what mechanism causes hair cell depolarization as stereocilia bend in response to sound waves?

Chapter 12: Auditory System: Structure and Function

12.1 The Vertebrate Hair Jail cell: Mechanoreceptor Mechanism, Tip Links, K⁺ and Ca²⁺ Channels

Figure 12.1
Mechanical Transduction in Hair Cells.

The cardinal structure in the vertebrate auditory and vestibular systems is the hair cell. The pilus prison cell first appeared in fish as part of a long, thin assortment forth the side of the body, sensing movements in the water. In higher vertebrates the internal fluid of the inner ear (not external fluid equally in fish) bathes the hair cells, but these cells still sense movements in the surrounding fluid. Several specializations brand human being pilus cells responsive to diverse forms of mechanical stimulation. Pilus cells in the Organ of Corti in the cochlea of the ear respond to sound. Hair cells in the cristae ampullares in the semicircular ducts reply to angular acceleration (rotation of the caput). Hair cells in the maculae of the saccule and the utricle reply to linear acceleration (gravity). (See the affiliate on Vestibular System: Construction and Function). The fluid, termed endolymph, which surrounds the pilus cells is rich in potassium. This actively maintained ionic imbalance provides an energy store, which is used to trigger neural action potentials when the hair cells are moved. Tight junctions between hair cells and the nearby supporting cells course a barrier between endolymph and perilymph that maintains the ionic imbalance.

Figure 12.1 illustrates the process of mechanical transduction at the tips of the pilus cell cilia. Cilia emerge from the apical surface of hair cells. These cilia increase in length along a consistent axis. At that place are tiny thread-like connections from the tip of each cilium to a non-specific cation channel on the side of the taller neighboring cilium. The tip links function like a string connected to a hinged hatch. When the cilia are aptitude toward the tallest i, the channels are opened, much like a trap door. Opening these channels allows an influx of potassium, which in turns opens calcium channels that initiates the receptor potential. This machinery transduces mechanical energy into neural impulses. An inward Grand⁺ current depolarizes the jail cell, and opens voltage-dependent calcium channels. This in plough causes neurotransmitter release at the basal end of the hair cell, eliciting an activeness potential in the dendrites of the VIIIth cranial nervus.

Printing the "play" button to run into the mechanical-to-electrical transduction. Pilus cells commonly take a small influx of Thou⁺ at balance, so there is some baseline action in the afferent neurons. Angle the cilia toward the tallest one opens the potassium channels and increases afferent activity. Bending the cilia in the contrary direction closes the channels and decreases afferent activeness. Bending the cilia to the side has no effect on spontaneous neural activity.

12.ii Sound: Intensity, Frequency, Outer and Center Ear Mechanisms, Impedance Matching by Area and Lever Ratios

The auditory organization changes a wide range of weak mechanical signals into a complex serial of electrical signals in the fundamental nervous arrangement. Sound is a series of pressure changes in the air. Sounds often vary in frequency and intensity over fourth dimension. Humans tin detect sounds that cause movements only slightly greater than those of Brownian movement. Plain, if nosotros heard that ceaseless (except at absolute zero) motion of air molecules we would accept no silence.

Figure 12.2
Air-conducted sounds eventually move the inner-ear fluid.

Effigy 12.2 depicts these alternate pinch and rarefaction (force per unit area) waves impinging on the ear. The pinna and external auditory meatus collect these waves, change them slightly, and direct them to the tympanic membrane. The resulting movements of the eardrum are transmitted through the iii center-ear ossicles (malleus, incus and stapes) to the fluid of the inner ear. The footplate of the stapes fits tightly into the oval window of the bony cochlea. The inner ear is filled with fluid. Since fluid is incompressible, as the stapes moves in and out there needs to be a compensatory move in the opposite direction. Find that the round window membrane, located beneath the oval window, moves in the opposite management.

