1. The effect of single or repetitive stimulation applied to efferent nerve fibres on afferent nerve activity and microphonic potentials was studied in the saccule of goldfish. 2. The sound-evoked excitatory post-synaptic potentials (e.p.s.p.s) recorded intracellularly from afferent eighth nerve fibres were reduced in size or completely abolished by efferent stimulation. The maximum inhibitory effect produced by repetitive efferent stimulation was equivalent to reducing the sound intensity by 10-25 db. Spontaneous miniature e.p.s.p.s were also suppressed by efferent stimulation. 3. The effect of single efferent stimulation appeared with a delay of 607 msec and lasted for about 40 msec, reaching its peak at about 12 msec. The slow and prolonged time course makes a sharp contrast with the very fast time course of afferent synaptic action. 4. The application of hyperpolarizing current through the recording micro-electrode revealed no sign of a post-synaptic increase in membrane conductance during inhibition. Hence, the inhibition was mostly attributable to a presynaptic action, i.e. to a suppression of transmitter release from hair cells. 5. Individual e.p.s.p.s were evoked in response to each wave of sound without any change in latency, but reached their peak much earlier during inhibition than in the control period. A likely explanation for this finding is that transmitter is released from hair cells during inhibition only in the early part of the stimulatory phase of the sound wave. 6. The extracellularly recorded microphonic potentials showed a slight increase in amplitude during efferent stimulation. 7. The nature and site of action of efferent nerve action are discussed along with some drug effects.
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