*

*
*

*

Chapter 12: hear System: structure and duty

Lincoln Gray, Ph.D., department of interaction Sciences and Disorders, James Madison university Reviewed and also revised 07 Oct 2020
*
*

The crucial structure in the vertebrate auditory and vestibular systems is the hair cell. The hair cell first appeared in fish as part of a long, thin range along the side of the body, sensing movements in the water. In greater vertebrates the internal liquid of the inner ear (not outside fluid together in fish) bathes the hair cells, yet these cell still sense motions in the bordering fluid. Numerous specializations make human being hair cell responsive come various develops of mechanically stimulation. Hair cell in the organ of Corti in the cochlea the the ear respond come sound. Hair cells in the cristae ampullares in the semicircular ducts respond come angular acceleration (rotation that 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 Function). The fluid, termed endolymph, which surrounds the hair cells is wealthy in potassium. This proactively maintained ionic imbalance provides an energy store, i beg your pardon is provided to create neural action potentials as soon as the hair cells space moved. Chop junctions between hair cells and the surrounding supporting cells type a barrier between endolymph and perilymph that maintains the ionic imbalance.

Figure 12.1 illustrates the procedure of mechanically transduction at the tips of the hair cell cilia. Cilia emerge from the apical surface of hair cells. This cilia rise in size along a continuous axis. There room tiny thread-like relations from the tip of every cilium to a non-specific cation channel on the next of the taller neighboring cilium. The guideline links role like a string connected to a hinged hatch. When the cilia space bent toward the tallest one, the networks are opened, much like a catch door. Opened these channels enables an influx of potassium, i beg your pardon in turns opens up calcium networks that initiates the receptor potential. This system transduces mechanical energy into neural impulses. An inward K+ present depolarizes the cell, and opens voltage-dependent calcium channels. This in turn reasons neurotransmitter relax at the basal end of the hair cell, eliciting an activity potential in the dendrites of the VIIIth cranial nerve.

Press the "play" button to view the mechanical-to-electrical transduction. Hair cells generally have a small influx the K+ at rest, so there is some baseline task in the afferent neurons. Bending the cilia toward the tallest one opens up the potassium channels and also increases afferent activity. Bending the cilia in opposing direction closes the channels and also decreases afferent activity. Bending the cilia come the side has actually no effect on voluntary neural activity.

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

The auditory system transforms a wide variety of weak mechanical signals right into a facility series of electric signals in the central nervous system. Sound is a series of pressure transforms 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 higher than those of Brownian movement. Obviously, if us heard that ceaseless (except at pure zero) movement of waiting molecules us would have no silence.


figure 12.2 depicts these alternate compression and rarefaction (pressure) tide impinging ~ above the ear. The pinna and also external listening meatus collection these waves, adjust them slightly, and direct them come the tympanic membrane. The resulting motions of the eardrum room transmitted with the 3 middle-ear ossicles (malleus, incus and also stapes) to the fluid of the inside ear. The footplate the the stapes fits tightly into the oval home window of the bony cochlea. The inside ear is filled through fluid. Due to the fact that fluid is incompressible, as the stapes moves in and also out there demands to be a compensatory motion in the opposite direction. Notification that the round home window membrane, situated beneath the oval window, move in the contrary direction.

Because the tympanic membrane has actually a bigger area 보다 the stapes footplate over there is a hydraulic amplification the the sound pressure. Additionally because the eight of the malleus come which the tympanic membrane is fastened is much longer than the eight of the incus to which the stapes is attached, over there is a slim amplification that the sound push by a lever action. These two impedance matching mechanisms successfully transmit air-born sound into the fluid of the inside ear. If the middle-ear apparatus (ear drum and also ossicles) to be absent, then sound getting to the oval and round windows would be mainly reflected.

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


The cochlea is a long coiled tube, through three channels divided by 2 thin membranes. The optimal tube is the scala vestibuli, i m sorry is connected to the oval window. The bottom pipe is the scala tympani, which is associated to the ring window. The middle tube is the scala media, which has the Organ that Corti. The organ of Corti sit on the basilar membrane, which develops the department between the scalae media and also tympani.

