Projections of the marginal nuclei in the spinal cord of the pigeon.

In the avian spinal cord, there are several groups of neurons lying outside the central gray substance. The most conspicuous ones lie at the very margin of the ventrolateral cord. In the lumbosacral spinal cord, these marginal nuclei protrude into the vertebral canal to form accessory lobes. The projections of these marginal nuclei were studied in the pigeon by neuroanatomical tracing methods. Anterograde transport of tracer injected into the lumbosacral accessory lobes showed that these neurons project to the contralateral medial ventral gray and to paragriseal cells located in the contralateral ventral and lateral white matter of lumbosacral segments. Double-labeling experiments disclosed that lumbosacral paragriseal cells projecting to the cerebellum are contacted by accessory lobe axon terminals. The projection of cervical marginal nuclei was studied with retrograde transport of tracers applied to the spinal tracts in the lateral funiculus. Retrogradely labeled cells were found in contralateral marginal nuclei of both rostral and caudal segments. All marginal nuclei have an ascending and a descending projection spanning about five segments each. The possible role of marginal nuclei in sensorimotor circuits is discussed.