The source of the respiratory drive to nasolabialis motoneurones in the rabbit; a HRP study.

Lower brainstem projections to the motoneurones of the nasolabialis muscles, which show rhythmic respiratory-phased activity were studied in the rabbit using the horseradish peroxidase (HRP) technique. The nasolabial motoneurone pool was first identified by the retrograde transport of HRP injected intramuscularly, and by antidromic stimulation and microelectrode recording techniques. The results from subsequent iontophoretic injection of HRP into the lateral division of the facial nucleus (the nasolabial pool) produced significant ipsilateral labelling in the nucleus ambiguus-retroambigualis (NA-NRA) complex. Labelled cells, predominantly ipsilateral, were also consistently observed in the parvocellular reticular nucleus. Smaller numbers of labelled cells were identified in the ventral, dorsal and gigantocellular nuclei of the reticular formation on both sides of the medulla. A large proportion of HRP-labelled cells of the NRA was located in the caudal medulla where the presence of propriobulbar and bulbospinal respiratory neurones has been well documented. These results suggest that some neurones of the NA-NRA complex may serve as upper respiratory motoneurones to the nasolabial musculature.