Otolith fibers and terminals in chick vestibular nuclei.

The distribution of gravity-sensing, otolith afferent fibers and terminals was studied in the vestibular nuclei of 4-5-day hatchling chicks by using single and double labeling of fibers and terminals with biocytin conjugated to Alexa Fluor and confocal imaging. The vestibular nuclei are represented in a series of five transverse sections of the brainstem immunolabeled with MAP2. Saccular fibers entered the medulla posterior to and at the level of the posterior tangential vestibular nucleus and coursed through ventral parts, producing ascending and descending branches. Small saccular terminals contacted a few dendrites in the tangential nucleus. In contrast, small saccular terminals contacted many dendrites and a few neuron cell bodies in the ventrolateral vestibular nucleus, vestibulocerebellar nucleus, and descending vestibular nuclei. Utricular fibers coursed through ventral parts of the central tangential nucleus before bifurcating into ascending and descending branches. In the tangential nucleus, utricular fibers formed a few large axosomatic terminals (spoon terminals) and a few small terminals on dendrites. In addition, small utricular terminals contacted numerous dendrites and a few neuron cell bodies in the ventrolateral, vestibulocerebellar, and descending vestibular nuclei. Thus, there was negligible overlap in the distribution of the otolith nerves, although each otolith afferent shared common regions with the canal afferents, previously shown, suggesting that some second-order vestibular neurons process convergent inputs from otolith and canal afferents. Taken together with previous results, the present findings identify discrete regions of the chick vestibular nuclei where second-order vestibular neurons likely process directly convergent otolith and canal inputs.