Projections from the spinal trigeminal nucleus to the entire length of the spinal cord in the rat

Using retrograde transport of horseradish peroxidase, projections were found from the spinal trigeminal nucleus to all levels of the spinal cord in rat. In addition to the previously described projections from nucleus caudalis, axons originate from two areas - one overlapping the nucleus caudalis and interpolaris and one in the ventral division of nucleus oralis - and descend as far as the lumbosacral cord. Projections from the nucleus caudalis and interpolaris are primarily ipsilateral to all levels of the spinal cord. Cells in the nucleus oralis innervating the cervical cord are distributed bilaterally. Projections from the nucleus oralis to the thoracic and lumbosacral segments are entirely crossed, lighter and less extensive than those to cervical levels. Trigeminospinal cells originate from several loci that strongly overlap the distributions of both trigeminotectal and trigeminocerebellar neurons. These findings thus reveal a new and more complexly organized system of trigeminospinal projections than previously recognized.