You are watching: As sound levels increase in the spiral organ of corti

Figure 12.3 illustrates a cross ar through the cochlea. The 3 scalae (vestibuli, media, tympani) are reduced in several locations as they spiral around a central core. The cochlea renders 2-1/2 transforms in the human (hence the 5 cuts in midline cross section). The tightly coiled shape provides the cochlea that is name, which method snail in Greek (as in conch shell). As defined in Tonotopic Organization, high frequency sound stimulate the basic of the cochlea, whereas short frequency sound stimulate the apex. This function is illustrated in the animation of figure 12.3 v neural impulses (having colors indigenous red come blue representing low to high frequencies, respectively) emerging from different turns of the cochlea. The task in figure 12.3 would be produced by white noise that has actually all frequencies at same amplitudes. The relocating dots room meant to suggest afferent activity potentials. Low frequencies are transduced at the apex the the cochlea and are represented by red dots. High frequencies are transduced at basic of the cochlea and are represented by blue dots. A an effect of this plan is that low frequencies are discovered in the main core of the cochlear nerve, with high frequencies top top the outside.


Figure 12.4 illustrates one cross ar of the cochlea. Sound waves reason the oval and also round home windows at the basic of the cochlea to move in opposite direction (See number 12.2). This reasons the basilar membrane to be displaced and also starts a traveling tide that sweeps native the base toward the apex the the cochlea (See number 12.7). The traveling wave boosts in amplitude together it moves, and reaches a top at a place that is straight related come the frequency the the sound. The illustration reflects a ar of the cochlea that is moving in an answer to sound.

Figure 12.5 illustrates a greater magnification of the body organ of Corti. The traveling wave reasons the basilar membrane and also hence the organ of Corti to relocate up and down. The organ of Corti has actually a central stiffening buttress created by paired tower cells. Hair cell protrude native the top of the organ of Corti. A tectorial (roof) membrane is hosted in ar by a hinge-like system on the next of the body organ of Corti and also floats above the hair cells. Together the basilar and also tectorial membranes move up and down through the traveling wave, the hinge mechanism causes the tectorial membrane to move laterally end the hair cells. This lateral shearing activity bends the cilia atop the hair cells, traction on the fine tip links, and opens the trap-door channels (See figure 12.1). The influx of potassium and also then calcium reasons neurotransmitter release, i m sorry in turn reasons an EPSP the initiates action potentials in the afferents the the VIIIth cranial nerve. Many of the afferent dendrites do synaptic contacts v the within hair cells.


Figure 12.6 looks under on the body organ of Corti. There space two varieties of hair cells, inner and also outer. There is one row of inner hair cells and also three rows of outer hair cells. Most of the afferent dendrites synapse on within hair cells. Many of efferent axons synapse ~ above the external hair cells. The external hair cells space active. They relocate in response to sound and also amplify the traveling wave. The external hair cells likewise produce sound that can be detect in the external auditory meatus through sensitive microphones. These internally generated sounds, termed otoacoustic emissions, are now used to display newborns because that hearing loss. Number 12.6 mirrors an immunofluorescent whole mount image of a neonatal mouse cochlea mirroring the 3 rows of external hair cells and also the single row of inside hair cells. The mature person cochlea would look approximately the same. Superimposed schematically-depicted neurons present the usual pattern the afferent connections. Ninety-five percent that the VIIIth nerve afferents synapse on inside hair cells. Each inner hair cell renders synaptic relations with countless afferents. Every afferent connect to just one within hair cell. About five percent the the afferents synapse on external hair cells. These afferents take trip a significant distance follow me the basilar membrane far from their ganglion cell to synapse top top multiple external hair cells. Much less than one percent (~0.5%) that the afferents synapse on multiple inner hair cells. The listed below micrograph is courtesy that Dr. Douglas Cotanche, room of Otolaryngology, Children"s Hospital the Boston, Harvard clinical School. Reprinted v permission.


Physical features of the basilar membrane reason different frequencies to reach maximum amplitudes at different positions. Lot as on a piano, high frequencies space at one end and low frequencies at the other. High frequencies room transduced in ~ the basic of the cochlea whereas low frequencies room transduced in ~ the apex. Figure 12.7 illustrates the way in which the cochlea acts as a frequency analyzer. The cochlea password the pitch of a sound by the location of maximal vibration. Keep in mind the place of the traveling tide at different frequencies. (Beware! that may originally seem backwards the low frequencies are not connected with the base.) choose different frequencies by turning the dial. If audio ~ above your computer system is enabled, you will certainly hear the sound friend selected. Listening loss at high frequencies is common. The typical loss of hear in American males is about a cycle per second per day (starting at around age 20, so a 50-year old would most likely have an obstacle hearing over 10 kHz). If you can"t hear the high frequencies, it might be as result of the speakers on her computer, yet it is constantly worth thinking around hearing preservation.

As you hear to these sounds, keep in mind that the high frequencies it seems to be ~ strangely similar. Think about cochlear-implant patients. This patients have lost hair-cell function. Your auditory nerve is created by a series of implanted electrodes. The implant can only be put in the basic of the cochlea, due to the fact that it is surgically difficult to thread the good wires more than around 2/3 that a turn. Thus, cochlear implant patients most likely experience something prefer high frequency sounds.

