Long descending projections of the hypothalamus in the pigeon, Columba livia

Abstract

An autoradiographic analysis was performed on the descending projections of nucleus periventricularis magnocellularis (PVM) of the hypothala‐mus in the pigeon. A PVM‐medullospinal pathway was observed coursing posteriorly through the lateral hypotlialamus, ventrolateral midbrain teg‐mentum, and into the spinal lemniscus (ls) in the ventrolateral pons and medulla. In the pons, some fibers course dorsomedially from ls and terminate at the lateral border of 1 he locus coeruleus. At medullary levels, fibers from ls sweep dorsomedially in the plexus of Horsley and project to certain regions of the nucleus of the solitary tract (NTS) and the dorsal motor nucleus of the vagus (NX). Specifically, PVM fibers project heavily into NTS subnuclei medialis superficialis, medialis ventralis, and lateralis (sulcalis) dorsalis as well as into the ventral parvocellular subnucleus of NX. Fibers in ls were traced caudally into the lateral funiculus as far as upper cervical levels of the spinal cord. Although autoradiographs of lower cervical or thoracic spinal cord sections were not available, PVM fibers do descend to thoracic spinal cord levels, as evidenced by the retrograde transport of horseradish peroxidase. In addition to the medullospinal pathway, the autoradiographs demonstrated PVM projections to septum, diencephalon, and midbrain. Labeled PVM fibers are found in the lateral septal nucleus, nucleus of the anterior pallial commisure, dorsomedial thalamic nucleus, dorsolateral anterior tha‐lamic nucleus (pars ventralis), median eminence, medial and lateral hypo‐thalamus, medial mammillary area, and nucleus intercollicularis and central gray of the midbrain. The projection of fibers to medullospinal regions and median eminence suggests that PVM is homologous to the mammalian par a ventricular nucleus. These projections to specific subnuclei of NTS and NX denote hypo‐thalamic control over certain autonomic functions.