The acoustic reflex (also known as the stapedius reflex, stapedial reflex, auditory reflex, middle-ear-muscle reflex (MEM reflex, MEMR), attenuation reflex, cochleostapedial reflex or intra-aural reflex) is an involuntary

muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of mus ...

contraction that occurs in the

middle ear The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear). The mammalian middle ear contains three ossicles, which transfer the vibrations of the eardrum into waves in the ...

in response to loud sound stimuli or when the person starts to vocalize. When presented with an intense sound stimulus, the

stapedius The stapedius is the smallest skeletal muscle in the human body. At just over one millimeter in length, its purpose is to stabilize the smallest bone in the body, the stapes or strirrup bone of the middle ear. Structure The stapedius emerges fr ...

and

tensor tympani The tensor tympani is a muscle within the middle ear, located in the bony canal above the bony part of the auditory tube, and connects to the malleus bone. Its role is to dampen loud sounds, such as those produced from chewing, shouting, or th ...

muscles of the

ossicles The ossicles (also called auditory ossicles) are three bones in either middle ear that are among the smallest bones in the human body. They serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea). The absence of the auditor ...

contract. The stapedius stiffens the ossicular chain by pulling the

stapes The ''stapes'' or stirrup is a bone in the middle ear of humans and other animals which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its annular ligament, which allows the foo ...

(stirrup) of the middle ear away from the oval window of the

cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory o ...

and the tensor tympani muscle stiffens the ossicular chain by loading the

tympanic membrane In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear from the middle ear. Its function is to transmit sound from the a ...

when it pulls the

malleus The malleus, or hammer, is a hammer-shaped small bone or ossicle of the middle ear. It connects with the incus, and is attached to the inner surface of the eardrum. The word is Latin for 'hammer' or 'mallet'. It transmits the sound vibrations f ...

(hammer) in toward the middle ear. The reflex decreases the transmission of vibrational energy to the

, where it is converted into electrical impulses to be processed by the

brain A brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as Visual perception, vision. I ...

Acoustic reflex threshold

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. People with normal hearing have an acoustic reflex threshold (ART) around 70–100 dB SPL. People with

conductive hearing loss Conductive hearing loss (CHL) occurs when there is a problem transferring sound waves anywhere along the pathway through the outer ear, tympanic membrane (eardrum), or middle ear (ossicles). If a conductive hearing loss occurs in conjunction with ...

(-i.e. bad transmission in the

) may have a greater or absent acoustic reflex threshold. 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 ears. The acoustic reflex threshold can be decreased 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 (i.e. the sound used to trigger the acoustic reflex).

Characteristics and effects

* 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 tympani. * The contraction of the stapedius muscle occurs bilaterally in normal ears, no matter which ear was exposed to the loud sound stimulation. * The prevalence of bilateral acoustic reflexes in persons 18–30 years old is 85.3% (82.9%, 87.4%) 95th percentile confidence interval N = 3280 and in all persons 74.6% (73.2%, 75.9%) N = 15,106. * The acoustic reflex mostly protects against low frequency sounds. *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 dB. * The acoustic reflex is also invoked when a person vocalizes. In humans, the vocalization-induced stapedius reflex reduces sound intensities reaching the

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in th ...

by approximately 20

decibel The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a ...

s. 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.

Hypothesized function

The main hypothesized function of the acoustic reflex is the protection of the

organ of Corti The organ of Corti, or spiral organ, is the receptor organ for hearing and is located in the mammalian cochlea. This highly varied strip of epithelial cells allows for transduction of auditory signals into nerve impulses' action potential. Trans ...

against excessive stimulation (especially that of the lower frequencies). This protection has been demonstrated both in humans and animals, but with limited effects. 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 more. 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 stimulus. Because of this latency, the acoustic reflex cannot protect against sudden intense noises. 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 fatigue. 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 seconds. In damage risk criteria for exposure to impulse noise, the acoustic reflex is integral to the

Auditory Hazard Assessment Algorithm for Humans The Auditory Hazard Assessment Algorithm for Humans (AHAAH) is a mathematical model of the human auditory system that calculates the risk to human hearing caused by exposure to impulse sounds, such as gunfire and airbag deployment. It was developed ...

model and the Integrated Cochlear Energy models. These two models estimate the response of the basilar membrane in response to an input stimulus and summate the vibration of the segments of the basilar membrane to predict the potential risk for hearing loss. The acoustic reflex can be activated before an impulse reaches the ear through an assumed conditioned response or it can be activated after the stimulus exceeds a specific level (e.g. 134 dB). Recent measurements of the acoustic reflex with a group of 50 subjects found that only 2 of the subjects exhibited any pre-activation of the reflex in the warned (countdown) or volitional control of the eliciting stimulus. An alternative hypothesis for the role of the acoustic reflex is the prevention of auditory masking of high-frequencies by low-frequencies, which are predominant in natural sounds.

Measurement

Most of the time, the stapedius reflex is tested with

tympanometry Tympanometry is an acoustic evaluation of the condition of the middle ear eardrum ( tympanic membrane) and the conduction bones by creating variations of air pressure in the ear canal. Tympanometry is an objective test of middle-ear function. It i ...

. The contraction of the stapedius muscle stiffens the middle-ear, thus decreasing middle-ear admittance; this can be measured thanks to tympanometry. The acoustic stapedius reflex can also be recorded by means of extratympanic manometry (ETM). The stapedial reflex can be measured with

laser Doppler velocimetry Laser Doppler velocimetry, also known as laser Doppler anemometry, is the technique of using the Doppler shift in a laser beam to measure the velocity in transparent or semi-transparent fluid flows or the linear or vibratory motion of opaque, refle ...

. Jones et al. focused a laser on the light reflex of the manubrium in awake human subjects. The amplitude of a 500 Hz probe tone was used to monitor the vibrations of the tympanic membrane. Various elicitors were presented to the subjects: 1000 Hz tone-burst for 0.5 s at 100 dB SPL, recorded .22 caliber gunshot noise with a peak level of 110 dB SPL. The amplitude of the 500 Hz probe tone was reduced in response to the eliciting stimuli. Time constants for the rate of onset and recovery were measured to be about 113 ms for the tone and 60-69 ms for the gunshot recordings. LDV_AR_measurement_USAARL

As the stapedius muscle is innervated by the

facial nerve The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of taste ...

, 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 (e.g., vestibular schwannoma). The acoustic reflex normally occurs only at relatively high intensities; contraction of middle ear muscles 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, superior olivary complex, and cochlear nucleus. Consequently, the absence of an acoustic reflex, by itself, may not be conclusive in identifying the source of the problem.

References

{{Reflex Reflexes Auditory system Audiology

Acoustic reflex threshold

Characteristics and effects

Hypothesized function

Measurement

See also

References