The primary octaval nuclei and inner ear afferent projections in the otophysan Ictalurus punctatus

Physical coupling between the swimbladder and the inner ear is believed to contribute to the enhanced hearing abilities of otophysans relative to those of teleosts lacking comparable otic specializations. We hypothesized that the auditory circuitry of otophysans might also exhibit derived features [McCormick and Braford, 1988]. As a first test of this hypothesis, we examined the normal anatomy and inner ear inputs of the primary octaval nuclei in the catfish Ictalurus punctatus. From Nissl-stained sections we conclude that Ictalurus, like other teleosts, has five first-order octaval nuclei: the anterior, magnocellular, tangential, descending, and posterior octaval nuclei. The overall projection pattern of the seven inner ear endorgans, determined using the exsanguinated horseradish peroxidase procedure [McCormick and Braford, 1984], is very similar to that hypothesized as primitive for gnathostome fishes. The otolithic endorgans project to dorsal areas of the descending and anterior nuclei, whereas the macula neglecta and cristae of the semicircular canals project more ventrally. Three zones are recognized within the descending nucleus-the dorsomedial, intermediate, and ventral zones. The dorsomedial zone, which is the main terminus of saccular fibers, has specialized morphological features: it extends far dorsally and contacts the medial portion of the cerebellar crest, and it is partitioned into lateral and medial portions by entering facial nerve fibers. The caudal anterior nucleus is likewise partitioned into lateral and medial portions by the facial nerve. Hypotheses addressing these derived features are presented.