Brainstem projections to the phrenic nucleus: A HRP study in the cat

Brainstem neurones which project to the phrenic nucleus were identified using retrogradely transported horseradish peroxidase (HRP) as a marker. Following iontophoretic injection of HRP into the phrenic nucleus, labelled cells were encountered throughout large areas of the medulla and pons, but occurred with characteristic high densities in those regions known to contain phasic respiratory neurones: namely, the ventrolateral solitary tract nucleus (vl-NTS), known as the dorsal respiratory group (DRG), the ambiguus complex or ventral respiratory group (VRG) and the parabrachial pontine nuclei (BCM-KF). In 12 cats a total of 1540 cells was identified within these regions, the relative contralateral and ipsilateral contributions were respectively 72%:28% (vl-NTS), 65%:35% for the ambiguus complex, and 5%:95% (BCM-KF). In addition, labelled cells, predominantly ipsilateral, were observed in the pontine and medullary reticular formation and the vestibular nuclei. The labelled cells of the DRG had round, oval or triangular perikarya. Their mean soma diameter was 18.3 micrometers. The HRP-positive cells of the VRG had slightly larger somas (mean 21.2 micrometers) and they were fusiform and triangular. The neurones labelled in the BCM-KF nuclei were more heterogeneous with a mean soma size of 14.9 micrometers. The bilateral projections to the phrenic nucleus from the DRG and the VRG, and the predominantly ipsilateral projection from the BCM-KF are discussed in relation to current electrophysiological and autoradiographic findings.