traced only as far caudally as T–10. At cervical levels, the majority of cortiospinal fascicles arborize within the medial portions of Rexed's laminae III through VI. A smaller contingent of fibers extends into the lateral portions of these same laminae. Of particular note were corticospinal fibers which appeared to end within lamina VII and, to some extent, within lamina VIII. Many of the fibers ramifying within lamina VII did so in close proximity to the motor neurons of lamina IX; none, however, could be identified within the confines of lamina IX. At thoracic levels, where lamina VI is indistinct, terminal degeneration of rapidly decreasing amount appeared within the medial portions of laminae IV, V and VII. At more caudal thoracic levels, the area of apparent corticospinal fiber termination was limited mainly to the medial and, to a much less extent, the lateral portions of lamina V.