Eardrum and columella displacement in single ossicle ears under quasi-static pressure variations

Although most birds encounter large pressure variations during flight, motion of the middle ear components as a result of changing ambient pressure are not well known or described. In the present study, motion of the columella footplate and tympanic membrane (extrastapedius) in domestic chickens (Gallus gallus domesticus) under quasi-static pressure conditions are provided. Micro-CT scans were made of cadaveric heads of chickens under positive (0.25 kPa, 0.5 kPa, 1 kPa, and 1.5 kPa) and negative (-0.25 kPa, -0.5 kPa, -1 kPa, and -1.5 kPa) middle ear pressure. Both extrastapedius and columella footplate displacements show a non-linear S-shaped curve as a function of pressure indicating non-linear response characteristics of the middle ear components. The S-curve is also seen in mammals, but unlike in mammals, the lateral piston-like displacement of both the columella footplate and extrastapedius, which is caused by an increased middle ear pressure are smaller than the medial piston-like displacements, caused by a decreased middle ear pressure of the same magnitude. Columella footplate piston displacements are always smaller than the extrastapedius piston displacements, indicating the flexibility of the extracolumella. The cone-shape of the avian tympanic membrane with inverted apex in comparison to the mammalian tympanic membrane can cause the inverted shape of the pressure response curve.