PONTOMEDULLARY EFFERENT PROJECTIONS OF THE VENTRAL RESPIRATORY NEURONAL SUBSETS OF THE RAT

The pontomedullary trajectories of projections efferent from the ventr al respiratory cell group were anterogradely labelled after discrete i njections of Fluoro Ruby into three morphophysiologically identified s ubdivisions (Botzinger complex, rostral inspiratory, and caudal expira tory cell groups). The anterogradely labelled varicosities were locate d in a variety of areas involved in cardiorespiratory function: other subdivisions of the ventral respiratory cell group, the parabrachial ( medial, central, and external lateral), Kolliker-Fuse, and lateral par agigantocellular nuclei, A5, and perifacial areas. Although the target areas were similar for the three studied subdivisions, some differenc es of the location and densities of labelled varicosities were found. Anterogradely labelled fibre bundles were found bilaterally after all of the tracer injections. Three caudally efferent bundles passed throu gh the ventral respiratory cell group, dorsal medullary, and paramedia n reticular nuclei. A labelled fibre bundle also took an ascending rou te through the ventral respiratory cell group: it surrounded the facia l nucleus, and then followed two different pathways, one coursing towa rds forebrain areas and the other to the parabrachial and Kolliker-Fus e complex. Bundles of efferent axons decussated mainly at medullary le vels and to a lesser extent in the pens. in the contralateral medulla and pens these labelled fibre bundles followed pathways similar to tho se observed ipsilaterally. The three ventral respiratory neuronal subs ets sent axonal projections through similar tracts, but within them th ey were topographically organized. The present data are discussed with respect to the circuitry involved in the mechanisms of cardiorespirat ory and other visceral functions. Copyright (C) 1997 Elsevier Science Inc.