Evidence for a lack of distinct rubrospinal somatotopy in the North American opossum and for collateral innervation of the cervical and lumbar enlargements by single rubral neurons.

Studies using axonal transport techniques on the North American opossum show that rubral neurons innervating the cervical cord are not distinctly separated from those which project to lumbar levels. The absence of clear rubrospinal somatotopy contrasts with that described for the placental mammals studied to date. Use of fluorescent markers in double-labeling experiments shows that most rubral neurons in the opossum still innervate either the cervical or lumbar enlargement alone, but that some supply collaterals to both levels.