Distribution of substance P‐like immunoreactive neurons and terminals throughout the nucleus of the solitary tract in the human brainstem

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Abstract
The anatomical distribution of substance P‐like immunoreactivity across the subnuclear divisions of the nucleus of the solitary tract has been examined in the human medulla oblongata. A differential distribution of neurons, fibers, and terminals was observed throughout the ten subnuclear divisions of this nucleus. Substance P‐like immunoreactive neurons were observed most frequently in the nucleus gelationsus, with moderate numbers in the medial, intermediate subnuclei and very few in the commissural, ventral, dorsal, and dorsolateral subnuclei. The paracommissural, ventrolateral, and interstitial subnuclei did not contain substance P‐like‐immunoreactive neurons. These neurons were typically bipolar and moderate‐sized to large, except for the neurons in the nucleus gelatinosus, which were substantially smaller. The highest densities of fibres and terminals were observed in the gelatinosus, medical, and intermediate nuclei, with moderate densities in the paracommissural and dorsal subnuclei. Sparse substance P‐like‐immunoreactive fibres and terminals were seen in the ventral and interstitial nuclei as well as within the solitary tract. The dorsolateral nucleus was characterized by a light distribution of fibres and terminals, except for a dense aggregation along its lateralmost border. A prominent innervation of pigmented neurons by substance P‐like‐immunoreactive terminals and fibres was also observed in the dorsolateral nucleus. The results reveal that the subnuclear complexity of the nucleus of the solitary tract is richly reflected by its differential pattern of substance P‐like‐immunoreactive structures.