Abstract
The present study examined the brainstem projections to the phrenic motor nucleus (PMN) in the rabbit and cat, and to the intermediolateral cell column (IML) in the rabbit using horseradish peroxidase (HRP) as a neuronal tracer. The intermediate and ventrolateral subnuclei of the nucleus of the tractus solitarius (NTS) were shown to project bilaterally to PMN in both species, although to a greater extent in the cat. The region of the nucleus ambiguus (NA) had comparable bilateral projections throughout its rostrocaudal extent in both species. Other regions that contained HRP labeled cells after PMN injections in both species were, the ventral reticular nucleus (VRN), the gigantocellular reticular nucleus (NRG), the raphe nuclei, and the parabrachial/Kolliker-Fuse (PBN/K-F) region. The projections of the respiratory cell groups in the rabbit were confirmed to be direct and specific to PMN by injection of HRP into medullary nuclei (NTS, NA, VRN-NRG, raphe) and examination of subsequent anterograde label in PMN. After HRP application to the cervical vagus nerve, the PMN-projecting cells in the region of NA of the rabbit were shown to overlap with vagal motor neurons at the mid-level of the medulla, but not caudally at the level of nucleus retroambigualis (nRA) or rostrally at the level of retrofacial nucleus (RFN).The projections to PMN and IML were compared in the rabbit by injection of HRP into these nuclei in separate animals. The NTS, VRN-NRG, raphe, and PBN/K-F regions were shown to project to both spinal levels. The ventral portion of nucleus reticularis lateralis (NRL) and an area directly caudal and dorsal to NRL project only to IML. The nRA, NA, and an area medial and ventral to RFN project only to PMN. The implications for the integrated control of the cardiovascular and respiratory systems are discussed in light of these results.