Stapedius reflex
The stapedius reflex (also acoustic reflex or middle ear reflex ) describes an involuntary reaction ( reflex ) within the ossicles in the middle ear that protects the inner ear from damage caused by loud sound pressure levels .
There are two small muscles in the middle ear :
- The tensor tympani muscle attaches to the hammer and tightens the eardrum .
- The stapedius muscle attaches to the stapes and tilts the stapes plate in the oval window.
In humans, only the stapedius muscle is involved in the stapedius reflex ; it contracts as a result of the reflex that is triggered by loud sound. In many animals, the tensor tympani muscle is also involved in the reflex (here it makes more sense to speak of the middle ear reflex, for example).
When the reflex is triggered, the acousto-mechanical coupling of the eardrum to the inner ear is reduced, the entire sound pressure is no longer transmitted to the inner ear, but part of it is reflected on the eardrum. In this way, the hearing can protect itself within certain limits from damage caused by excessively high sound levels. The afferent leg of the reflex is formed by the auditory nerve ( cochlear nerve ), the efferent leg by the stapedius nerve .
A sound stimulus given in only one ear triggers the reflex in both ears.
The stapedius reflex starts at sound levels of 70 to 95 dB (stapedius reflex threshold) and is effective about 50 ms after the sound begins.
Stapedius reflex threshold measurement
By measuring the impedance of the external auditory canal , the use of the stapedius reflex can be observed and this can be used for diagnostic purposes.
For this purpose, the probe of a measuring device ( tympanometer ) is inserted airtight into the external auditory canal . The reflex is triggered by tones of variable volume and can be measured at different frequencies. The stapedius reflex increases the acoustic resistance ( acoustic impedance ) of the ossicular chain including the eardrum. This means that more sound is reflected from the eardrum, which can be measured using a microphone in the measuring device. The recorded information is output as impedance or compliance of the eardrum and displayed graphically.
In the case of conductive hearing loss , the reflex threshold is increased depending on the extent of the hearing loss or the reflex cannot be triggered at all. In the case of sensorineural hearing loss, the reflex threshold can be normal or even reduced as a result of recruitment . This is called Metz recruitment after the first person to describe it. In the case of neural hearing loss (damage to the auditory nerve), the reflex is usually not detectable or the reflex threshold is increased. The rapid fatigue of the reflex ( reflex decay ) is also typical of neural hearing loss .
In the case of paralysis of the facial nerve (facial nerve), measurement of the stapedius reflex threshold can be used for topical diagnostics, i.e. to localize the damage. If the damage is in the peripheral part of the facial nerve after the origin of the stapedius nerve, the reflex can be detected despite the paralysis; if the damage is central to the origin of the stapedius nerve, the stapedius reflex is missing. The stapedius reflex can also be absent in diseases of the hindbrain , where the facial nerve originates.
After an operation for otosclerosis (stapesplasty), in which the tendon of the stapedius muscle is severed, no stapedius reflex can be detected either. Under direct vision, however, a movement of the tendon can be seen with sound. Under certain circumstances, this can also be demonstrated in the case of a large perforation of the eardrum (for example, in the case of chronic mucosal expansion ).
The measurement of the stapedius reflex threshold does not require active cooperation on the part of the patient, so this method can also be used with small children. This procedure can also provide an indication in cases where it is suspected that the patient is only simulating a hearing loss.
Trivia
Mercedes-Benz uses the reflex in a technology known as pre-safe, in that the reflex is triggered by a noise shortly before an accident. The reflex triggering noise should be significantly quieter than the noise of the accident and thus protect the inner ear from the latter.
See also
Individual evidence
- ↑ Djupesland G .: Electromyography of the tympanic muscles in man. Int. Audiol. 4, 33 (1965)
- ↑ a b Notes on the Acoustic Middle Ear Reflex. American academy of audiology
- ↑ K. Chiveralls; R. Fitzsimons: Stapedial reflex action in normal subjects. Brit. J. Audiol. 7, 105 (1973)
- ^ Metz O .: Threshold of reflex contractions of muscles of the middle ear and recruitment of loudness. Arch. Otolaryngol. 55, 536 (1952)
- ↑ PRE-SAFE system. In: mercedes-benz.com
