ORGANIZATION AND CONNECTIONS OF OCTAVAL AND LATERAL-LINE CENTERS IN THE MEDULLA OF A CLUPEID, DOROSOMA-CEPEDIANUM

In the clupeid fishes, the functionally specialized utricle and cephal ic lateral line respond to sound pressure by virtue of their mechanica l coupling to the auditory bullae. The cytoarchitecture of, and primar y inputs to, the octavolateralis area were studied in the gizzard shad , Dorosoma cepedianum, in order to determine whether first-order acous tic and lateral line areas of the medulla are likewise specialized. Th e octavolateralis area of Dorosoma is composed of the nuclei that have been observed in other teleosts: nucleus medialis, the descending and anterior octaval nuclei, nucleus magnocellularis, nucleus tangentiali s, and a caudal granular-cell region that likely represents nucleus ca udalis and the posterior octaval nucleus. The descending octaval nucle us can be divided into dorsomedial, intermediate, and ventral zones us ing cytoarchitectonic criteria, whereas the anterior octaval nucleus c an be divided into caudal, rostral, and medial portions. Primary input s to the octavolateralis area were determined by means of in vitro app lication of horseradish peroxidase to nerves from the otolithic endorg ans of the inner ear and the lateral line neuromasts. These primary in puts are generally organized like those of other teleosts: the otolith ic endorgans supply the posterior, descending, magnocellular, and ante rior nuclei, whereas the majority of lateral line fibers project to nu cleus medialis, nucleus caudalis, and to the magnocellular nucleus. Ho wever, other characteristics of these projections may be unique to clu peids. The medial subdivision of the dorsomedial zone of the descendin g nucleus is dominated by a bilateral projection from at least a porti on of the utricle, while the lateral subdivison of the dorsomedial zon e is supplied by the saccule and lagena. This pattern is not present i n non-clupeid fishes; in many species, the saccule has the most dorsom edial projection zone within the descending nucleus. In Dorosoma, both lateral line nerves contribute a light, bilateral projection to the m edial and lateral subdivisions of the dorsomedial zone. The apparently specialized, bilateral utricular and lateral line inputs to the dorso medial zone of the descending nucleus may be related to the specialize d sensitivity of the utricle and the cephalic lateral line to sound pr essure. A prominent group of neurons, tentatively identified as a seco ndary octaval population, is also described. Like the secondary octava l population of otophysans, the presumed secondary octaval population of Dorosoma is composed of a dorsal, fusiform region, an intermediate spherical cell region, and a ventral fusiform cell region.