The canine nucleus tractus solitarii: light microscopic analysis of subnuclear divisions.

The nucleus tractus solitarii (nTS) is a complex structure situated in the dorsal medulla oblongata. This region receives primary visceral and gustatory sensory afferent fibers and has widespread interconnections with brainstem structures, hypothalamus, and limbic forebrain. In both rat and cat distinct subnuclei correlate with specific functions of the nTS. Since the canine model is used extensively for physiological study and evidence from this laboratory supports a critical role for the canine nTS in cardiovascular function, we examined its morphological organization. Light microscopic analysis of cellular and fiber patterns of the nTS revealed nine discrete regions based on cytoarchitecture: the commissural, lateral, ventral, dorsal, intermediate, interstitial and medial subnuclei, the subnucleus gelatinosa, and the dorsal parasolitary region. Analysis of each subnucleus revealed that both the lateral and ventral subnuclei contained two distinct neuronal groups based on cell size. Neurochemical and functional correlates are being provided by ongoing analyses of each subnucleus of the nTS.