Localization of Swimbladder and Pectoral Motoneurons Involved in Sound Production in Pimelodid Catfish

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Abstract
We localized the motoneurons and occipital and true spinal innervation of sound-producing organs in pimelodid catfish. Pimelodids have a stridulatory organ composed of the pectoral girdle and the first pectoral fin ray, a swimbladder with extrinsic muscles to produce drumming sounds, and a tensor tripodis (TT) muscle that inserts on the swimbladder. Sonic muscles are innervated by three branches (rostral, dorsal and caudal) of the occipital nerve (Oc) and the first two true spinal nerves (S1 and 2): pectoral spine muscles (abductor, adductor and ventral rotator) by the rostral branch of Oc and S1 and 2, drumming muscle by the caudal branch of Oc and twigs of the S1 and 2, and TT by the dorsal branch of Oc. Sonic nuclei form ipsilateral medial, intermediate and ventrolateral columns in the caudal medulla and spinal cord. Pectoral neurons form a ventrolateral motor column, and neurons for the first spine occupy the rostral part of the column. The medial division of the swimbladder drumming motor nucleus (DMm) is situated on the midline between the central canal and the medial longitudinal fasciculus. The rostral pole of the DM nucleus expands ventrolaterally to include a population of neurons of intermediate position (DMi). The TT nucleus also assumes an intermediate position ventrolateral to DMm. The pectoral, TT, and DMi have a restricted rostrocaudal extent, whereas DMm extends further caudally. These data demonstrate that fish can evolve multiple sonic motor nuclei and that sound producing organs can be innervated in parallel by occipital and spinal nerves.

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