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Chapter 12: auditory System: structure and role

Lincoln Gray, Ph.D., department of interaction Sciences and also Disorders, James Madison university Reviewed and revised 07 Oct 2020
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The crucial structure in the vertebrate auditory and vestibular solution is the hair cell. The hair cell an initial appeared in fish as component of a long, thin range along the next of the body, sensing movements in the water. In higher vertebrates the internal fluid of the inside ear (not outside fluid together in fish) bathes the hair cells, yet these cells still sense movements in the bordering fluid. Number of specializations make person hair cells responsive to various creates of mechanical stimulation. Hair cells in the body organ of Corti in the cochlea of the ear respond come sound. Hair cell in the cristae ampullares in the semicircular ducts respond come angular acceleration (rotation of the head). Hair cells in the maculae of the saccule and also the utricle respond to direct acceleration (gravity). (See the thing on Vestibular System: Structure and also Function). The fluid, termed endolymph, which surrounding the hair cell is rich in potassium. This proactively maintained ionic imbalance provides an power store, i m sorry is provided to create neural action potentials once the hair cells are moved. Tight junctions in between hair cells and the nearby supporting cells type a obstacle between endolymph and also perilymph that maintains the ionic imbalance.

Figure 12.1 illustrates the procedure of mechanically transduction in ~ the tips of the hair cabinet cilia. Cilia arise from the apical surface of hair cells. This cilia boost in size along a continual axis. There room tiny thread-like connections from the pointer of each cilium to a non-specific cation channel top top the side of the taller neighboring cilium. The pointer links function like a string associated to a hinged hatch. When the cilia space bent toward the tallest one, the networks are opened, much like a catch door. Opening these channels allows an influx of potassium, which in turns opens up calcium networks that initiates the receptor potential. This device transduces mechanical energy into neural impulses. One inward K+ existing depolarizes the cell, and also opens voltage-dependent calcium channels. This in turn causes neurotransmitter relax at the basal finish of the hair cell, eliciting an activity potential in the dendrites that the VIIIth cranial nerve.

Press the "play" switch to watch the mechanical-to-electrical transduction. Hair cells typically have a little influx of K+ at rest, so over there is part baseline task in the afferent neurons. Bending the cilia towards the tallest one opens up the potassium channels and increases afferent activity. Bending the cilia in the contrary direction closes the channels and decreases afferent activity. Bending the cilia to the side has no impact on spontaneous neural activity.

12.2 Sound: Intensity, Frequency, Outer and Middle Ear Mechanisms, Impedance matching by Area and Lever Ratios

The auditory system alters a wide selection of weak mechanically signals into a complicated series of electrical signals in the central nervous system. Sound is a series of pressure alters in the air. Sounds often vary in frequency and also intensity end time. Humans have the right to detect sound that reason movements only slightly better than those of Brownian movement. Obviously, if us heard that ceaseless (except at absolute zero) movement of waiting molecules we would have no silence.


number 12.2 depicts these alternate compression and also rarefaction (pressure) waves impinging top top the ear. The pinna and also external auditory meatus collect these waves, readjust them slightly, and also direct them come the tympanic membrane. The resulting activities of the eardrum room transmitted v the three middle-ear ossicles (malleus, incus and stapes) come the fluid of the inside ear. The footplate of the stapes fits tightly into the oval window of the bony cochlea. The inside ear is filled v fluid. Because fluid is incompressible, together the stapes move in and also out there requirements to it is in a compensatory activity in the contrary direction. An alert that the round home window membrane, located beneath the oval window, move in opposing direction.

Because the tympanic membrane has actually a larger area than the stapes footplate over there is a hydraulic amplification of the sound pressure. Likewise because the eight of the malleus come which the tympanic membrane is attached is much longer than the eight of the incus come which the stapes is attached, over there is a slim amplification that the sound press by a bar action. These 2 impedance matching mechanisms efficiently transmit air-born sound into the liquid of the inside ear. If the middle-ear device (ear drum and ossicles) were absent, then sound getting to the oval and also round home windows would be greatly reflected.

12.3 The Cochlea: three scalae, basilar membrane, activity of hair cells


The cochlea is a lengthy coiled tube, v three channels divided by two thin membranes. The top tube is the scala vestibuli, i beg your pardon is linked to the oval window. The bottom tube is the scala tympani, i m sorry is connected to the ring window. The middle tube is the scala media, which contains the Organ that Corti. The body organ of Corti sits on the basilar membrane, which develops the division between the scalae media and tympani.

