Synaptic delay and time course of postsynaptic potentials at the junction between hair cells and eighth nerve fibers in the goldfish.

Abstract

1. Synaptic transmission from hair cells to afferent eighth nerve fibers was studied in goldfish's sacculus (inner ear) by recording the microphonic potential and excitatory postsynaptic potentials (EPSPs) simultaneously. The microphonic potential was recorded extracellularly from the base of hair cells, and EPSPs were recorded intracellularly from individual nerve fibers.2. There was an excellent one to one correspondence between microphonic deflections and EPSPs. An EPSP followed each microphonic deflection with a delay of about 0.5 msec. The delay was found to stay unchanged for different frequencies of sound.3. Spontaneous miniature EPSPs were observed. Duration of these miniature potentials, about 0.5 msec at their half-width, roughly coincided with the duration of the sound-evoked EPSPs. A close correlation was found between the duration of a microphonic deflection and that of the corresponding EPSP.4. The EPSP changed in size when the membrane at individual afferent nerve terminals was polarized by a flow of extrinsic current. A hyperpolarizing current produced an augmentation of the EPSP, while a depolarizing current produced its diminution. The EPSP was reversed in sign when a strong depolarizing current was applied.5. These results conform to the view that the transmission from hair cells to afferent nerves is chemically mediated and that the depolarization of hair cells as represented by the positive deflection in the intramacular microphonic potential would trigger the release of the transmitter.