The stapedial reflex arc involves afferent input via the auditory nerve and efferent output via the facial nerve to the stapedius muscle. Which of the following correctly describes its ipsilateral and contralateral components?

• A Afferent from cochlear nerve → cochlear nucleus → ipsilateral superior olivary complex → ipsilateral facial nerve motor nucleus only
• B Afferent from cochlear nerve → inferior colliculus → medial geniculate → bilateral facial nerve nuclei
• C Afferent from cochlear nerve → trapezoid body → ipsilateral facial nerve only
• D Afferent from cochlear nerve → cochlear nucleus → both superior olivary complexes → bilateral facial nerve motor nuclei ✓

Explanation

The acoustic (stapedial) reflex arc: sound stimulates the cochlear nerve, whose fibres project to the cochlear nucleus. From here fibres project bilaterally to both superior olivary complexes via the trapezoid body. Each superior olivary complex then activates the ipsilateral facial nerve motor nucleus, which sends efferents via the facial nerve to the stapedius. This bilateral projection explains why a loud sound in one ear elicits contraction of the stapedius in BOTH ears (ipsilateral and contralateral reflex). This is clinically used in impedance audiometry.

Reference: Dhingra Diseases of Ear, Nose and Throat, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.