STRUCTURE AND INNERVATION OF THE INNER-EAR SENSORY ORGANS IN AN OTOPHYSINE FISH, THE UPSIDE-DOWN CATFISH (SYNODONTIS-NIGRIVENTRIS DAVID)

All the sensory epithelia of the inner ear in the upside-down catfish (Synodontis nigriventris David) were examined by light microscopy. The morphology of the membranous labyrinth and the orientation of the hai r cells is similar to what has been found in other otophysine fishes. The sensory cells are of variable size both inter- and intraepithelial ly; particularly the macula sacculi is equipped with heterogeneous rec eptors. Regional differences in the hair cell density are presented fo r all the otolith organs plus the papilla neglecta. Nerve stainings re veal regional differentiation. The central areas are innervated by sto ut and stubbly nerve endings intermingled with a few thin nerve fibres while the peripheral parts are reached exclusively by thin axons. In the anterior region of the macula sacculi are found unique cup-shaped axon terminations which surround the basal parts of a single or a few sensory cells. The number and diameter range of the myelinated nerve f ibres as well as the hair cell/axon ratio are presented. Electron micr oscopy demonstrates the presence of unmyelinated axons in all inner ea r nerve ramuli.