Effect of Tympanic Muscle Activity on Movement of the Eardrum, Acoustic Impedance and Cochlear Microphonics

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Abstract
The functions of the tympanic muscles were studied in anesthetized cats and rabbits. The change in air pressure produced in the closed external auditory meatus and in the acoustic impedance near the eardrum was recorded during (1) independent contraction of the two tympanic muscles, and during (2) contraction of the two muscles together. The change in cochlear microphonics following contraction of the tympanic muscles was also studied. As expected, it was found that a contraction of the tensor tympani always produced a negative pressure change in the meatus indicating an inward movement of the eardrum. A contraction of the stapedius muscle produced no change or considerably smaller pressure change which could be either positive or negative. It was further found that simultaneous contraction of the two muscles produced a negative change in the air pressure the magnitude of which was smaller than that produced by the tensor tympani alone. Independent contraction of the stapedius and the tensor tympani muscles produced a change within the same range of magnitude in the acoustic impedance as well as in the cochlear microphonics potential. The effect on the acoustic impedance and the cochlear microphonics of both muscles acting together was greater than that produced by either of the muscles when contracting alone. The results of the experiments indicate that a contraction of the stapedius muscle counteracts the effect of the tensor tympani on the movement of the eardrum and further that the combined action of the two muscles on acoustic impedance and cochlear microphonics is greater than that of the two muscles acting alone.