Projections ascending from the spinal cord to the brain in petromyzontid and myxinoid agnathans.

The course of projections ascending through the rostral spinal cord to nuclei in the brains of petromyzontid and myxinoid agnathans was examined with silver staining of anterograde degeneration and horseradish peroxidase histochemistry. As in jawed vertebrates, the ascending spinal projections of lampreys and hagfishes appear to be organized into two major systems, the spinal lemniscal and dorsal column pathways. The spinal lemniscal pathway, extending rostrally along the ventrolateral margin of the spinal and medullary central gray, consists of a spinoreticular and possibly a spinovestibular projection in both aganthan groups. In Pacific hagfish, spinal lemniscal fibers reach the ipsilateral mesencephalic tectum, but no spinal projection to the thalamus was evident. The spinal lemniscus of lampreys ascends to the region of the isthmus and may extend into the mesencephalic tegmentum. Anterograde and retrograde tracing methods indicate that a very small population of cells in the far rostral cord of lampreys may project to the optic tectum and diencephalon; however, spinotectal and spinothalamic projections, if present, are limited in extent. The dorsal column pathway in agnathans, consisting in part of primary spinal afferents, ascends in the dorsal funiculus of the cord. The dorsal column fibers of agnathans, like those of some other anamniotes, continue beyond the spinomedullary junction through the length of the hindbrain, possibly conveying ascending somatosensory input to the sensory nuclei of the alar medulla.