Novel sources of descending input to the spinal cord of the hatchling chick

Nuclear groups contributing supraspinal input to the spinal cord of the hatchling chick ( Gallus domesticus ) were determined by using the enzyme tracer horseradish peroxidase processed with tetramethylbenzidine histochemistry. Five sources of projections to the spinal cord were found which have not been previously described in any species. All are probably related to autonomic function. They include ipsilateral hypothalamic projections from the lateral mamillary n., suprachiasmatic n., and n. of the lateral tubercle. There is a bilateral projection from the large interstitial cells of the mesencephalic posterior commissure, and in the myelencephalon, a mainly contralateral projection from interstitial cells of the vagus‐glossopharyngeal nerve. Two other projections observed here have not been described in other avian species, one from the accessory vestibular n., the other, from the n. ambiguus. In the cerebellum, projections arise from the main and ventrolateral divisions of the fastigial n., and from “border cells” between the fastigial and interpositus n. The large‐celled submedial vestibular n. projects bilaterally. Several projections previously described only in the pigeon, were confirmed here: the hypothalamic nucleus over the supramammilary decussation, the n. intercollicularis, the tangential n., and the n. alatus, a cell group between the hypoglossal and vagal nuclei. Four sources of input projected only as far as mid‐cervical cord. These are n. intercollicularis, fastigial n., accessory vestibular n., and tangential n. All remaining projections reached to lower lumbosacral cord. Sources of descending input are remarkably similar in mammals and avians. Where homologous nuclei exist, virtually identical projections to the cord are present.