Origin and identification of fibers in the cranial nerve IX-X complex of Xenopus laevis: Lucifer Yellow backfills in vitro.

The central projections of individual components of the IX-X nerve complex in the South African clawed frog, Xenopus laevis, were mapped by dye diffusion with Lucifer Yellow in an isolated brain preparation. The method reliably revealed fiber tracts, termination zones, and detailed cell morphology. In addition, motor neurons could be doubly labelled by retrograde transport of horseradish peroxidase from muscle targets followed by backfilling the appropriate nerves with Lucifer Yellow. The most anterior root associated with the nerve IX-X complex, root 1, is composed of lateral line afferents that terminate in the medial medulla. Root 2 contains sensory fibers that terminate in the nucleus tractus solitarii and axons of lateral line efferent neurons. Root 3 is composed of sensory and motor fibers, including a major somatosensory component that terminates in posterior medulla and anterior spinal cord, and axons from cranial nerve nucleus IX-X. The most posterior root of the IX-X nerve complex, root 4, contains axons of laryngeal motor neurons and of general visceral efferent neurons.