Rubrospinal projections in the rat

Following stereotaxically placed lesions in the magnocellular and parvicellular portions of the red nucleus in a series of young adult albino rats, the site of termination of the rubrospinal fibers in the cervical cord has been studied using the Nauta‐Gygax and Fink‐Heimer silver methods and electron microscopy. Following three days survival the degenerating rubrospinal tract could be identified in the dorsal aspect of the lateral funiculus, principally contralateral to the lesion. Degenerating axons could be seen projecting to the base of the dorsal horn and intermediate regions of the ventral horn. Degenerating synaptic terminals were electron dense, contained remnants of synaptic vesicles and mitochondria, and usually made synaptic contact with small to medium size dendrites. This study finds that the rubrospinal tract in the rat projects chiefly to the base of the dorsal horn and intermediate regions of the ventral horn. The system forms axodendritic synapses with spinal neurons. This projection is in contrast to the dorsomedial projection of corticospinal terminals seen in this species. The significance of these apparently diverging pathways is discussed.