Development, functional organization, and evolution of vertebrate axial motor circuits

Organization of axial MNs in tetrapods and fish. a In jawed vertebrates, axial muscles are separated into dorsal epaxial and ventral hypaxial groups, separated by the horizontal myoseptum (HM). Each muscle group is innervated by separate spinal nerves. Dorsal root ganglia (drg) and sympathetic chain ganglia (scg) are shown. b MN columnar subtypes at trunk levels. In tetrapods, as well as some cartilaginous fish, MNs innervating dorsal epaxial muscles are organized in the medial motor column (MMC). MNs projecting to ventral hypaxial muscles are contained within the hypaxial motor column (HMC). Autonomic preganglionic column (PGC) neurons, which project to scg, are shown in gray. c Organization of MN pools at thoracic levels. MNs innervating specific types of axial muscle are organized in pool-like clusters. Some MNs within the HMC project to dorsally located axial muscles, such as serratus, but are nevertheless supplied by axons originating from the ventral ramus. Abbreviations: tv, transversospinalis; long, longissimus; ilio, iliocostalis; lc, levator costae; sr, caudal serratus; ii, internal intercostal; sc, subcostalis; ei, external intercostal; eo, external oblique. Not all trunk muscles are shown. Diagram based on data from rat in [13]. d Organization of MNs in adult zebrafish. MNs innervating fast, intermediate, and slow muscle are organized along the dorsoventral axis. Fast MNs include primary MNs and some secondary MNs, intermediate and slow are all secondary MNs. These MN types project to specific types of trunk-level axial muscles. Diagram based on data in [14