The acoustic reflex (or stapedius reflex, attenuation reflex, or auditory reflex) is an involuntary muscle contraction that occurs in the middle ear of mammals in response to high-intensity sound stimuli.
When presented with a high-intensity sound stimulus, the stapedius and tensor tympani muscles of the ossicles contract. The stapedius stiffens the ossicular chain by pulling the stapes (stirrup) of the middle ear away from the oval window of the cochlea and the tensor tympani muscle stiffens the ossicular chain by loading the eardrum when it pulls the malleus (hammer) in toward the middle ear. The reflex decreases the transmission of vibrational energy to the cochlea, where it is converted into electrical impulses to be processed by the brain. The acoustic reflex normally occurs only at relatively high intensities; activation for quieter sounds can indicate ear dysfunction. The pathway involved in the acoustic reflex is complex and can involve the ossicular chain (malleus, incus and stapes), the cochlea (organ of hearing), the auditory nerve, brain stem, facial nerve and other components. Consequently, the absence of an acoustic reflex, by itself, may not be conclusive in identifying the source of the problem.
Vocalization-induced stapedius reflex
The stapedius reflex is also invoked when a person vocalizes. In humans, the vocalization-induced stapedius reflex reduces sound intensities reaching the inner ear by approximately 20 decibels. The stapedius reflex causes an acousto-mechanical increase in impedance.
The components of the ear are connected in series, basic signal flow through the ear is as follows: The outer ear → the eardrum → the ossicles → the cochlea. A traveling wave of displacement occurs inside the fluid-filled cochlea, and deflects the inner hair cells' hairs, which causes the primary auditory neurons to send impulses to the brain. The vocalization-induced stapedius reflex is an active effect which acts upon the ossicles of the middle ear, decreases signal flow and ultimately leads to less signal sent to the brain. The reflex is triggered in anticipation of the onset of vocalization.
While the vocalization-induced stapedius reflex in humans results in an approximate 20 dB reduction in transduction to the inner ear, birds have a stronger stapedius reflex that is invoked just before the bird tweets.
The stapedius reflex in injury to the facial nerve
As the stapedius muscle is innervated by the facial nerve, a measurement of the reflex can be used to locate the injury on the nerve. If the injury is distal to the stapedius muscle, the reflex is still functional.
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