A review of the organization and evolution of motoneurons innervating the axial musculature of vertebrates

In most anamniotes the axial musculature is myomeric and is functionally subdivided into superficial red and deep white muscle. In those anamniotes that have been studied the organization of the motor column is related to this functional subdivision. The motoneurons innervating red and white muscle differ in size, distribution in the motor column, and developmental history. There is no obvious topographic relationship between the location of motoneurons in the motor column and the dorsoventral location of the muscle they innervate in the myomeres; epaxial motoneurons are not segregated from hypaxial ones. Among amniotes, the myomeres divide to form a number of discrete muscles that may be complexly arranged. This breakup of the musculature is correlated with a subdivision of the motor column into discrete motor pools serving the different muscles. Unlike anamniotes, the motor pools are topographically organized. The epaxial pools are segregated from hypaxial ones, and within the epaxial and hypaxial pools the location of motoneurons innervating any particular muscle is related to the location of the muscle's precursor in the embryonic muscle masses.Thus adjacent motor pools innervate muscles arising from adjacent positions in the myotome. These dramatic differences between the motor columns in anamniotes and amniotes imply that the medial motor column has undergone a major restructuring during the evolution of vertebrates. The available evidence — which is tentative because of the few species that have been studied — suggests that a topographically organized motor column was absent in early vertebrates. A motor column/myotome map appears to have arisen just prior to, or in conjunction with the origin of amniotic vertebrates. The details of this map were conserved in different amniotes in spite of major structural and functional changes in the musculature. The map may be important for the proper control of the many muscles arising from the myotomes in amniotes because it facilitates the development and evolution of motor systems in which anatomically and functionally different muscles have spatially separate motor pools in the cord.