12.5 The variety of sounds to Which us Respond; Neural Tuning Curves

figure 12.8 shows the range of frequencies and also intensities the sound to which the person auditory system responds. Our absolute threshold, the minimum level of sound that we have the right to detect, is strong dependent top top frequency. In ~ the level the pain, sound level are about six assignment of magnitude above the minimal audible threshold. Sound push level (SPL) is measure up in decibels (dB). Decibels space a logarithmic scale, with each 6 dB boost indicating a doubling of intensity. The regarded loudness that a sound is pertained to its intensity. Sound frequencies space measured in Hertz (Hz), or cycles every second. Normally, us hear sounds as low as 20 Hz and as high as 20,000 Hz. The frequency of a sound is associated with that pitch. Listening is ideal at about 3-4 kHz. Listening sensitivity to reduce at higher and reduced frequencies, but an ext so at higher than lower frequencies. High-frequency listening is typically lost together we age.


The neural password in the central hear system is complex. Tonotopic organization is preserved throughout the listening system. Tonotopic organization method that cells responsive to different frequencies are discovered in different places at every level of the main auditory system, and that over there is a typical (logarithmic) relationship in between this position and also frequency. Each cell has actually a characteristic frequency (CF). The CF is the frequency to which the cell is maximally responsive. A cell will generally respond to other frequencies, yet only at better intensities. The neural tuning curve is a plot the the amplitude of sounds at various frequencies important to elicit a an answer from a main auditory neuron. The tuning curves for several various neurons are superimposed top top the audibility curve in figure 12.8. The portrayed neurons have CFs that differ from short to high frequencies (and are shown with red come blue colors, respectively). If we taped from every auditory neurons, we would certainly basically fill the area within the audibility curves. When sounds room soft they will certainly stimulate only those few neurons through that CF, and also thus neural task will be confined come one collection of fibers or cells at one specific place. As sounds gain louder castle stimulate various other neurons, and the area of activity will increase.

Graduate Students sarah Baum, heath Turner, Nadeeka Dias, Deepna Thakkar, Natalie Sirisaengtaksin and Jonathan Flynn that the Neuroscience Graduate routine at cg-tower.comealth Houston further define the structures, functions and also pathways that the auditory system in an animated video clip "The trip of Sound".

Test Your understanding


High frequencies space transduced

A. In ~ the apex of the cochlea

B. At the basic of the cochlea

C. Throughout the cochlea

D. Through vibrations that the stapes

E. At the superior temporal gyrus


High frequencies space transduced

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

It might seem "backwards" but although the Cochlear duct seems to acquire smaller towards the apex, the basilar membrane actually gets wider.

B. At the base of the cochlea

C. Throughout the cochlea

D. By vibrations the the stapes

E. In ~ the premium temporal gyrus


High frequencies room transduced

A. In ~ the apex that the cochlea

B. In ~ the base of the cochlea This answer is CORRECT!

C. Transparent the cochlea

D. Through vibrations of the stapes

E. At the remarkable temporal gyrus


High frequencies room transduced

A. In ~ the apex the the cochlea

B. At the basic of the cochlea

C. Throughout the cochlea This answer is INCORRECT.

High frequencies perform not travel much along the basilar membrane. (As an aside, low frequencies traverse the length of the Cochlea, and hence reason the most damages if they space sufficiently loud.)

D. By vibrations the the stapes

E. In ~ the superior temporal gyrus


High frequencies room transduced

A. In ~ the apex that the cochlea

B. In ~ the basic of the cochlea

C. Transparent the cochlea

D. By vibrations that the stapes This prize is INCORRECT.

See more: 1 Clove Minced Garlic Is Equal To How Much Is Two Garlic Cloves Minced ?

Sound is transmitted to the fluid of the inner ear with vibrations that the tympanic membrane, malleus, incus and stapes. Transduction, the adjust from mechanical energy to neural impulses, takes place in the hair cells, especially through potassium networks at the tips of the stereocilia.

E. In ~ the superior temporal gyrus


High frequencies room transduced

A. At the apex that the cochlea

B. At the base of the cochlea

C. Transparent the cochlea

D. By vibrations of the stapes

E. At the premium temporal gyrus This price is INCORRECT.

Auditory afferents eventually reach the main auditory cortex in Heschel"s gyrus in ~ insular cortex, and this area is tonotopically organized. Stimulation the this area leader to conscious awareness the the sound, however the transduction from mechanically vibrations come neural activity occurs in the inside ear.