The avian stapedius muscle. Influence on auditory sensitivity and sound transmission

The influence of the solitary avian middle ear muscle, the m. stapedius, on auditory sensitivity and sound transmission was investigated in the domestic chicken, Gallus gallus. Electrophysiological recordings of inner ear microphonic potentials (MP) were made in order to determine the effects of calibrated, mechanically induced stapedius muscle (SM) tension changes on sounds reaching the auditory receptor cells. The maximum MP responses recorded during the pre-tension measurements were on the order of 200 microV and were linear (on a log-log scale) over a range of 40-100 dB SPL. Tension levels of 50-400 mN in the SM caused a reduction of up to 20 dB in the MP at frequencies throughout the auditory spectrum. It is concluded that the SM of Aves serves to protect the inner ear receptor cells against overstimulation. In addition to attenuating the amplitude of the MP response, SM tension changes caused significant changes in the phase of the signals reaching the inner ear. The magnitude of attenuation in the ipsilateral MP response to 200-400 mN of tension was found to be similar to the interaural attenuation that occurs when sound is transmitted to the ipsilateral ear from the contralateral ear via the intracranial passageway. The similarity in MP amplitude changes resulting from SM tension and intracranial transmission suggests that the SM may be involved in interaural interaction and thereby may aid in sound localization.