Acoustic reflex
Encyclopedia
The acoustic 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 pulls the stapes (stirrup) of the middle ear away from the oval window of the cochlea
and the tensor tympani muscle pulls the malleus
(hammer) away from ear drum. 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 and absence of acoustic reflex can indicate neural hearing loss.
s. The stapedius reflex causes an acousto-mechanical increase in impedance
.
Continuing the electronics analogy (impedance is "resistance" for AC signals), understanding the basic block diagram is important. Essentially everything in the ear is connected "in series": The outer ear → the eardrum
→ the ossicles
→ the fluid-filled cochlea
. Inside the cochlea, a traveling wave of displacement occurs in the fluid, and deflects the inner hair cells' hairs, which causes the primary auditory neurons to send impulses to the brain.
If any effect decreases transduction from one element to the next (in the items of the above paragraph), there will ultimately be less signal sent to the brain. The stapedius reflex that is invoked upon vocalization works in the ossicles of the middle ear, and is an active effect. A muscle is tightened in anticipation of the onset of vocalizing.
While the vocalization-induced stapedius reflex in humans results in about a 20 dB reduction in transduction to the inner ear, birds have a stronger stapedius reflex that is invoked just before the bird tweets.
Humming when you don't want to hear someone else really works — and the reason is the stapedius reflex. While rude, the behavior provides the opportunity to make two points: first, the effect is active, not passive; second, and a more subtle point, is that the reflex is not a psychological effect or perceptual masking effect or a question of poorer signal-to-noise ratio
when "noise" is boosted. The effect is not that of a sound seeming less loud; rather, the effect is that less of the sound waves is transducted into the inner ear — thus causing less bending of the hairs of the hair cells.
The vocalization-induced stapedius reflex can indeed be used for hearing protection purposes. Just before an impulse noise
(door slam, electromagnet lock slapback, gunshot, pound of hammer on nail) one could vocalize (or cough or hum) to protect one's hearing from the sound pressure that the impending sound would create. The reflex is not a perceptual reduction in sound; the reflex is a real reduction in sound level reaching the inner ear — an actual reduction in how far one's delicate hair cells will be bent by that sound. An identical hammer blow when one engaged in no vocalization is more damaging to one's hearing than that same hammer blow if one began vocalizing just a few tens of milliseconds prior to the blow.
The stapedius reflex can also be inconvenient in conversations where inadequate time is allowed between one person vocalizing and the next person beginning his/her vocalization. For example, in a telephone conversation in which one's credit card number is read out in sets of four numbers at a time, if the other person starts echoing them back to you in sets of four and you begin vocalizing the next set of four numbers before the other person's stapedius reflex has subsided, it is likely that the other person will mis-hear the first one or two syllables you spoke.
Finally, the stapedius reflex is not very effective for very low frequency sounds because they are mainly transmitted by bone-conduction to the ear. For such sounds, intra-aural hearing protection devices (earplugs), or even full-face helmets, can provide attenuation.
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
contraction that occurs in the middle ear
Middle ear
The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...
of mammals in response to high-intensity sound stimuli
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....
.
When presented with a high-intensity sound stimulus, the stapedius
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....
and tensor tympani
Tensor tympani
The tensor tympani, the larger of the two muscles of the tympanic cavity, is contained in the bony canal above the osseous portion of the auditory tube...
muscles of the ossicles
Ossicles
The ossicles are the three smallest bones in the human body. They are contained within the middle ear space and serve to transmit sounds from the air to the fluid-filled labyrinth . The absence of the auditory ossicles would constitute a moderate-to-severe hearing loss...
contract. The stapedius pulls the stapes (stirrup) of the middle ear away from the oval window of the cochlea
Cochlea
The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns around its axis, the modiolus....
and the tensor tympani muscle pulls the malleus
Malleus
The malleus or hammer is a hammer-shaped small bone or ossicle of the middle ear which connects with the incus and is attached to the inner surface of the eardrum...
(hammer) away from ear drum. The reflex decreases the transmission of vibrational energy to the cochlea
Cochlea
The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns around its axis, the modiolus....
, where it is converted into electrical impulses to be processed by the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
. The acoustic reflex normally occurs only at relatively high intensities; activation for quieter sounds can indicate ear dysfunction and absence of acoustic reflex can indicate neural hearing loss.
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 decibelDecibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
s. The stapedius reflex causes an acousto-mechanical increase in impedance
Acoustic impedance
The acoustic impedance at a particular frequency indicates how much sound pressure is generated by a given air vibration at that frequency. The acoustic impedance Z is frequency dependent and is very useful, for example, for describing the behaviour of musical wind instruments...
