Effects of pars flaccida on sound conduction in ears of Mongolian gerbil: acoustic and anatomical measurements.

This paper presents evidence on how the pars flaccida of the tympanic membrane affects the acoustic input to the middle and inner ear. Measurements of middle-ear acoustic input admittance and sound-pressure levels in both the ear canal and the middle-ear cavity were made in ears of eight gerbils before and after manipulations of the middle ear and tympanic membrane. The results are interpreted in terms of a model proposed by Kohllöffel [Hear, Res. 13(1984) 83-88]. The input-admittance measurements show that the pars flaccida of gerbil acts as a resonator with a resonance frequency of approx. 500 Hz. The admittance of this resonator appears in parallel with the input admittance of the pars tensa and its ossicular and cochlear load. At frequencies below the resonance, the pars flaccida admittance is compliance-like and its magnitude is comparable to that of the pars tensa and its load; consequently, the presence of pars flaccida increases the overall middle-ear input admittance and decreases the pressure difference across the tympanic membrane. At higher frequencies, the admittance of pars flaccida is mass-like and small in magnitude, and it has negligible influence on the overall middle-ear input admittance and the pressure difference across the tympanic membrane. These results suggest that the presence of pars flaccida reduces low-frequency acoustic input to the middle- and inner-ear and consequently decreases hearing sensitivity in this frequency range. Our measurements suggest that with a constant sound pressure stimulus, stiffening the gerbil pars flaccida would increase the motion of the pars tensa by 3-10 dB at frequencies below the normal flaccida resonance.