MICROARCHITECTURE AND INNERVATION OF THE HUMAN LATISSIMUS-DORSI MUSCLE

Re-investigation of the adult human latissimus dorsi muscle (HLD) by m icrodissection of acetylcholinesterase (AChE)-stained material reveale d a complicated microstructure of this muscle. Motor endplates distrib ute over the entire muscle; numerous Ache-stained myomyonal and myoten dinous junctions are interspersed. After teasing muscle fascicles from selected representative areas, the following results were obtained. M ost of the single muscle fibers constituting the fascicles are shorter than the length of the fascicles. They are linked together by myomyon al junctions or by myotendinous intersections end-to-end, end-to-side, or via muscular crossbridges side-to-side, thus forming chains and ne ts of varying appearance and length. Additionally, many fiber furcatio ns were found. These observations throw light on the microarchitectura l basis of local mass changes of the triangular HLD occurring during i ts convergence from its wide lumbar and thoracic attachment line to it s narrow tendon attached to the humeral crista tuberculi minoris. The microstructural observations also explain why motor endplates spread o ver the whole muscle, instead of being restricted to a single central endplate-band, as is found in the majority of other muscles. To clarif y postnatal development, the topography of endplates in adult muscles was compared with fetal and infantile muscles. Primarily, the endplate bands were clearly demarcated; their width then broadened, and they g radually scattered into groups of single endplates. Simultaneous with these developmental changes, a corresponding branching of the thoracod orsal nerve occurs.