Organization and connections of octaval and lateral line centers in the medulla of a clupeid, Dorosoma cepedianum

Abstract

In the clupeid fishes, the functionally specialized utricle and cephalic lateral line respond to sound pressure by virtue of their mechanical coupling to the auditory bullae. The cytoarchitecture of, and primary inputs to, the octavolateralis area were studied in the gizzard shad, Dorosoma cepedianum, in order to determine whether first-order acoustic and lateral line areas of the medulla are likewise specialized. The 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 tangentialis, and a caudal granular-cell region that likely represents nucleus caudalis and the posterior octaval nucleus. The descending octaval nucleus can be divided into dorsomedial, intermediate, and ventral zones using cytoarchitectonic criteria, whereas the anterior octaval nucleus can be divided into caudal, rostral, and medial portions. Primary inputs to the octavolateralis area were determined by means of in vitro application of horseradish peroxidase to nerves from the otolithic endorgans of the inner ear and the lateral line neuromasts. These primary inputs are generally organized like those of other teleosts: the otolithic endorgans supply the posterior, descending, magnocellular, and anterior nuclei, whereas the majority of lateral line fibers project to nucleus medialis, nucleus caudalis, and to the magnocellular nucleus. However, other characteristics of these projections may be unique to clupeids. The medial subdivision of the dorsomedial zone of the descending nucleus is dominated by a bilateral projection from at least a portion of the utricle, while the lateral subdivison of the dorsomedial zone is supplied by the saccule and lagena. This pattern is not present in non-clupeid fishes; in many species, the saccule has the most dorsomedial projection zone within the descending nucleus. In Dorosoma, both lateral line nerves contribute a light, bilateral projection to the medial and lateral subdivisions of the dorsomedial zone. The apparently specialized, bilateral utricular and lateral line inputs to the dorsomedial zone of the descending nucleus may be related to the specialized sensitivity of the utricle and the cephalic lateral line to sound pressure. A prominent group of neurons, tentatively identified as a secondary octaval population, is also described. Like the secondary octaval 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.