The function of the middle ear muscle reflex pathway appears to be protective contraction of the middle ear muscles results in frequency-dependent sound attenuation in the presence of intense acoustic stimuli, an effect that is more pronounced for lower sound frequencies. 90,91 Two major functions of the stapedius reflexes have been proposed: (1) modulation of middle ear impedance and attenuation of acoustic energy that reaches the cochlea 92–94 and (2) high-pass filtration of low-frequency sound (background noise) to prevent masking of speech frequencies. 88–90 Unlike the case in some animal models, in which both the stapedius and tensor tympani contract in response to sound, the stapedius reflex is the dominant sound-evoked pathway in humans. Stapedius motoneurons project to the middle ear by way of the facial nerve to innervate the stapedius muscle (stapedial nerve), which is attached to the posterior neck of the stapes capitulum contraction stiffens the stapes superstructure and increases middle ear impedance.Īcoustic impedance measurements have proven that the stapedius is the primary sound-evoked middle ear muscle. 128.14), although these central pathways are not completely understood at this point. Interneurons that have not yet been identified but may be located in the ventral cochlear nucleus, 87 projecting either directly or indirectly from the cochlear nuclei to the stapedius motoneurons (see Fig. The reflex therefore begins as afferent auditory input from the cochleae, and the signal is transmitted along the auditory nerve to the cochlear nuclei. Acoustic stimuli presented to either ear activate stapedius muscle contraction in both ears, similar to the consensual pupillary response to light. 86 The detailed pathway of the stapedial reflex arc is shown in Fig. Contraction of the stapedius and tensor tympani muscles exerts forces perpendicular to the stapes and malleus, respectively, to increase the impedance of the ossicular chain. Activation of this neural pathway results in contraction of the middle ear muscles in response to specific sound stimuli. The stapedius and tensor tympani muscles are the target organs of the middle ear muscle reflex and are innervated by the efferent fibers that originate in the motoneurons around and near the facial and trigeminal nerve nuclei, respectively. The pathways for the stapedius reflex to one side (ipsilateral side) are shown in Fig. The middle ear muscle reflex is another major feedback system to the auditory periphery. Flint MD, FACS, in Cummings Otolaryngology: Head and Neck Surgery, 2021 Middle Ear Muscle Reflex Pathways

Auditory signal path