stapedius

The stapedius is a small muscle of the middle ear that functions to stabilize the stapes (stirrup bone) and regulate sound transmission to the inner ear. It originates from the pyramidal eminence on the posterior wall of the tympanic cavity and inserts on the neck of the stapes, with its tendon passing near the incudostapedial joint.

The stapedius is innervated by the nerve to stapedius, a motor branch of the facial nerve (cranial nerve VII). It is considered the smallest skeletal muscle in humans. Its contraction pulls the stapes posteriorly, reducing the oscillatory motion of the auditory ossicles and thereby dampening sound transmission, especially at higher intensities. This action is part of the acoustic (stapedial) reflex, a protective mechanism that helps shield the inner ear from damage due to loud sounds.

The reflex arc involves afferent input from the cochlear nerve to the cochlear nucleus and superior olivary complex, with bilateral efferent signals via the facial nerve to the stapedius on both ears. The tensor tympani, supplied by the trigeminal nerve, is the other middle-ear muscle that participates in a similar reflex pathway.

Clinical significance includes hyperacusis (increased sensitivity to sound) when the stapedius or its innervation is deficient, as seen in facial nerve palsy or stapedius nerve paralysis. The stapedius is a key component of middle-ear impedance matching and protective auditory reflexes.