Distribution and structural characterization of neurons giving rise to descending spinal projections in the turtle, Pseudemys scripta elegans

Descending spinal projections were investigated in the turtle Pseudemys scripta elegans following injections of horseradish peroxidase and/or radioactive wheat germ agglutinin into the spinal cord at various levels. Using various planes of section the cells of origin in the brainstem, cerebellum, and diencephalon were characterized according to their size, dendritic tree, and precise location. Projections to levels as far caudal as the lumbar spinal cord were found to arise from medial and lateral rhombencephalic reticular fields, including the perihypoglossal complex, the nucleus raphe inferior, and the locus coeruleus; from certain subdivisions of the vestibular complex (ipsilateral subnucleus (subn.) ventrolateralis, contralateral subn. ventromedialis, and possibly subn. tangentialis); from the motor trigeminal nucleus; from the contralateral red nucleus, the ipsilateral nucleus (n.) interstitialis of the fasciculus longitudinalis medialis (flm) and from the hypothalamus. Fibers to high cervical levels arose from neurons within the dorsolateral and superior vestibular nuclei, the lateral cerebellar nucleus, the mesencephalic trigeminal nucleus, and from neurons of the optic tectum. Low cervical and thoracic spinal levels were reached by fibers from the torus semicircularis n. laminaris, the n. of the flm, the medial cerebellar nucleus as well as from the n. vestibularis inferior and the principal and descending trigeminal nuclei.