Origin of the rubrospinal tract of the rat.

The location of cells of origin of the rat rubrospinal tract has been determined by the use of retrograde axonal transport of horseradish peroxidase. Rubral neurons projecting to cervical spinal levels are found in dorsal and medial portions of the nucleus while those sending fibers to lumbar cord levels are located ventrally and ventrolaterally within the nucleus. No cells in the rostral one-half of the nucleus were labelled in this study. The results indicate that the rat rubrospinal tract originates from somatotopically-organized groups of neurons in the caudal one-half of the red nucleus.