Deformation of avian middle ear structures under static pressure loads, and potential regulation mechanisms.

Static pressure changes can alter the configuration and mechanical behavior of the chain of ossicles, which may affect the acoustic transfer function. In mammals, the Eustachian tube plays an important role in restoring ambient middle ear pressure, hence restoring the acoustic transfer function and excluding barotrauma of the middle and inner ear. Ambient pressure fluctuations can be potentially extreme in birds and due to the simple structure of the avian middle ear (one ossicle, one muscle), regulation of the middle ear pressure via reflexive opening of the pharyngotympanic tube appears all the more important. In this study the deformations of the chicken (Gallus gallus domesticus) middle ear structures, as a result of middle ear pressure alterations, are quantified, using micro-CT scanning. It was experimentally tested whether reflexive opening of the pharyngotympanic tube to restore ambient middle ear pressure is present in chicken and mallard (Anas platyrhynchos) and whether this mechanism depends on sensing middle ear pressure indirectly via deformations of the middle ear components or sensing the middle ear pressure directly. A translation of the columella footplate was observed when middle ear pressure was kept at 1kPa and -1kPa relative to ambient pressure. Deformation of the tympanic membrane was larger than the columella footplate translation. Bending and deformation of the extracolumella was observed. Opening of the pharyngotympanic tube occurred at random pressure for both chicken and mallard when middle ear pressure was raised and lowered by 1.5kPa relative to ambient pressure. We also did not find a difference in middle ear venting rate when middle ear pressure was held constant at 0.5, 1, 1.5, -0.5, -1 and -1.5kPa for chickens and at 1, 2, 4, -1, -2 and -4kPa for mallards. As a result, no statement can be made about pressure within the avian middle ear being measured directly or indirectly. Our experiments do not support the presence of a short-loop reflexive control of pressure equilibration via the pharyngotympanic tube. However, it is still possible that triggering this loop requires additional sensorial input (e.g. visual, vestibular) or that it occurs voluntarily (being controlled at a higher brain level).