Summary

Abstract

Throughout tetrapods a basic pattern in the organization of descending supraspinal pathways is present. The most notable difference between nonmammalian tetrapods and mammals is the apparent absence of somatomotor cortical areas giving rise to long descending projections to the spinal cord. The phylogenetic constancy of descending supraspinal pathways, at least of those arising in the brain stem, probably implies a comparable pattern of development, presumably a developmental sequence in the formation of these central motor pathways. For studies on the development of motor systems, anurans such as the clawed toad, Xenopus laevis, chicken embryos, and opossums are very attractive animals. Moreover, in these species as well as in rodents in vitro approaches can be used. In the present survey, current knowledge on the neurogenesis, axonal outgrowth, synaptogenesis, and developmental plasticity of the central motor pathways in tetrapods including the sparse data available for man, is discussed. These data are placed in the perspective of the development of the spinal cord and, where possible, correlated with functional data. Emphasis is on the clawed toad, X. laevis, chicken embryos, and opossum and rodent data.