Analog modelling of cochlear adaptation

Adaptation phenomena in the peripheral hearing organ originate from the properties of the haircell-neuron synaps as revealed by their temperature dependency. In order to verify this hypothesis a model experiment is set up. The model consists of programming on an analog computer the equations describing the reaction kinetics from the frogs myoneural junction. A model neuron is attached to the synapsmodel in which two independent noise sources are incorporated. This noise addition serves to simulate the stochastic behavior of the transmitter release in the synaps resulting in a fluctuating generator potential and independently thereof to reflect the varying threshold of the nerve fiber. The reaction rate constants in the synaptic model were modified with respect to the original ones in order to get a coincidence of in vivo- and model results. The compound model primarily is used to simulate the quite different synchronization between the auditory nerve fibers occurring during intensity changes and during changes of the stimulus repetition rate. These results were known to be different in the animal experiments and were also generated by the model. It is shown clearly that neither synaptic noise alone nor membrane noise alone can account for the observations made in the animal experiments. Therefore a combination of both types of noise is used in this model. The model experiment also visualizes that amplitude changes for compound AP's during adaptation can be explained in toto by a decreasing synchronization, i.e. the broadening of the latency distribution function for the nerve fibers.