Cochlea

The cochlea (Latin cochlea of Greek Κοχλιάριον Kochliárion, kóchlos of κόχλος "Snail, snail shell ") is a part of the inner ear and represents the receptive field for the auditory perception out its vibration mechanical properties, for their enlightenment Georg von Békésy 1961 received the Nobel Prize, contribute to resolution at different pitches; Also, the various types of hair cells and their neuronal interconnection.

• 3.1 pitch
• 3.2 Volume and Tone
• 3.3 Signal processing of hearing
• 3.4 Technical application of inner ear effects

Structure of the cochlea

The cochlea is a snail- shaped cavity in the temporal bone with two and a half turns. It is surrounded by bone material which is the hardest material in the human body by the teeth. As part of deafness associated with congenital malformation ( Mondini dysplasia ), the worm can be reduced to one and a half turns.

The bony axis of the cochlea is called the modiolus ( worm gear ). He communicates with the internal auditory canal connected and contains the root of the Höranteils of the vestibulocochlear nerve ( VIII cranial nerve).

Interior is divided into three superposed fluid-filled passages the cochlea. they are called

• Scala vestibuli (atrial stairs; Scala, Latin: stairs, conductor; vestibule, Latin: atrium)
• Scala media or cochlear duct ( worm gear) and
• Scala tympani ( scala tympani ).

The base of the cochlea adjacent to the middle ear with the ossicles. The footplate of the stapes in the oval window ( fenestra vestibuli or oval window ) is movably fitted. Behind the oval window is the Scala vestibuli. This is at the top of the screw (Latin apex) on the helicotrema (Greek: Schneckenloch ) connected to the scala tympani. The latter is adjacent to the base of the round window ( fenestra cochleae ), which ( tympanic membrane secundaria ) is closed to the middle ear through the free-swinging secondary tympanic membrane. A print of the ossicles to the oval window runs as a traveling wave on the Scala vestibuli toward the tip of the screw and lead to the displacement of the basilar membrane, which the registered pressure on the scala tympani transmits. About the round window, this pressure can be compensated.

The scala media is given by the Reissner membrane ( after Ernst Reissner ) from the scala vestibuli and the osseous spiral lamina (Latin: spirally Bone Leaf ) tympani separated and the basilar membrane (lamina or basilar membrane basilar ) of La Scala. The scala vestibuli and the scala tympani are filled with perilymph, which exchanges over the helicotrema between the two transitions. The scala media contains endolymph. Both liquids differ in their composition: The perilymph is similar to the extracellular milieu, whereas the endolymph has a high potassium concentration. In this it resembles the cytosol.

From the sound to the nerve impulse

On the basilar membrane, the organ of Corti is located with four rows of hair cells, which have different properties:

• The outer hair cells ( three rows ) are used to amplify the sound traveling waves within the cochlea (so-called cochlear amplifier). You work as an acoustic pre-filter, which thanks to the ability of the outer hair cells directly controlled by the frequency spectrum length adjustment is also a fast adaptive filter.
• The inner hair cells ( one row) afford the conversion of mechanical vibrations into nerve impulses (called transduction ), which are transmitted to the brain.

The conversion of sound into nerve impulses depends mainly on the following effects:

• Electrical and mechanical vibration properties of the cell body and hair bundle of the outer hair cells of constant gradient along the screw channel
• Implementation of the mechanical excitation of the inner hair cells into nerve impulses

Vibration Mechanical properties of the inner ear

If the sound into the inner ear one, it produces there a wave that travels through the inner ear. One speaks of the traveling wave. You articulated by the tectorial membrane, the sensory hairs of the outer, but not the inner hair cells (they have, as opposed to the outer, no contact with the tectorial membrane ) from.

Basilar membrane and cochlear canal act here as a mechanical resonator. Since the width of the basilar membrane of the oval window to helicotrema increases towards the expense of osseous spiral lamina, the diameter of the bone screw channel, however, decreases the mechanical properties ( mass coating, stiffness, damping), and thus the vibration characteristics of the system vary depending on the distance to helicotrema. In the vicinity of the oval window, the basilar membrane is stiff and therefore resonant with high frequencies near the helicotrema it is flexible and resonant with low frequencies. Conversely, the liquid is in the screw channel through their inertia for high frequencies stiff and increasingly for low yielding. With decreasing frequency waves can penetrate deeper into the screw channel. Before a wave of a certain frequency is applied to the place where it is resonant, it causes no big mass forces, but their energy is hydraulically transported by opposite longitudinal movements of the liquid column inside. For spectral selectivity contributes to that behind the location of the resonance, the membrane more forgiveness, because it is wider, and the liquid column stiffer because narrower, so that the shaft hardly spreads further ( a short circuit in terms of the transmission line theory - the lowest frequencies are the helicotrema short-circuited in order to avoid damage).

