Brainstem connections of the macula lagenae in the chicken

The macula lagenae, an otolithic hair-cell organ with probable vestibular function, lies close to the apical end of the avian auditory hair-cell epithelium, the papilla basilaris. In an earlier study in the pigeon in which lesioning techniques were used, Boord and Rasmussen ([1963] J. Comp. Neurol. 120:463-473) reported finding a projection of lagenar fibers to parts of the cochlear nuclei (nucleus magnocellularis and nucleus angularis). Subsequent to this report, it has been generally assumed that at least part of the cochlear nuclei has a vestibular or a combined vestibular-auditory function. In this study, we labeled fibers innervating the macula lagenae of the chicken by using a lipophilic fluorescent tracer. The analysis of Vibratome sections of the brainstem with epifluorescence illumination showed no projection to the cochlear nuclei. In cases in which the apical part of the papilla basilaris was contaminated with tracer, however, we found labeling of the cochlear nuclei in the same areas as described with the lesioning technique in the pigeon. Our results thus imply that there is no processing of information from the macula lagenae in the cochlear nucleus of the chicken. In addition, we studied the origin of the few labeled efferent neurons in the brainstem. The location of all somata encountered was restricted to an area medial to the nucleus facialis dorsalis and corresponded to the dorsal efferent cell group, from which efferents to other vestibular organs also originate.