Location of dye-coupled second order and of efferent vestibular neurons labeled from individual semicircular canal or otolith organs in the frog.

Vestibular nerve branches innervating the sensory epithelia of the three semicircular canals or of the three otolith organs of frogs were selectively labeled in-vitro with biocytin. Labeled afferent fibers from the semicircular canals, utricle, and lagena were encountered in each of the four vestibular nuclei and their projections overlapped considerably. Saccular afferent fibers projected to the dorsal (acoustic) nuclei and smaller projections to the vestibular nuclei were regionally restricted. Per semicircular canal or otolith organ about equal numbers (11-14) of medium sized vestibular neurons (between 7.5 and 17 microm in diameter) were dye-coupled to afferent fibers. Most of these dye-coupled vestibular neurons were located in the lateral and descending vestibular nuclei between the VIIIth and IXth nerves. The superior vestibular nucleus was relatively free of dye-coupled vestibular neurons. The location of this subpopulation of central vestibular neurons supports the notion that these neurons are part of a particular vestibulospinal pathway. In addition, from each of the canal and/or otolith organs about 3-4 efferent vestibular neurons were labeled retrogradely. These neurons (between 15 and 26 microm in diameter) were located ventral to the vestibular nuclear complex. The branching of efferent vestibular neurons was shown by the presence of neurons that were double labeled by two different fluorescent dyes applied in the same experiment to the anterior and posterior ramus of the same VIIIth nerve, respectively. The branching of these efferent neuron axons explained the presence of collaterals and terminals in the sensory epithelia of a number of untreated ipsilateral endorgans.