Meeting Abstract
The vestibular sensory epithelia of amniotes offer unique opportunities to explore, in a relatively simple circuit, questions of sensory transduction and encoding, synaptic transmission and spike patterning. The vestibular afferent neurons form bouton synaptic contacts on type II hair cells and unique calyceal terminals on type I hair cells, in diverse combinations and morphologies. Synaptic transmission from the type I hair cells onto the calyceal terminals relies on both quantal transmission (release of glutamate from synaptic vesicles) and an unusual nonquantal transmission. Recent work (especially Contini et al., J Physiol 595:777, 2017) shows that the nonquantal response depends on potassium ion (K+) efflux from the hair cell through numerous low-voltage-activated K+-selective ion channels into the extensive synaptic cleft formed by the calyx. Nonquantal and quantal responses can be recorded together or separately in single calyceal terminals; how they are coordinated in daily living is not known. By comparing quantal and nonquantal postsynaptic currents and potentials evoked by deflecting the type I hair cell’s hair bundle, we see that nonquantal transmission works in both directions between hair cell and calyx, unlike quantal transmission, and can carry faster signals and drive precise spike timing. The calyx terminal also expresses many ion channels, which have been implicated in synaptic transmission and/or the initiation and patterning of action potentials (spikes). Systematic differences in the ion channels and morphologies of the afferent arbors play complementary roles in setting up the diversity of sensitivity and spike patterning in the vestibular afferent nerve.
