EFFERENT CONTROL OF POSTERIOR CANAL AFFERENT RECEPTOR DISCHARGE IN THE FROG LABYRINTH

EPSP and spike discharges were intracellularly recorded from 90 afferent fibres of the posterior nerve in the isolated frog labyrinth and the effects of electrical activation of the efferent system were tested. Posterior canal efferent synapses were activated, via an axon reflex, by electrical shocks to the anterior-horizontal nerves. The afferent resting discharge of all fibres tested was affected by efferent stimulation: 39 units were inhibited (43%) and 51 (57%) were facilitated. The efferent system was activated with several stimulation frequencies in the 10-200 Hz range applied for different times (250 ms-10 s). By changing the stimulus parameters, inhibition did not reverse to facilitation or vice versa. Facilitation appeared within the train above a threshold frequency of about 10 Hz. The peak response was readily reached within the first second and then, with long lasting stimulation, a marked adaptation ensued. The increase in firing rate was independent of previous resting activity. The relationship between frequency and facilitatory response is described by a logarithmic function over the 30-200 Hz range tested. At the end of short trains a consistent post-stimulation after-discharge appeared, whose intensity is positively related to stimulation frequency and time. Inhibition was achieved at stimulation rates above 10 Hz, and a post-stimulation rebound discharge was evident, which was linearly dependent on previous stimulation rate. The latency values of both inhibitory and facilitatory effects were measured by taking into account either the EPSP release rate or the spike discharge modifications at all the frequencies tested. Latency proved to decrease exponentially with increasing stimulation time from a minimal value of 3 ms to a maximum of 200 ms, with minor differences between inhibition or facilitation. These long latency values, the presence of a threshold frequency and the stimulus- and frequency dependence indicate that the efferent synapses must be activated repetitively to produce detectable effects on the afferent discharge; this is in line with the discharge pattern of the efferent system fibres physiologically measured in some systems. The present results show that the dual central control of the crista ampullaris of frog posterior canal is potentially capable of setting the receptor population at a variable level of sensitivity to mechanical stimuli, with profound modifications in the canal transfer function and the spike encoding mechanism.