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Acoustic reflex

acoustic reflex, acoustic reflex pathway
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 in response to high-intensity sound stimuli or when the person starts to vocalize

When presented with a high-intensity sound stimulus, the stapedius and tensor tympani muscles of the ossicles contract1 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 tympanic membrane 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

Contents

  • 1 Acoustic reflex threshold
  • 2 Characteristics and effects
  • 3 Protection role
  • 4 Measurement
  • 5 See also
  • 6 References

Acoustic reflex thresholdedit

The acoustic reflex threshold ART is the sound pressure level SPL from which a sound stimulus with a given frequency will trigger the acoustic reflex The ART is a function of sound pressure level and frequency

Individuals with normal hearing have an acoustic reflex threshold ART around 70-100 dB SPL People with conductive hearing loss -ie bad transmission in the middle ear have a higher acoustic reflex threshold2

The acoustic reflex threshold is usually 10-20 dB 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 just as harmful to their ears3

The acoustic reflex threshold can be lowered by the simultaneous presentation of a second tone facilitator The facilitator tone can be presented to either ear This facilitation effect tends to be greater when the facilitator tone has a frequency lower than the frequency of the elicitor ie the sound used to trigger the acoustic reflex4

Characteristics and effectsedit

  • For most animals, the acoustic reflex is the contraction of both middle ear muscles: the stapedius and tensor tympani muscles However in humans, the acoustic reflex only involves the contraction of the stapedius muscle -not the tensor tympani5
  • The contraction of the stapedius muscle occurs bilaterally in normal ears, no matter which ear was exposed to the loud sound stimulation2
  • The acoustic reflex mostly protects against low frequency sounds6
  • When triggered by sounds 20 dB above the reflex threshold, the stapedius reflex decreases the intensity of the sound transmitted to the cochlea by around 15 dB7
  • The acoustic reflex is also invoked when a person vocalizes8 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 vocalization8 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 tweets9

Protection roleedit

The protection of the organ of Corti, provided by the acoustic reflex against excessive stimulation especially that of the lower frequencies has been demonstrated both in man and animals But this protection effect is limited6

According to the article Significance of the stapedius reflex for the understanding of speech, the latency of contraction is only about 10ms, but maximum tension may not be reached for 100 ms or more6 According to the article Le traumatisme acoustique, the latency of contraction is 150 ms with noise stimulus which SPL is at the threshold ATR, and 25-35 ms at high sound pressure levels Indeed, the amplitude of the contraction grows with the sound pressure level stimulus10

Because of this latency, the acoustic reflex cannot protect against sudden intense noises106 However, when several sudden intense noises are presented at a pace higher than 2–3 seconds of interval, the acoustic reflex is able to play a role against auditory fatigue10

Moreover, the full tension of the stapedius muscle cannot be maintained in response to continued stimulation Indeed, the tension drops to about 50% of its maximum value after a few seconds6

Measurementedit

Most of the time, the stapedius reflex is tested with tympanometry The contraction of the stapedius muscle stiffens the middle-ear, thus decreasing middle-ear admittance; this can be measured thanks to tympanometry2 The acoustic stapedius reflex can also be recorded by means of extratympanic manometry ETM7

As the stapedius muscle is innervated by the facial nerve,11 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 neuroma2

The acoustic reflex normally occurs only at relatively high intensities; contraction of middle ear muscles for quieter sounds can indicate ear dysfunction eg 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 problem11

See alsoedit

  • Tensor tympani
  • Otoacoustic emission
  • Equal-loudness contours
  • Audiometry
  • Hyperacusis

Referencesedit

  1. ^ Fox, Stuart 2006 Human Physiology ninth ed New York: McGraw-Hill pp 267–9 ISBN 0-07-285293-3 
  2. ^ a b c d "Impedance Audiometry" MedScape 
  3. ^ W Niemeyer 1971 "Relations between the Discomfort Level and the Reflex Threshold of the Middle Ear Muscles" Internal journal of audiology 10: 172–176 doi:103109/00206097109072555 
  4. ^ Kawase, Tetsuaki; Takasaka, Tomonori; Hidaka, Hiroshi June 1997 "Frequency summation observed in the human acoustic reflex" Hearing Research 108: 37–45 doi:101016/s0378-59559700039-7 
  5. ^ "Notes on the Acoustic Middle Ear Reflex" American academy of audiology 
  6. ^ a b c d e G Lidén; J E Hawkins; B Nordlund 1964 "Significance of the Stapedius Reflex for the Understanding of Speech" Acta Oto-laryngologica 57: 275–279 doi:103109/00016486409134576 
  7. ^ a b Brask, Torben 1978 "The Noise Protection Effect of the Stapedius Reflex" Acta Oto-laryngologica 86: 116–117 doi:103109/00016487809123490 
  8. ^ a b Møller, Aage 2000 Hearing: It's Physiology and Pathophysiology illustrated ed Academic Press pp 181–90 ISBN 978-0125042550 
  9. ^ Borg, E; Counter, S A 1989 "The Middle-Ear Muscles" Scientific American 261 2: 74–78 PMID 2667133 doi:101038/scientificamerican0889-74 
  10. ^ a b c Dancer, Armand 1991 "Le traumatisme acoustique" PDF médecine/sciences in French 7: 357–367 doi:104267/10608/4361 
  11. ^ a b Probst, Rudolf; Gerhard Grevers; Heinrich Iro 2006 Basic Otorhinolaryngology: A Step-by-Step Learning Guide second, illustrated, revised ed Thieme pp 185–6 ISBN 1588903370 

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