Cells of origin of descending pathways to the spinal cord in the clawed toad (Xenopus laevis)

The cells of origin of pathways descending to the spinal cord in the clawed toad Xenopus laevis have been demonstrated with the horseradish peroxidase technique. A technique has been used taking advantage of the phenomenon that damaged axons can take up horseradish peroxidase and transport this enzyme to their parent cell bodies. The following descending supraspinal pathways could be demonstrated: a striatospinal pathway to the rostral part of the cord; distinct hypothalamospinal projections; a projection as far as the lumbar cord from the ventral thalamic nucleus; distinct projections from the mesencephalic tegmentum; a contralateral cerebellospinal projection from the cerebellar nucleus; a projection from neurons directly medial to the nucleus isthmi which shows resemblance to the coeruleospinal pathway of higher veretebrates; massive reticulospinal projections; a vestibulospinal projection arising in the nucleus ventralis VIII and pathways arising in nuclei receiving lateral line afferents. Furthermore, spinal projections from the nucleus of the solitary tract and the nucleus descendons nervi trigemini were observed.Rather massive projections were found to arise in the midbrain tegmentum: a mainly ipsilateral projection from the interstitial nucleus of the fasciculus longitudinalis medialis, a contralateral projection as far as the lumbar cord from a cell group which presumably represents the anuran homologue of the red nucleus of higher vertebrates, and projections from various other parts of the midbrain tegmentum, mainly to more rostral levels of the cord. Only a very small tectospinal projection could be demonstrated.A comparison with experimental data in higher vertebrates makes it likely that the pathways demonstrated from the hypothalamus and brain stem in Xenopus laevis show remarkable similarities to pathways in reptiles, birds and mammals.