3.4 Protective reflexes

Two tiny muscles in the middle ear can stiffen the ossicular chain on demand. The stapedius (innervated by the facial nerve, VII) inserts on the neck of the stapes; the tensor tympani (innervated by the trigeminal, V) inserts on the malleus. Their job, broadly, is to attenuate the ossicular transmission in response to loud sounds — and, in the case of the tensor tympani, also in response to one’s own voice and to chewing. The reflex onset latency is on the order of 10 ms, far too slow to protect against an impulsive sound like a gunshot but useful for sustained loud noise.

The attenuation provided by the stapedius reflex is modest — about 10–15 dB and only at low frequencies. It is not a high-fidelity protection mechanism; it is a low-pass-filtering protection mechanism. High-frequency transients pass right through.

A separate clinical fact worth flagging: the stapedius reflex is bilateral. A loud sound presented to one ear will trigger stapedius contraction in both ears. This is the basis of the acoustic reflex test in audiology, which probes brainstem-level auditory processing. If a sound in one ear cannot evoke a reflex in the contralateral stapedius, something is wrong somewhere in the brainstem auditory pathway — not necessarily in either cochlea.