Timothy C. Hain, MD Page last modified: June 18, 2009
There are two muscles within the middle ear that function to protect it from loud noses -- the Tensor Tympani and the Stapedius.
Cartoon of the middle ear showing muscles that attach to ossicles (ear bones), and ear drum. The stapedius is attached to the stapes (of course -- horseshoe object above), while the tensor tympani (TT) is attached to the ear drum. The picture above, provided with permission from Loyola University's anatomy program, from: http://www.meddean.luc.edu/lumen/meded/grossanatomy/dissector/mml/images/stap.jpg is actually very inaccurate as it does not show the sharp turn of the tendon as it enters the middle ear cavity or the insertion of the TT onto the manubrium near it's root. The manubrium is one of the ossicles of the middle ear.
The stapedius muscle is also shown drawn in the illustration above from Loyola University Medical School in an innacurate fashion. It is the small red muscle attached to the stapes (horseshoe shaped structure). It goes backward rather than forward as drawn.
If there is anyone who can contribute a more accurate illustration of the anatomy - -please let me know and help make the web a more accurate place.
The Tensor Tympani muscle (the big brown muscle shown above) originates from the auditory tube (eustachian tube) and inserts onto the manubrium of the ear drum. Note the comments about the innacuracy of the drawing. It is innervated by the mandibular division of the trigeminal nerve (V), through the trigeminal ganglion.
In many people with hyperacusis, there is increased reactivity of the TT reflex as part of the startle response to loud sound. The lowered reflex threshold for TT contraction can be activated by the anticipation of loud sound. In some people, the TT can be contracted simply by thinking about a loud noise.
Following contraction of the TT, there may be a thump, ear pain or fluttering or fullness. See the tinnitus page for more about these noises.
Occasional people can voluntarily contract the TT.
This is another muscle in the middle ear that protects to loud noises -- primarily chewing. It is a much smaller muscle than the TT, and runs from the stapes to a close wall of the middle ear. It is innervated by the tympanic branch of the 7th nerve. When it is paralyzed, as is sometimes (but not always) the case in Bell's Palsy, there may be heightened perception of loud noises on one ear vs. the other.
The stapedius muscle may be overactive, causing a high-pitched "tick" sound. See the tinnitus page for more detail about stapedius muscle myoclonus.
The stapedius reflex can be measured using the "Acoustic Reflex test".