REVIEW PAPER The frog labyrinth cytoneural junction and its efferent control

Hair cells in the frog labyrinth synapse with the nerve terminals of the VIIIth nerve fibres and receive an efferent innervation originating in the brain. Subthreshold potentials recorded from single fibres of the posterior nerve proved to be dependent on Ca2+ and Mg2+ levels in the bath and independent of the presence of tetrodotoxin, thus revealing their true nature of excitatory postsynaptic potentials (EPSPs). EPSP and spike rates are profoundly affected by electrical activation of the posterior canal efferent systems. Two subpopulations of hair cells were detected: one inhibited and the other facilitated by efferent stimulation. Pharmacological treatments reveal that efferent inhibition is cholinergic in nature, whereas good evidence is provided that ATP is involved in efferent facilitation. The high EPSP frequency (>100/sec) prevents a correct evaluation of their numbers by a direct examination of the tracings. By treating the summated signal as `shot noise' it is demonstrated that many properties of the spike discharge response, such as asymmetry, adaptation and phase lead, are presynaptic in origin, whereas rectification arises at the encoder. Furthermore, statistical analysis of Wiener-filtered recordings reveals that the EPSP amplitude variability is related to the postsynaptic summation of single events occurring at random and generated by the independent release of single transmitter quanta by the hair cells.