Electrophysiological study of the tangential vestibular nucleus of the chick embryo "in vitro".

The two main neuron types of the tangential vestibular nucleus, the principal and the elongate cells, were investigated using intracellular recordings on brain slice preparations of 15-16 day chick embryos. Their identification was confirmed by intracellular injections of the fluorescent dye, Lucifer yellow. The passive membrane properties of these two neuron types were not sufficient criteria for their identification, whereas their responses to current pulse injections were distinctive. The elongate cells showed a nonlinear current-voltage relationship to hyperpolarizing currents and they fired repetitively at high frequencies without accommodation when the cells were depolarized. Neither of these two properties were observed in principal cells. Synaptic responses to vestibular nerve stimulation were different for the two neurons. The elongate cells responded gradually to increasing stimulus intensity, while the principal cells exhibited two clear postsynaptic responses: a low-threshold action potential initiated by an all-or-none EPSP, and a second, higher threshold, longer latency and graded EPSP. The latter EPSP is characterized by a slow rise time and a long duration. The findings obtained at this embryonic age are consistent with data concerning principal and elongate cell innervation by different caliber fibers comprising the vestibular nerve. The early EPSP recorded from the principal cells is attributed to the colossal fibers, which are vestibular afferents of large diameter forming calyciform contacts, the spoon endings, exclusively on the principal cells. The other synaptic responses recorded from the elongate and principal cells are due to the firing of different classes of small vestibular afferents. These experiments were performed in this new preparation at a critical age for the development of the spoon endings.(ABSTRACT TRUNCATED AT 250 WORDS)