Theoretical mechanical frequency response of the otolithic organs.

A distributed parameter model, which consisted of three coupled partial differential equations, describing the mechanical response of the otolithic organs, was used to develop a system mechanical transfer function. The equations were Laplace transformed and combined to yield a transfer function for otoconial layer displacement relative to an acceleration stimulus. Acceleration is either skull acceleration or gravity. In addition, transfer functions for the gel layer and endolymph fluid layer were also developed which included a spatial coordinate as a transfer function variable. Frequency response diagrams were constructed from the transfer functions, using numerical values of nondimensional parameters developed in an earlier study. The otoconial layer transfer function was compared to physiologic data on utricular primary afferent neurons and there was good agreement except at low frequencies. The discrepancy between the theoretical transfer function and the primary afferent response can be attributed to contributions of the spike encoder, which has been measured experimentally, additional non-mechanical components in the transduction process, and even non-linear elements in the mechanical system.