Notes on the Acoustic Middle Ear Reflex
Moller (2012) reports that the acoustic middle ear reflex in normal hearing people usually occurs at about 85 dB HL, although there is significant variation across populations. The strength of the stapedial contraction increases as the stimuli increases (above reflex threshold) and the latency (of the contraction) decreases from typical threshold responses of some 100 milliseconds to 25 milliseconds for stimuli of higher intensity.
Typically, when the acoustic reflex is triggered in one ear, both stapedius muscles contract (ipsi and contralateral). Of note, the acoustic middle ear reflex only includes the stapedius muscle in humans—not the tensor tympani. The stapedius muscle and the tensor tympani respond in tandem (to acoustic stimuli) in many lab animals, but not in humans. The stapedius nerve is a branch of the facial nerve (the seventh cranial nerve) and the stapedius nerve innervates the stapedius muscle. The tensor tympani muscle is innervated by the trigeminal nerve (the fifth cranial nerve) and this reflex can typically be stimulated via a puff of air on the cornea.
Moller notes that although the acoustic middle ear reflex may offer some minimal protection against noise induced hearing loss (NIHL), it is evolutionarily unlikely to be the purpose of the acoustic middle ear reflex, as sounds that trigger the acoustic reflex (i.e., long duration and high intensity) are quite rare in nature.
For More Information, References, and Recommendations
Kei J. (2012) Acoustic Stapedial Reflexes in Healthy Neonates- Normative Date and Test-Retest Reliability. Journal of the American Academy of Audiology 23(1):46–56
Moller A. (2012) Acoustic Middle Ear Reflex. In Hearing- Anatomy, Physiology and Disorders of the Auditory System. Plural Publishing.