The effects of slight pressure oscillations in the far infrasound frequency range on the pars flaccida in gerbil and rabbit ears

This study was designed to clarify whether the pars flaccida (PF) as a flexible part of the tympanic membrane is capable of reacting to pressure oscillations (PO) with amplitudes and frequencies typical for natural atmospheric pressure fluctuations in the far infrasound frequency range (APF). If so, the PF mechanical reactions to APF might be involved in the overall physiologic regulation processes, which make organisms susceptible to APF. The displacements of the PF in response to PO were measured in vitro in ears of gerbils and rabbits by means of laser Doppler vibrometry. The index of the PF reactivity (R_a) was determined as the ratio of the amplitude of the PF oscillations (PFO) to the amplitude of the PO. All kinds of PO applied caused PFO. The amplitude of the PFO increased when the amplitude of the PO was increased. In gerbils, a decrease in R_a with the increase in amplitude of the PO was observed. In the range of PO lowest amplitudes (4–20 Pa) R_a proved to be 1.4 times higher than in the range of highest amplitudes (90–105 Pa). Considering that the natural APF are usually within the range of ±20 Pa, this fact points to an important contribution of the PF to the pressure dynamics in the middle ear (ME) of gerbils. In rabbit ears, R_a was lower and recovery from plastic deformation was slower than in gerbils. Our findings are in line with the suggestion that the PF might play an important role in respect of adaptation to natural APF.