You are watching: Sensory transduction in the auditory system is much like transduction of

Figure 12.3 illustrates a cross ar through the cochlea. The three scalae (vestibuli, media, tympani) are reduced in several areas as castle spiral roughly a central core. The cochlea provides 2-1/2 transforms in the person (hence the 5 cuts in midline cross section). The strict coiled shape provides the cochlea that is name, which means snail in Greek (as in conch shell). As described in Tonotopic Organization, high frequency sound stimulate the base of the cochlea, whereas short frequency sounds stimulate the apex. This function is illustrated in the animation of number 12.3 v neural impulses (having colors native red to blue representing short to high frequencies, respectively) emerging from various turns that the cochlea. The activity in number 12.3 would be created by white noise that has actually all frequencies at equal amplitudes. The moving dots are meant to suggest afferent action potentials. Low frequencies room transduced at the apex of the cochlea and are represented by red dots. High frequencies room transduced at base of the cochlea and also are stood for by blue dots. A consequence of this arrangement is that low frequencies are uncovered in the main core that the cochlear nerve, with high frequencies on the outside.


Figure 12.4 illustrates one cross ar of the cochlea. Sound waves reason the oval and also round windows at the basic of the cochlea to relocate in opposite directions (See number 12.2). This causes the basilar membrane to it is in displaced and also starts a traveling tide that sweeps indigenous the base toward the apex of the cochlea (See number 12.7). The travel wave increases in amplitude together it moves, and also reaches a peak at a place that is straight related to the frequency the the sound. The illustration reflects a section of the cochlea that is moving in response to sound.

Figure 12.5 illustrates a higher magnification that the body organ of Corti. The travel wave reasons the basilar membrane and also hence the body organ of Corti to move up and also down. The organ of Corti has a central stiffening buttress created by paired shaft cells. Hair cell protrude from the peak of the body organ of Corti. A tectorial (roof) membrane is hosted in place by a hinge-like system on the next of the organ of Corti and also floats above the hair cells. As the basilar and also tectorial membranes relocate up and down through the traveling wave, the hinge mechanism causes the tectorial membrane to move laterally over the hair cells. This lateral shearing movement bends the cilia atop the hair cells, traction on the fine guideline links, and opens the trap-door networks (See number 12.1). The influx of potassium and then calcium reasons neurotransmitter release, i m sorry in turn causes an EPSP that initiates activity potentials in the afferents the the VIIIth cranial nerve. Many of the afferent dendrites make synaptic contacts with the within hair cells.


Figure 12.6 looks under on the body organ of Corti. There space two types of hair cells, inner and outer. Over there is one heat of within hair cells and also three rows of outer hair cells. Most of the afferent dendrites synapse on inside hair cells. Many of efferent axons synapse top top the outer hair cells. The external hair cells room active. They move in an answer to sound and amplify the traveling wave. The external hair cells likewise produce sound that deserve to be recognize in the outside auditory meatus with sensitive microphones. These internally generated sounds, termed otoacoustic emissions, are now used to display newborns because that hearing loss. Number 12.6 reflects an immunofluorescent entirety mount photo of a neonatal computer mouse cochlea mirroring the 3 rows of outer hair cells and also the single row of within hair cells. The mature person cochlea would certainly look around the same. Superimposed schematically-depicted neurons display the usual pattern of afferent connections. Ninety-five percent of the VIIIth nerve afferents synapse on inner hair cells. Every inner hair cell provides synaptic connections with countless afferents. Each afferent connects to just one inside hair cell. Around five percent of the afferents synapse on outer hair cells. These afferents travel a considerable distance along the basilar membrane away from their ganglion cell to synapse top top multiple external hair cells. Less than one percent (~0.5%) of the afferents synapse ~ above multiple inner hair cells. The listed below micrograph is courtesy the Dr. Douglas Cotanche, room of Otolaryngology, Children"s Hospital the Boston, Harvard clinical School. Reprinted with permission.


Physical characteristics of the basilar membrane reason different frequencies to with maximum amplitudes at various positions. Lot as ~ above a piano, high frequencies room at one end and low frequencies at the other. High frequencies space transduced in ~ the base of the cochlea whereas short frequencies room transduced in ~ the apex. Number 12.7 illustrates the method in i m sorry the cochlea acts as a frequency analyzer. The cochlea codes the pitch of a sound by the ar of maximal vibration. Note the place of the traveling tide at different frequencies. (Beware! the may at first seem backwards that low frequencies space not connected with the base.) select different frequencies by transforming the dial. If audio top top your computer system is enabled, you will certainly hear the sound friend selected. Listening loss in ~ high frequencies is common. The median loss of hearing in American males is about a bicycle per second per day (starting at around age 20, therefore a 50-year old would most likely have an obstacle hearing end 10 kHz). If friend can"t hear the high frequencies, it may be as result of the speakers on your computer, but it is always worth thinking about hearing preservation.

