Brain stem projections by axonal collaterals to the rostral and caudal ventral respiratory group in the rat

The location of neurons projecting by axonal collaterals to the rostral and caudal ventral respiratory group (VRG) regions was determined after discrete injections of Fast blue and Diamidino yellow into the physiologically identified rostral inspiratory VRG and the caudal expiratory VRG areas, respectively. In contrast with single fluorochrome labeled neurons found throughout the rostro-caudal extent of the medulla and pons (in a variety of areas known to have cardiorespiratory function), double-labeled neurons were located in discrete pontomedullary regions. The largest number of the double-labeled neurons was counted within the peripheral facial area, lateral paragigantocellular nucleus, and the VRG region, ipsi- and contralaterally to the injected side. Only a few double-labeled neurons were found within the ventrolateral and intermediate subnuclei of the solitary tract, medial parabrachial, and Kölliker-Fuse nuclei. The possible physiological implications of this neuronal network have also been emphasized.