Brainstem connections of the rostral ventral respiratory group of the rat.

The pontomedullary connections of the rostral division of the ventral respiratory group (rVRG), the largest medullary population of inspiratory bulbospinal and propriobulbar neurons, were identified in the rat by retrograde and anterograde tracing techniques. These experiments revealed that: (i) the Kölliker-Fuse nucleus, portions of the medial and lateral parabrachial nuclei, and all levels of the ipsilateral and contralateral VRG complex have dense reciprocal connections with the rVRG; (ii) the lateral paragigantocellular nucleus has reciprocal but less dense connections with rVRG; (iii) portions of the nucleus of the solitary tract have prominent projections to, but weaker inputs from rVRG; (iv) the raphe, magnocellular tegmental field and spinal trigeminal nuclei have minor projections to rVRG and receive only sparse inputs from rVRG, and; (v) the retrotrapezoid nucleus, pontine lateral tegmental field and area postrema each have only efferent projections to rVRG. These findings are consistent with previous studies of pontomedullary connections of rVRG in the cat, and further document the extensive reciprocal connections between principal respiratory groups. These connections are likely to be important for generation of the respiratory pattern, for the coordination of effector responses of the cranial and spinal respiratory motor neurons to afferent stimuli, and coordination of the central respiratory and cardiovascular control systems.