Localizing spinal-cord-projecting neurons in adult albino rats.

Following horseradish peroxidase injection into the cervical and lumbosacral cords of adult albino rats, labeled neurons were seen in the first cervical segment, brain stem, and cerebellar and diencephalic nuclei. A new pathway, the faciospinal projection, originating in the medial portion of the rostral part of the facial nucleus, was traced. Another new pathway, the olivospinal pathway, is probably also present. Our results for neurons projecting to the spinal cord (spinal-projecting neurons) from the nucleus ambiuus, dorsal motor nucleus of the vagus, superior vestibular nucleus and nucleus f, nucleus Darshevch, nucleus Rolleri, nucleus prepositus hypoglossi, and nucleus of the posterior commissure have been reported before in other mammals but not in rats. Projections from the following regions are in general agreement with previous results in rats, but show significant topographical differences: the first cervical segment; nuclei gracilis, cuneatus, and cuneatus lateralis; the midline and lateral reticular nuclear complex; the trigeminal nuclear complex (spinal, principal, and mesencephalic); nucleus of the tractus solitarius; the medial, lateral, and descending vestibular nuclei, nuclei coeruleus and subcoeruleus; superior colliculus; interstitial nucleus of Cajal, and the deep cerebellar nuclei. The distribution of labeled neurons in the nucleus parabrachialis, nucleus tegmentolaterodorsalis, nucleus Kölliker-Fuse, nucleus Edinger-Westphal, and the hypothalamic nuclear complex confirmed that of previous reports in rats. With the exception of a few nuclear groups which project primarily to either lumbosacral (e.g., the paraventricular nucleus of the hypothalamus) or cervical segments (e.g., the facial motor nucleus and the superior colliculus) most of the other nuclear groups project to both the lumbar and cervical levels. There is no distinct somatotopy in the neuronal groups projecting to both cervical and lumbosacral levels. With only a few exceptions (e.g., the superior vestibular nucleus) most of the spinal-projecting neurons are bilaterally distributed, some with contralateral and others with ipsilateral predominance.