The effect of immobilizing the gerbil's pars flaccida on the middle-ear's response to static pressure.

The pars flaccida of the tympanic membrane has a small role in regulating middle-ear static pressure (Acta Physiol. Scand. 118 (1983) 337; Hear. Res. 118 (1998) 35) and can also modify the response of the middle ear to low-frequency sound pressures by shunting ear-canal volume velocity around the pars tensa (Hear. Res. 13 (1984) 83; Hear. Res. 106 (1997) 39; Diversity in Auditory Mechanics (1997) 129; Audiol. Neuro-Otol. 4 (1999) 129). It has been hypothesized that these two functions can interact to reduce the effect of middle-ear static pressure on sound transmission through the middle ear (Hear. Res. 153 (2001) 146). This paper tests this hypothesis by measuring the effect of static pressure on the sensitivity of the p. tensa and the coupled malleus to sound, before and after immobilizing the p. flaccida. The results are consistent with a limited role of the p. flaccida in influencing the effect of static pressure on the p. tensa's acoustic response. However, this effect is only observed at low frequencies and over the +/-1 cm H(2)0 range of middle-ear static pressures. The results also suggest that large negative middle-ear pressures can induce a change in the mode of tympanic membrane motion regardless of the state of the p. flaccida.