Primary afferent projections of the major splanchnic nerve to the spinal cord and gracile nucleus of the cat.

Splanchnic afferent projections to the spinal cord and gracile nucleus were labeled following the application of HRP to the central cut end of the major splanchnic nerve. Labeled afferent fibers were detected in the ipsilateral dorsal column, in Lissauer's tract (LT), in laminae 1, 5, 7, and 10, and in the dorsal gray commissure at T1-T13 levels of the spinal cord. Afferent projections were not identified in laminae 2-4. Collaterals from LT projected ventrally along the lateral and medial margins of the dorsal horn (called lateral and medial pathways, respectively). Afferents in the lateral pathway formed small bundles, spaced rostrocaudally at intervals of 300-1,000 microns, which passed medially at the base of the dorsal horn into laminae 5, 7, and 10 and to the contralateral spinal cord. Some afferents in the lateral pathway projected to the intermediolateral nucleus where labeled sympathetic preganglionic neurons were located. Afferents in the medial pathway entered the lateral aspect of the dorsal column and projected as a group near the midline rostrally to the medulla. The dorsal column pathway terminated in the ventral gracile nucleus in four or five clusters, each occupying a region ranging in size from 0.01-0.1 mm3 and separated in the rostrocaudal axis by distances of 400-800 microns. These clusters were concentrated in the middle and caudal portions of the nucleus below the obex. A comparison of the present results with those from earlier experiments on the central projections of afferent fibers from the heart, kidney, and pelvic organs demonstrates a consistent pattern of visceral afferent termination in the thoracolumbar and sacral segments of the spinal cord. This is not unexpected, since visceral afferent pathways to different organs perform similar functions, such as the transmission of nociceptive information and the initiation of autonomic reflexes.