IDENTIFICATION AND CONNECTIONS OF INSPIRATORY PREMOTOR NEURONS IN SONGBIRDS AND BUDGERIGAR

Recordings of extracellular unit activity in the ventrolateral medulla and of electyromyographic activity in either the M. scalenus, a princ ipal inspiratory muscle, or the abdominal expiratory muscles, were use d to identify inspiratory related (IR) neurons. IR neurons extended fr om levels caudal to the obex through the caudal level of the descendin g vestibular nucleus. This distribution was found to correspond to tha t of a subset of cells retrogradely labeled from injections of neurona l tracers into the upper thoracic spinal cord, where motoneurons inner vating the M. scalenus were located by retrograde transport. Injection s of biotinylated dextran amine at the recording sites resulted in pro jections to the spinal cord and brainstem. Bulbospinal axons traveled in the lateral funiculus, predominantly contralaterally, and terminate d in relation to the dendrites and cell bodies of motoneurons innervat ing the M. scalenus. Brainstem nuclei receiving projections from injec tions at IR loci included the retroambigualis, tracheosyringeal motor nucleus, ventrolateral nucleus of the rostral medulla, infra-olivaris superior, ventrolateral parabrachial nucleus, and the dorsomedial nucl eus of the intercollicular complex. In the finches, there were also bi lateral projections to nucleus uvaeformis of the posterior thalamus. T he spinal and brainstem projections are similar to those found in pige on (Reinke and Wild, [1997] J. Comp. Neurol. 379:347-362), and probabl y mediate the intricate coordination of the vocal (syringeal) and resp iratory systems for the control of vocalization. The distribution of I R neurons in birds is similar to that of the rostral ventral respirato ry group (rVRG) in mammals. (C) 1998 Wiley-Liss, Inc.