Because the tympanic membrane has a larger area than the stapes footplate in that location is a hydraulic distension of the sound pressure. Also because the arm of the malleus to which the tympanic membrane is attached is longer than the arm of the incus to which the stapes is attached, there is a slight amplification of the sound force per unit area by a lever action. These two impedance matching mechanisms effectively transmit air-born audio into the fluid of the inner ear. If the middle-ear apparatus (ear drum and ossicles) were absent-minded, then sound reaching the oval and round windows would be largely reflected.

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

Figure 12.iii
Cross-section of the coiled Cochlea.

The cochlea is a long coiled tube, with 3 channels divided by two thin membranes. The top tube is the scala vestibuli, which is connected to the oval window. The bottom tube is the scala tympani, which is connected to the round window. The middle tube is the scala media, which contains the Organ of Corti. The Organ of Corti sits on the basilar membrane, which forms the partitioning between the scalae media and tympani.

Effigy 12.iii illustrates a cantankerous section through the cochlea. The 3 scalae (vestibuli, media, tympani) are cut in several places as they spiral around a primal core. The cochlea makes ii-one/2 turns in the human (hence the five cuts in midline cross department). The tightly coiled shape gives the cochlea its proper noun, which means snail in Greek (as in conch shell). As explained in Tonotopic Arrangement, loftier frequency sounds stimulate the base of the cochlea, whereas depression frequency sounds stimulate the apex. This characteristic is depicted in the animation of Figure 12.3 with neural impulses (having colors from red to blue representing low to loftier frequencies, respectively) emerging from different turns of the cochlea. The activity in Figure 12.three would exist generated by white racket that has all frequencies at equal amplitudes. The moving dots are meant to indicate afferent action potentials. Low frequencies are transduced at the apex of the cochlea and are represented by red dots. High frequencies are transduced at base of the cochlea and are represented by blue dots. A upshot of this organisation is that low frequencies are constitute in the fundamental core of the cochlear nerve, with loftier frequencies on the exterior.

Figure 12.4
Detailed cantankerous-section of one turn of the Cochlear duct.

Figure 12.iv illustrates one cross section of the cochlea. Sound waves cause the oval and round windows at the base of the cochlea to movement in opposite directions (See Figure 12.2). This causes the basilar membrane to exist displaced and starts a traveling moving ridge that sweeps from the base toward the apex of the cochlea (See Effigy 12.7). The traveling wave increases in amplitude as it moves, and reaches a peak at a identify that is directly related to the frequency of the sound. The illustration shows a department of the cochlea that is moving in response to audio.

Figure 12.five illustrates a higher magnification of the Organ of Corti. The traveling wave causes the basilar membrane and hence the Organ of Corti to move up and down. The organ of Corti has a central stiffening buttress formed by paired pillar cells. Hair cells protrude from the top of the Organ of Corti. A tectorial (roof) membrane is held in place by a swivel-similar mechanism on the side of the Organ of Corti and floats above the hair cells. Equally the basilar and tectorial membranes move up and downward with the traveling wave, the hinge mechanism causes the tectorial membrane to motility laterally over the pilus cells. This lateral shearing motion bends the cilia atop the hair cells, pulls on the fine tip links, and opens the trap-door channels (See Figure 12.i). The influx of potassium and then calcium causes neurotransmitter release, which in turn causes an EPSP that initiates activeness potentials in the afferents of the VIIIth cranial nerve. Most of the afferent dendrites make synaptic contacts with the inner hair cells.

Figure 12.6 looks downward on the Organ of Corti. There are ii types of pilus cells, inner and outer. There is one row of inner hair cells and three rows of outer hair cells. Most of the afferent dendrites synapse on inner pilus cells. Most of efferent axons synapse on the outer hair cells. The outer pilus cells are active. They move in response to sound and amplify the traveling wave. The outer hair cells also produce sounds that tin exist detected in the external auditory meatus with sensitive microphones. These internally generated sounds, termed otoacoustic emissions, are now used to screen newborns for hearing loss. Figure 12.6 shows an immunofluorescent whole mount image of a neonatal mouse cochlea showing the three rows of outer hair cells and the single row of inner hair cells. The mature human cochlea would expect approximately the same. Superimposed schematically-depicted neurons show the typical pattern of afferent connections. Ninety-v percent of the VIIIth nervus afferents synapse on inner hair cells. Each inner hair cell makes synaptic connections with many afferents. Each afferent connects to only 1 inner hair jail cell. Near five percent of the afferents synapse on outer hair cells. These afferents travel a considerable distance along the basilar membrane away from their ganglion cells to synapse on multiple outer hair cells. Less than one pct (~0.5%) of the afferents synapse on multiple inner pilus cells. The below micrograph is courtesy of Dr. Douglas Cotanche, Department of Otolaryngology, Children's Hospital of Boston, Harvard Medical School. Reprinted with permission.

