Adaptation and Dynamic Response Occurring at Hair Cell-Afferent Fiber Synapse

Abstract

The rate of afferent discharges in the cochlear nerve increases at the start of sound, but gradually declines thereafter until it reaches a steady level. Upon cessation of the sound, the rate goes down for a while below the level of spontaneous firing (Harris and Dallos, 1979). Also when a small increment or decrement is added to the intensity of the sound, afferent discharges show characteristic dynamic changes (Smith and Zwislocki, 1975). Since no corresponding changes were found in the microphonic potentials, i.e., in the activity of sensory hair cells, it is generally assumed that these adaptive phenomena may occur at the synapse between hair cells and afferent fibers. We report here results on the goldfish saccule, in which adaptive processes with the above mentioned properties were demonstrated as changes in the amplitude of the excitatory postsynaptic potentials (e.p.s.p.s) produced in the primary afferent fiber terminals. We analyzed the phenomena in some detail and reached the conclusion that the observed adaptive and dynamic changes are produced in relation to the mechanism of the transmitter release at the afferent synapse (Furukawa and Matsuura, 1978; Furukawa et al., 1978).