.
Continuing the electronics analogy (impedance is "resistance" for AC signals), understanding the basic block diagram is important. Essentially everything in the ear is connected "in series": The outer ear → the eardrum
Eardrum
The eardrum, or tympanic membrane, is a thin membrane that separates the external ear from the middle ear in humans and other tetrapods. Its function is to transmit sound from the air to the ossicles inside the middle ear. The malleus bone bridges the gap between the eardrum and the other ossicles...
→ the ossicles
Ossicles
The ossicles are the three smallest bones in the human body. They are contained within the middle ear space and serve to transmit sounds from the air to the fluid-filled labyrinth . The absence of the auditory ossicles would constitute a moderate-to-severe hearing loss...
→ the fluid-filled cochlea
Cochlea
The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns around its axis, the modiolus....
. Inside the cochlea, a traveling wave of displacement occurs in the fluid, and deflects the inner hair cells' hairs, which causes the primary auditory neurons to send impulses to the brain.
If any effect decreases transduction from one element to the next (in the items of the above paragraph), there will ultimately be less signal sent to the brain. The stapedius reflex that is invoked upon vocalization works in the ossicles of the middle ear, and is an active effect. A muscle is tightened in anticipation of the onset of vocalizing.
While the vocalization-induced stapedius reflex in humans results in about a 20 dB reduction in transduction to the inner ear, birds have a stronger stapedius reflex that is invoked just before the bird tweets.
Humming when you don't want to hear someone else really works — and the reason is the stapedius reflex. While rude, the behavior provides the opportunity to make two points: first, the effect is active, not passive; second, and a more subtle point, is that the reflex is not a psychological effect or perceptual masking effect or a question of poorer signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
when "noise" is boosted. The effect is not that of a sound seeming less loud; rather, the effect is that less of the sound waves is transducted into the inner ear — thus causing less bending of the hairs of the hair cells.
The vocalization-induced stapedius reflex can indeed be used for hearing protection purposes. Just before an impulse noise
Impulse noise (audio)
Impulse noise is a category of noise which includes unwanted, almost instantaneous sharp sounds . Noises of the kind are usually caused by electromagnetic interference, scratches on the recording disks, and ill synchronization in digital recording and communication...
(door slam, electromagnet lock slapback, gunshot, pound of hammer on nail) one could vocalize (or cough or hum) to protect one's hearing from the sound pressure that the impending sound would create. The reflex is not a perceptual reduction in sound; the reflex is a real reduction in sound level reaching the inner ear — an actual reduction in how far one's delicate hair cells will be bent by that sound. An identical hammer blow when one engaged in no vocalization is more damaging to one's hearing than that same hammer blow if one began vocalizing just a few tens of milliseconds prior to the blow.
The stapedius reflex can also be inconvenient in conversations where inadequate time is allowed between one person vocalizing and the next person beginning his/her vocalization. For example, in a telephone conversation in which one's credit card number is read out in sets of four numbers at a time, if the other person starts echoing them back to you in sets of four and you begin vocalizing the next set of four numbers before the other person's stapedius reflex has subsided, it is likely that the other person will mis-hear the first one or two syllables you spoke.
Finally, the stapedius reflex is not very effective for very low frequency sounds because they are mainly transmitted by bone-conduction to the ear. For such sounds, intra-aural hearing protection devices (earplugs), or even full-face helmets, can provide attenuation.
The stapedius reflex in injury to the facial nerve
The stapedius muscle is innervated by the facial nerve, and 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.See also
- Tensor tympaniTensor tympaniThe tensor tympani, the larger of the two muscles of the tympanic cavity, is contained in the bony canal above the osseous portion of the auditory tube...
- Otoacoustic emissionOtoacoustic emissionAn otoacoustic emission is a sound which is generated from within the inner ear. Having been predicted by Thomas Gold in 1948, its existence was first demonstrated experimentally by David Kemp in 1978 and otoacoustic emissions have since been shown to arise by a number of different cellular...
- Equal-loudness contours
- AudiometryAudiometryAudiometry is the testing of hearing ability, involving thresholds and differing frequencies. Typically, audiometric tests determine a subject's hearing levels with the help of an audiometer, but may also measure ability to discriminate between different sound intensities, recognize pitch, or...
- HyperacusisHyperacusisHyperacusis is a health condition characterized by an over-sensitivity to certain frequency ranges of sound...