Peripheral and central aspects of the acoustic and lateral line system of a bottom dwelling catfish, Ancistrus sp.

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Abstract
The topographical relationship between the swim bladder, the inner ear, and the otic lateral line was studied in the bottom dwelling catfish, Ancistrus sp. In addition, afferent and efferent subcomponents of the eighth and lateral line nerves were labelled with horseradish peroxidase (HRP) or with differently fluorescing dextran amines. The swim bladder of Ancistrus consists of two separate, transversely oriented parts each of which is connected to the sinus impar of the inner ears via two Weberian ossicles and the perilymphatic sac. The osseous capsula of the ear has two foramina other than the nerve foramina. One is for the sinus impar. The other foramen, which also separates two fluid-filled spaces, exits where the horizontal canal of the ear contacts the otic lateral line. Both the otic and the postotic lateral line canal run deep below the epidermis. Each canal contains a neuromast that is innervated by the middle lateral line nerve. Further caudally, the otic lateral line canal gives rise to the postotic and finally to the trunk canal whose nonossified anterior part travels through an ossified chamber that surrounds the swim bladder. Thus the anterior part of each trunk lateral line canal is in contact with a bipartite sound pressure receiver, the swim bladder. Anterior and posterior lateral line afferents terminate ipsilaterally throughout the neuropil of the electroreceptive lateral line nucleus and the mechanoreceptive nuclei medialis and caudalis of the medulla. Middle lateral line afferents terminate between the projection sites of anterior and posterior lateral line afferents. Some primary mechanosensory anterior lateral line nerve fibers continue into the ipsilateral eminentia granularis and the valvula cerebelli, In the electroreceptive lateral line projection, anterior lateral line fibers terminate more medially and posterior fibers more laterally. This somatotopy is not as clear-cut in the mechanosensory lateral line. Afferents of the sacculus and the lagena terminate predominantly in the saccular nucleus. Afferents of the utriculus, the horizontal canal, and the anterior vertical canal terminate in the magnocellular vestibular nucleus and in the medial octavolateral nucleus. The projection sites of the anterior part and the posterior part of the eighth nerve show little overlap. Eighth nerve projections to the valvula cerebelli are less prominent than the projections from the lateral line. Eighth nerve and lateral line nerve efferents arise from a common nucleus, the octavolateralis efferent nucleus. Axons of efferent cells may divide to supply two or more branches of the eighth nerve and some axons supply both lateral line and eighth nerve endorgans. The anatomical relationships between the head lateral line and the inner ear suggest that parts of the lateral line of Ancistrus participate in sound pressure perception. The assumed pressure sensitivity of the sacculus and the lagena probably is reflected in the distinct projection of saccular and lagena fibers to a definable auditory nucleus, the saccular nucleus. In contrast and irrespective of the supposed auditory function for parts of the head lateral line, no primary lateral line afferents were found to project to an acoustic or vestibular nucleus. Physiological studies are under way to find out whether and how the lateral line of Ancistrus is involved in sound pressure perception.