The outer hair cells

The outer hair cells react to even a slight deflection of their hair bundles with an active length change their entire cell body. For this purpose, the outer hair cells by a particular membrane protein capable Prestin. This is a contractile protein in the plasma membrane, the shrinking of potential- dependent. Transgenic mice lacking the gene for Prestin, have a greatly reduced hearing sensitivity. This is regarded as proof that the prestin motors strengthen in the cell membrane of the outer hair cells of the sound within the inner ear and increase the frequency selectivity.

The outer hair cells affect the mechanical oscillations of the basilar membrane model system the screw channel. At the resonance point, the vibrations are amplified and thereby stimulates the inner hair cells stronger. Beyond the point of resonance oscillations are strongly damped, the corresponding frequency spreads hardly further. Thereby, the frequency selectivity of the inner ear is larger, the separation of sounds or of human speech into individual audio frequencies is facilitated ( " cochlear amplifier ").

Another effect is that high frequencies have its resonance point in the vicinity of the oval window, do not cause any excitation of the inner hair cells for low frequencies. Low frequencies that produce an excitation maximum only near the Heliocotremas, excite the other hand also responsible for high frequencies hair cells.

The inner hair cells

The individual frequencies of dismantled in this way sound irritate specialized in the respective frequencies of inner hair cells. The stimulus triggers an electrical signal in the hair cells ( mechano- electrical transduction ). These provide a chemical signal ( transmitter glutamate ) to a auditory nerve ( transformation ), each auditory nerve fiber is responsible for a single frequency. The auditory nerve fibers respond electrically ( action potential ) and extend to the brain stem. In this way, the tone frequencies can be transmitted separately and electrically to the brain.

The excitement of a hair cell depends on the previous history. If a mechanical excitation after a certain resting phase, so "fires" the hair cell particularly intense. If the pickup made ​​a certain amount of time, so does the number of nerve impulses (known as adaptation). Only after a certain time, the original high excitation poor nerve impulse number is reached again. This situation is modeled including in psychoacoustic models with digital signal processors, which are used for the audio data compression in the sound recording.

Innervation of the hair cells

The hair cells are supplied from the afferent and efferent nerves. While the afferent fibers come from the spiral ganglion, the efferent fibers come from the olive pits ( tractus olivocochlearis or Rasmussen bundle ).

The spiral ganglion consists of over 30,000 bipolar neurons. More than 90 % of which are myelinated neurons (type I), which are in contact with the inner hair cells. The smaller, unmyelinated neurons (type II ) supply the outer hair cells. Both types send impulses to the Cochleariskerne in the medulla oblongata.

The efferent fibers first run with the vestibular nerve in the internal auditory canal, then branch but from about the Oort anastomosis for Cochlearisteil of the vestibulocochlear nerve. The physiological function is largely unclear.

The inner hair cells are supplied by the radial afferent fibers and the lateral efferent fibers, the outer hair cells of the spiral afferent and efferent fibers of the media.

Inner hair cells: All of the spiral ganglion -derived type I neurons are connected only to the inner hair cells with synapses. Their dendrites form the radial afferent system. The axons collect in the modiolus and run with the cochlear nerve to the Cochleariskernen. Each inner hair cell has about ten afferent fibers contact, this figure is in the range of best hearing is much higher. As a neurotransmitter glutamate is assumed for these synapses.

Part of the efferent fibers ( lateral efferent system ) obtained contact with the heads of the synapses afferent fibers to the inner hair cells and form synapses with them, so these fibers have no direct contact with hair cells.

Outer hair cells: the outer hair cells have only a relatively small supply of afferent fibers. Only in the apical turn there is a greater supply of afferent fibers. The outer hair cells form synapses exclusively to the unmyelinated type II fibers of the spiral ganglion. These fibers run in the bottom of Corti tunnel of the outer hair cells towards the modiolus, they accompany the outer hair cells of spiral ( spiral afferent system ), and each individual fiber has multiple cells synaptic contact.

The medial efferent fibers run free as radiating fibers through the tunnel of Corti tunnel and form the bases of the outer hair cell synapses.

Influences on the acoustic perception

The manner, such as sound signals are converted into nerve impulses, and where nerve impulses in the inner ear occur influences the acoustic perception.

Pitch

The pitch that is perceived in sound reinforcement with a certain frequency depends closely with the place on the basilar membrane together, an excitation maximum is at the at this frequency. As the basilar membrane at the oval window is narrow and thick, its natural frequency is high here (at low amplitude). Continue towards helicotrema where it is wider and thinner, it vibrates at a lower frequency (with larger amplitude ).

