Organization of spinal motor nuclei in the stingray, Dasyatis sabina

The Atlantic stingray, Dasyatis sabina, has enlarged pectoral fins consisting of a series of antagonist dorsal (elevator) and ventral (depressor) muscles. Each muscle is divided into superficial and deep components. The retrograde transport of horseradish peroxidase (HRP) was used to determine the organization of motoneuron pools innervating fin and epaxial muscles. HRP applied to a single peripheral nerve labeled motoneurons within a single spinal segment. Following intramuscular injection of HRP, 3 distinct cell groups were identified in the transverse plane. Motoneurons innervating elevator muscles were lateral in the ventral horn, while motoneurons innervating depressor muscles were dorsomedial. The epaxial muscles were found to be innervated by a distinct cell column along the ventral border of the ventral horn. Separate injections of the superficial and deep bundles of the elevator muscle resulted in considerable overlap in the distribution of labeled motoneurons. Soma areas for both elevator and depressor motoneurons were unimodally distributed. The mean cell diameters were 33.6 and 31.8 micron respectively. Motoneurons innervating the superficial and deep bundles of elevator muscle also had similar size distributions. The location of motoneurons innervating elevator and depressor fin muscles in the stingray supports the hypothesis that motoneurons innervating muscle derived from the dorsal premuscle mass are located laterally in the ventral horn while motoneurons innervating muscle derived from the ventral premuscle mass are located medially.