Characterization of respiratory neurons in the rostral ventrolateral medulla, an area critical for vocal production in songbirds

Abstract

Much is known about the neuronal cell types and circuitry of the mammalian respiratory brainstem and its role in normal, quiet breathing. Our understanding of the role of respiration in the context of vocal production, however, is very limited. Songbirds contain a well-defined neural circuit, known as the song system, which is necessary for song production and is strongly coupled to the respiratory system. A major target of this system is nucleus parambigualis (PAm) in the ventrolateral medulla, a structure that controls inspiration by way of its bulbospinal projections but is also an integral part of the song-pattern generation circuit by way of its "thalamocortical" projections to song-control nuclei in the telencephalon. We have mapped out PAm to characterize the cell types and its functional organization. Extracellular single units were obtained in anesthetized adult male zebra finches while measuring air sac pressure to monitor respiration. Single units were characterized by their discharge patterns and the phase of the activity in the respiratory cycle. Several classes of neurons were identified and were analogous to those reported for mammalian medullary respiratory neurons. The majority of the neurons in PAm was classified as inspiratory augmenting or preinspiratory, although other basic discharge patterns were observed as well. The well-characterized connectivity of PAm within the vocal motor circuit and the similarity of its neural firing patterns to the rostral ventral respiratory group and pre-Bötzinger complex of mammals make it an ideal system for investigating the integration of breathing and vocalization.