Human middle-ear muscle pulls change tympanic-membrane shape and low-frequency middle-ear transmission magnitudes and delays

The three-bone flexible ossicular chain in mammals may allow independent alterations of middle-ear (ME) sound transmission via its two attached muscles, for both acoustic and non-acoustic stimuli. The tensor tympani (TT) muscle, which has its insertion on the malleus neck, is thought to increase tension of the tympanic membrane (TM). The stapedius (St) muscle, which has its insertion on the stapes posterior crus, is known to stiffen the stapes annular ligament. We produced ME changes in human cadaveric temporal bones by statically pulling on the TT and St muscles. The 3D static TM shape and sound-induced umbo motions from 20 Hz to 10 kHz were measured with optical coherence tomography (OCT); stapes motion was measured using laser-Doppler vibrometry (LDV). TT pulls made the TM shape more conical and moved the umbo medially, while St pulls moved the umbo laterally. In response to sound below about 1 kHz, stapes-velocity magnitudes generally decreased by about 10 dB due to TT pulls and 5 dB due to St pulls. In the 250 to 500 Hz region, the group delay calculated from stapes-velocity phase showed a decrease in transmission delay of about 150 mu s by TT pulls and 60 mu s by St pulls. Our interpretation of these results is that ME-muscle activity may provide a way of mechanically changing interaural time-and level-difference cues. These effects could help the brain align head-centered auditory and ocular-centered visual representations of the environment.(c) 2023 Published by Elsevier B.V.

Automated summary

The flexible ossicular chain in mammals, consisting of the tensor tympani muscle and the stapedius muscle, allows independent alterations of middle-ear sound transmission. Pulling on these muscles can change the shape of the tympanic membrane and the motion of the ossicles, affecting sound-induced vibrations and transmission delay. These effects may play a role in aligning auditory and visual representations of the environment.