Segmental level discrepancy of human iliocostalis muscles and their innervation

Detailed studies on intramuscular innervation of human thoracic iliocostalis (IC) muscles were done through macroscopic dissection of six sides of three Japanese adult cadavers. Human IC muscles possessed a segmental nature in that muscular segments sequentially originated from individual ribs, although their structure was complex, with many long sequentially overlapping multisegmental bundles and their diverse multilayered insertions. Nerves to IC muscles (NICs) arose metamerically from the spinal nerve at every thoracic axial level. Nerves to IC muscles ran caudally, and their entry points into the fascia of IC muscles were shifted inferiorly by one segment level compared with their origin. Each NIC supplied a few muscular segments that were defined by their costal origin, but the distribution boundary often did not match the muscular segmental boundary. At the distribution boundaries of individual NICs, the muscular layers were often innervated by two NICs from adjacent levels. The level of costal origin of the IC muscular segment was lower than that of the segmental origin of the NIC, with the difference in level, which is one at the higher thoracic region, increasing caudally up to three on average at the lower thoracic region. It is thus noteworthy that while the IC muscles and NICs were both segmental in nature, their segmentations exhibited level discrepancies, which were not coordinated with each other due to their indistinct boundaries as well as the inconsistent differences in level.