|The medial wall and part of the posterior and anterior walls of the right tympanic cavity, lateral view.|
|Bones and muscles in the tympanic cavity in the middle ear|
|Gray's||subject #230 1046|
|Origin||Walls of pyramidal eminence|
|Insertion||Neck of stapes|
|Artery||Stapedial branch of posterior auricular artery|
|Nerve||Facial nerve (nerve to stapedius)|
|Actions||Control the amplitude of sound waves to the inner ear|
The stapedius emerges from a pinpoint foramen in the apex of the pyramidal eminence (a hollow, cone-shaped prominence in the posterior wall of the tympanic cavity), and inserts into the neck of the stapes.
It reflexively dampens the vibrations of the stapes by pulling on the neck of that bone. It prevents excess movement of the stapes, helping to control the amplitude of sound waves from the general external environment to the inner ear. The stapedius muscle dampens the ability of the stapes vibration and protects the inner ear from high noise levels, primarily the volume of your own voice. Compare also the role of the tensor tympani muscle, which primarily dampens those sounds associated with chewing.
The stapedius is innervated by the nerve to stapedius, a branch of cranial nerve VII, the facial nerve. This is the first branch of the facial nerve after it exits the facial canal; the second branch is the chorda tympani which carries special sense (taste) and parasympathetic fibres of cranial nerve VII. Chorda tympani provides taste sensation to the anterior 2/3 of the tongue by joining with the lingual nerve – a branch of the mandibular nerve (V3) – in the infratemporal fossa.
Paralysis of the stapedius, such as in injury to the facial nerve (CN VII) distal to the geniculate ganglion prior to its branch to stapedius muscle (which would also cause Bell's Palsy), allows wider oscillation of the stapes, resulting in heightened reaction of the auditory ossicles to sound vibration. This condition, known as hyperacusis, causes normal sounds to be perceived as very loud.
Like the stapes bone to which it attaches, the stapedius muscle shares evolutionary history with other vertebrate structures.
The mammalian stapedius evolved from a muscle called the depressor mandibulae in other tetrapods, the function of which was to open the jaws (this function was taken over by the digastric muscle in mammals). The depressor mandibulae arose from the levator operculi in bony fish, and is equivalent to the epihyoidean in sharks. Like the stapedius, all of these muscles derive from the hyoid arch and are innervated by cranial nerve VII.