DISTRIBUTION AND CONNECTIONS OF INSPIRATORY PREMOTOR NEURONS IN THE BRAIN-STEM OF THE PIGEON (COLUMBA-LIVIA)

We have recorded extracellular, inspiratory-related (IR) unit activity in the medulla at locations corresponding to those of neurons retrogr adely labeled by injections of retrograde tracers in the lower brachia l and upper thoracic spinal cora, injections that covered cell bodies and dendrites of motoneurons innervating inspiratory muscles. Bulbospi nal neurons were distributed throughout the dorsomedial and ventrolate ral medulla, from the spinomedullary junction through about 0.8 mm ros tral to the obex. Almost all of the 104 IR units recorded were located in corresponding parts of the ventrolateral medulla, rostral to nucle us retroambigualis, where expiratory related units are found. Injectio ns of biotinylated dextran amine at the recording sites labeled projec tions both to the spinal cord and to the brainstem. In the lower brach ial and upper thoracic spinal cord, bulbospinal axons traveled predomi nantly in the contralateral dorsolateral funiculus and terminated incl ose relation to the dendrites of inspiratory motoneurons retrogradely labeled with cholera toxin B-chain. In the brainstem, there were predo minantly ipsilateral projections to the nucleus retroambigualis, trach eosyringeal motor nucleus (XIIts), ventrolateral nucleus of the rostra l medulla, infraolivary superior nucleus, ventrolateral parabrachial n ucleus, and dorsomedial nucleus of the intercollicular complex. In all these nuclei, except XIIts, retrogradely labeled neurons were also fo und, indicating reciprocity of the connections. These results suggest the possibility of monosynaptic connections between inspiratory premot or neurons and inspiratory motoneurons, which, together with connectio ns of IR neurons with other brainstem respiratory-vocal nuclei, seem l ikely to mediate the close coordination that exists in birds between t he vocal and respiratory systems. The distribution of IR neurons in bi rds is similar to that of the rostral ventral respiratory group (rVRG) in mammals. (C) 1997 Wiley-Liss, Inc.