The acoustic reflex (also known as the stapedius reflex, middle-ear-muscles (MEM) 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).
Miscellaneous essencial informations
- The acoustic reflex is the contraction of both the stapedius and tensor tympani muscles for most animals; however with humans, only the stapedius muscle contracts -not the tensor tympani.
- The contraction of the stapedius muscle occurs bilaterally in normal ears, no matter which ear was exposed to the loud sound stimulation.
- The acoustic reflex mostly protects agains low frequency sounds.
- Individuals with normal hearing have an acoustic reflexes threshold around 70-100dB SPL. People with conductive hearing loss (-i.e. bad transmission in the middle ear) have a higher acoustic reflex threshold.
- The acoustic reflex threshold is usually 10-20dB below the discomfort threshold. However the discomfort threshold is not a relevant indicator of the harmfulness of a sound: industry workers tend to have a higher discomfort threshold, but the sound is as much harmful to their ears.
- The acoustic reflex threshold can be lowered by the simultaneous presentation of a second tone (facilitator). The facilitator tone can be presented to either ears. This facilitation effect tends to be more important when the facilitator tone has a frequency lower than the frequency of the elicitor (i.e. the sound used to trigger the acoustic reflex).
- The stapedius reflex decreases the intensity of the sound transmitted to the cochlea, by around 15dB.
- The acoustic 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 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 measurement
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.
A measurement of the reflex can also be used to suggest a retrocochlear lesion (eg, vestibular schwannoma, acoustic neuroma).
The acoustic reflex normally occurs only at relatively high intensities; activation for quieter sounds can indicate ear dysfunction (e.g. tonic tensor tympani syndrome -TTTS).
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.
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