In animal studies could determine the location on the basilar membrane, in which certain frequency for a maximum excitation of the inner hair cells is achieved. Hence the location of the excitation maximum was derived in humans based on physiological comparisons. With the help of listening tests can in turn be determined which frequency leads to what pitch sensation. They found a linear relationship between the position of maximum excitation on the basilar membrane (calculated as distance from helicotrema ) and the perceived pitch.

The human ear is able to detect frequency differences of only 3 Hz; The audible range is approximately from 20 Hz to 18 kHz, wherein the hearing threshold is different for different frequencies and at 2-4 kHz is the lowest; biographical hear mainly the high frequencies progressively worse ( presbycusis ). Local principle: If the basilar membrane is excited with a certain frequency, it will oscillate there most where they can best vibrate with this frequency. For a frequency therefore only a few inner hair cells are responsible (which is also a particularly low threshold have for this frequency ); due to the interconnection in the auditory pathway are also in the primary auditory cortex neurons responsible only for certain specific frequencies ( tonotopy ). This oscillation maximum of the basilar membrane is outlined in addition to this passive component is in particular because sharp because the Outer hair cells are excited at the site of the vibration maximum and the vibration by contracting about a thousand fold increase ( cochlear amplifier ), so that only in a very small area of ​​the Inner hair cells are very strongly excited. Still comes to this " mechanically active " component, a neural: lateral inhibition along the auditory pathway (mainly in the cochlear spiral ganglion ), ie, strongly excited neurons inhibit neighboring easily excited neurons ( which would be transmitted to the directly neighboring frequency range). This contrast is used for noise reduction.

For frequencies above 4 kHz ( the well still be in the field of languages ​​), a problem arises from the fact that the inner hair cells "only" a maximum of about 4000 times can fire per second, a vibration of 4.2 kHz so can not 1:1 to the cochlear nerve are passed. Now, the human ear does but the principle of phase coding advantage: Lying between two sensor potentials of a hair cell about once a 6 ms, once 15 and once 9, the central nervous system calculates the greatest common divisor and receives 3 ms as the distance between the wave crests.

Volume and tone

The number of nerve impulses generated by a total frequency band is a measure of the perceived loudness of a sound signal strength. The number of nerve impulses emitted in turn depends on the strength of the excitation of the inner hair cells and thus on the vibration characteristics of the basilar membrane.

The excitation pattern that causes a certain sound, you can understand the basis of masking experiments. Is the presence of a tone, a second tone quieter imperceptible, so this indicates that the first tone has the nerve cells that are responsible for the perception of the second sound, already much more excited than can the second tone.

Due to the oscillatory behavior of the basilar membrane rain single tones even nerve cells that are above their frequency, ie belong to frequencies that are not included in the acoustic signal. Wherein sound signals with a flat frequency response without nerve cells are energized outside the frequency response of the sound signal. This means that individual sounds (or sound signals with strong tonal shares ) will be perceived louder than band sound signals with the same sound.

On the other hand, the timing of a sound signal influences the number of nerve impulses emitted. Sets ( in a frequency range ) a sound signal after prolonged rest, which nerve cells fire particularly strong. For longer periods of sound, the number of nerve impulses decreases again to an average value.

This results in sound signals with sudden sound inserts ( eg hammers ) are perceived as much louder than uniform sound signals with the same sound.

As the volume and the perceived sound is influenced by this, tonal shares and sound inserts thus determine the sound impression much stronger than it would yield the physical spectrum of a sound signal.

Signal processing of the auditory

The position of the maximum excitation on the basilar membrane not only determines the perceived pitch (see above), but also which signal components are evaluated jointly by the ear.

For this, the hearing shall notify the basilar membrane in approximately 24 sections of equal length, so-called frequency groups. The nerve impulses from one frequency group are evaluated together in order to determine from volume, tone and direction of the sound signal in this frequency range.

The width of a frequency group is about 100 Hz at frequencies up to 500 Hz and a minor third above 500 Hz (this corresponds to about 1 Bark or Mel 100 )

The healthy person can usually perceive frequencies from 20-18000 Hz. The frequency range decreases with age.

Technical application of inner ear effects

Be Exploited inner ear effects in data reduction techniques such as MP3.

Here, just as in hearing, signal ranges analyzed in frequency groups. Signal ranges (ie due to the inner ear mechanism) is not audible due to the masking effects, are removed from the signal, or transmitted with a lower quality. This reduces the amount of data, but a difference from the original signal is not perceptible to humans.

The healthy ear has a frequency discrimination at 1 kHz of about / - 3 Hz. If there is a hearing disorder, the frequency discrimination may be reduced depending on the type and extent of hearing impairment.