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Abstract
Brainstem neurones which project to the immediate vicinity of the spinal motoneurones
which supply the intercostal and abdominal respiratory muscles were identified by
means of the retrograde transport of horseradish peroxidase (HRP). A combined electrophysiological
and histological technique was used in which recording of phasic inspiratory or expiratory
motoneurone activity within upper (T3-T4) or lower (T8-T9) thoracic segments was followed
by the ion-tophoretic injection of HRP at these recording sites. HRP labelled cells
were concentrated in those brainstem regions known to contain phasic respiratory neurones,
namely the ventrolateral nucleus of the solitary tract (vl-NTS) or dorsal respiratory
group (DRG), the ambiguus complex or ventral respiratory group (VRG) and the parabrachial
pontine (PB) nuclei. In 18 cats, 248 cells were labelled in these three respiratory
regions of the brainstem while 668 were much more diffusely distributed in other regions
of the medulla and pons. The ipsilateral and contralateral contributions within the
respiratory regions were respectively; 23%:77% (DRG), 33%:67% (VRG), 95%:5% (PB).
These results are considered in the general context of previous electrophysiological
and histological findings, but also with particular reference to a related study of
the projections from brainstem neurones to the phrenic nucleus [32].