Volume displacement of the gerbil eardrum pars flaccida as a function of middle ear pressure

The pars flaccida (PF) is a small region of the eardrum, with elasticity and histology completely different from the rest of the membrane, which has often been attributed a pressure regulating function for the middle ear (ME). In this paper, the volume displacement of the PF as a function of ME pressure is discussed. The deformation of the PF was measured in vitro in five Mongolian gerbil ears, by means of an opto-electronic moiré interferometer. Volume displacement was determined at small intervals in three sequential pressure cycles, in the range of +/- 0.4 kPa, +/- 2 kPa, and again +/- 0.4 kPa. The displacement was found to be a highly non-linear function of pressure, with a strong increase up to 0.4 kPa ME over- or underpressure and remaining nearly unchanged for pressures beyond 0.4 kPa. In all animals, maximal volume displacement was less than 0.5 microl, or 0.2% of total ME air volume. Clear hysteresis was found between the deformations at the same pressure level in the increasing and decreasing parts of the pressure cycles. Membrane behavior in the first 0.4 kPa pressure cycle was significantly different from that in the second 0.4 kPa cycle, which followed the 2 kPa pressure cycle. The results indicate that the ME pressure change regulation function of the PF is limited to very small pressure changes of a few hundred Pa around ambient pressure, and that larger ME pressures cause at least short-term changes in the membrane's behavior.