VOLUME DISPLACEMENT OF THE GERBIL EARDRUM PARS FLACCIDA AS A FUNCTIONOF MIDDLE-EAR PRESSURE

The pars flaccida (PF) is a small region of the eardrum, with elastici ty and histology completely different from the rest of the membrane, w hich 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 m easured in vitro in five Mongolian gerbil ears, by means of an opto-el ectronic moire interferometer. Volume displacement was determined at s mall intervals in three sequential pressure cycles, in the range of +/ - 0.4 kPa, +/- 2 kPa, and again +/- 0.4 kPa. The displacement was foun d to be a highly non-linear function of pressure, with a strong increa se up to 0.4 kPa ME over-or underpressure and remaining nearly unchang ed for pressures beyond 0.4 kPa. In all animals, maximal volume displa cement was less than 0.5 mu l, or 0.2% of total ME air volume. Clear h ysteresis was found between the deformations at the same pressure leve l in the increasing and decreasing parts of the pressure cycles. Membr ane behavior in the first 0.4 kPa pressure cycle was significantly dif ferent from that in the second 0.4 kPa cycle, which followed the 2 kPa pressure cycle. The results indicate that the ME pressure change regu lation function of the PF is limited to very small pressure changes of a few hundred Pa around ambient pressure, and that larger ME pressure s cause at least short-term changes in the membrane's behavior. (C) 19 98 Elsevier Science B.V.