As you listen to these sounds, note that the high frequencies it seems to be ~ strangely similar. Think around cochlear-implant patients. This patients have actually lost hair-cell function. Their auditory nerve is stimulated by a collection of implanted electrodes. The implant can only be placed in the base of the cochlea, since it is surgically impossible to thread the well wires much more than around 2/3 of a turn. Thus, cochlear implant patients probably experience something favor high frequency sounds.

12.5 The variety of sound to Which we Respond; Neural Tuning Curves

figure 12.8 reflects the selection of frequencies and also intensities that sound to which the human being auditory device responds. Our absolute threshold, the minimum level of sound the we have the right to detect, is strong dependent on frequency. In ~ the level that pain, sound level are around six assignment of magnitude above the minimal audible threshold. Sound push level (SPL) is measured in decibels (dB). Decibels space a logarithmic scale, v each 6 dB boost indicating a doubling of intensity. The perceived loudness the a sound is concerned its intensity. Sound frequencies space measured in Hertz (Hz), or cycles every second. Normally, we hear sounds as low as 20 Hz and as high as 20,000 Hz. The frequency the a sound is linked with the pitch. Hear is finest at around 3-4 kHz. Hearing sensitivity to reduce at greater and lower frequencies, but much more so at greater than reduced frequencies. High-frequency hear is frequently lost together we age.


The neural code in the central auditory system is complex. Tonotopic company is preserved throughout the auditory system. Tonotopic organization way that cells responsive to various frequencies are uncovered in different places at each level of the main auditory system, and also that there is a standard (logarithmic) relationship between this position and frequency. Each cell has actually a characteristic frequency (CF). The CF is the frequency come which the cabinet is maximally responsive. A cell will usually respond to various other frequencies, but only at better intensities. The neural tuning curve is a plot of the amplitude of sounds at various frequencies crucial to elicit a solution from a central auditory neuron. The tuning curves for several various neurons are superimposed on the audibility curves in figure 12.8. The depicted neurons have actually CFs that differ from low to high frequencies (and are displayed with red come blue colors, respectively). If we recorded from every auditory neurons, we would basically to fill the area within the audibility curves. When sounds are soft they will stimulate only those few neurons with that CF, and thus neural activity will be confined come one collection of yarn or cell at one specific place. As sounds gain louder they stimulate various other neurons, and the area of activity will increase.

Graduate Students buy it Baum, heath Turner, Nadeeka Dias, Deepna Thakkar, Natalie Sirisaengtaksin and also Jonathan Flynn that the Neuroscience Graduate regimen at 2175forals.comealth Houston further describe the structures, functions and pathways of the auditory mechanism in an animated video clip "The journey of Sound".

Test Your expertise


High frequencies room transduced

A. At the apex the the cochlea

B. In ~ the base of the cochlea

C. Transparent the cochlea

D. Through vibrations that the stapes

E. In ~ the remarkable temporal gyrus


High frequencies room transduced

A. At the apex that the cochlea This answer is INCORRECT.

It may seem "backwards" however although the Cochlear duct seems to acquire smaller toward the apex, the basilar membrane in reality gets wider.

B. At the base of the cochlea

C. Throughout the cochlea

D. By vibrations that the stapes

E. In ~ the superior temporal gyrus


High frequencies are transduced

A. At the apex the the cochlea

B. At the base of the cochlea This answer is CORRECT!

C. Throughout the cochlea

D. Through vibrations that the stapes

E. In ~ the premium temporal gyrus


High frequencies room transduced

A. At the apex of the cochlea

B. In ~ the basic of the cochlea

C. Throughout the cochlea This price is INCORRECT.

High frequencies perform not travel far along the basilar membrane. (As one aside, short frequencies traverse the length of the Cochlea, and also hence cause the most damage if they room sufficiently loud.)

D. Through vibrations of the stapes

E. In ~ the exceptional temporal gyrus


High frequencies space transduced

A. In ~ the apex that the cochlea

B. At the base of the cochlea

C. Transparent the cochlea

D. By vibrations the the stapes This answer is INCORRECT.

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Sound istransfer come the liquid of the within ear through vibrations the the tympanic membrane, malleus, incus and also stapes. Transduction, the readjust from mechanical energy to neural impulses, takes ar in the hair cells, especially through potassium networks at the tips of the stereocilia.

E. At the remarkable temporal gyrus


High frequencies room transduced

A. At the apex that the cochlea

B. In ~ the base of the cochlea

C. Transparent the cochlea

D. Through vibrations the the stapes

E. In ~ the superior temporal gyrus This answer is INCORRECT.

Auditory afferents at some point reach the primary auditory cortex in Heschel"s gyrus within insular cortex, and this area is tonotopically organized. Stimulation the this area leader to mindful awareness of the sound, but the transduction from mechanical vibrations come neural activity occurs in the inside ear.