Secondary connections of the dorsal and ventral facial lobes in a teleost fish, the rockling (Ciliata mustela).

In the rockling, Ciliata mustela (Teleostei), a portion of the dorsal fin is a specialized chemosensory organ possessing solitary chemoreceptor cells innervated by a recurrent branch of the facial nerve. Previous studies have demonstrated that the specialized solitary chemoreceptor cell system is represented in the dorsal segment of the medullary facial lobe (DFL), whereas the taste buds in the remainder of the facial-nerve-innervated skin are represented in the ventral division of the lobe (VFL). The carbocyanine dye DiI was used to investigate the secondary and higher order brain connections of these two distinct subdivisions of the facial lobe. Both segments of the facial lobe sent fibers into the contralateral DFL via a dorsocaudal facial commissure and to the contralateral vagal lobes and VFL via fibers arching ventrally through the reticular formation. Ascending fibers from both facial lobe segments were traced into the secondary gustatory nucleus and into the lateral superficial facial nucleus, a small area in the dorsolateral brainstem laterally adjacent to the nucleus medialis of the octavolateral complex. Additionally, the VFL had reciprocal connections with a newly described nucleus adjacent to the incoming facial nerve root. Both DFL and VFL had descending fibers reaching two portions of the funicular nuclear complex, although the VFL contribution to this area is far more extensive than the DFL input. Thus, substantial overlap exists in the connections of the two facial subsystems; i.e., the solitary chemoreceptor information is not processed in nuclei distinct from those making up the usual gustatory lemniscus.