Figure 12.6
Pilus cells on the mammalian basilar membrane.

12.four Tonotopic Organization

Figure 12.vii
Tonotopic organization of the mature man Cochlea.

Concrete characteristics of the basilar membrane cause unlike frequencies to reach maximum amplitudes at different positions. Much equally on a pianoforte, loftier frequencies are at i end and low frequencies at the other. High frequencies are transduced at the base of the cochlea whereas depression frequencies are transduced at the noon. Figure 12.7 illustrates the fashion in which the cochlea acts as a frequency analyzer. The cochlea codes the pitch of a sound by the place of maximal vibration. Annotation the position of the traveling wave at dissimilar frequencies. (Beware! It may initially seem backwards that low frequencies are not associated with the base.) Select different frequencies by turning the dial. If audio on your computer is enabled, you lot will hear the sound you selected. Hearing loss at high frequencies is common. The average loss of hearing in American males is nigh a bike per second per day (starting at about age twenty, so a 50-year one-time would likely have difficulty hearing over 10 kHz). If you can't hear the loftier frequencies, it may exist due to the speakers on your computer, only it is always worth thinking about hearing preservation.

Equally yous listen to these sounds, note that the loftier frequencies seem strangely similar. Think virtually cochlear-implant patients. These patients accept lost pilus-cell office. Their auditory nervus is stimulated by a serial of implanted electrodes. The implant tin can only be placed in the base of the cochlea, because information technology is surgically incommunicable to thread the fine wires more than than nearly 2/iii of a turn. Thus, cochlear implant patients probably experience something like high frequency sounds.

12.five The Range of Sounds to Which We Respond; Neural Tuning Curves

Effigy 12.8 shows the range of frequencies and intensities of sound to which the human auditory system responds. Our absolute threshold, the minimum level of sound that we can discover, is strongly dependent on frequency. At the level of pain, audio levels are about half dozen orders of magnitude above the minimal audible threshold. Audio pressure level (SPL) is measured in decibels (dB). Decibels are a logarithmic scale, with each six dB increment indicating a doubling of intensity. The perceived loudness of a audio is related to its intensity. Sound frequencies are measured in Hertz (Hz), or cycles per 2d. Normally, we hear sounds as low as 20 Hz and as high as 20,000 Hz. The frequency of a sound is associated with its pitch. Hearing is all-time at most 3-four kHz. Hearing sensitivity decreases at higher and lower frequencies, but more so at college than lower frequencies. High-frequency hearing is typically lost as we age.

Effigy 12.8
Audiometric bend for a normal hearing subject and some neural tuning curves.

The neural lawmaking in the cardinal auditory organization is circuitous. Tonotopic organization is maintained throughout the auditory system. Tonotopic organisation ways that cells responsive to different frequencies are institute in unlike places at each level of the cardinal auditory arrangement, and that there is a standard (logarithmic) relationship between this position and frequency. Each cell has a characteristic frequency (CF). The CF is the frequency to which the prison cell is maximally responsive. A cell will usually reply to other frequencies, merely only at greater intensities. The neural tuning bend is a plot of the amplitude of sounds at various frequencies necessary to elicit a response from a central auditory neuron. The tuning curves for several different neurons are superimposed on the audibility curves in Figure 12.8. The depicted neurons have CFs that vary from depression to loftier frequencies (and are shown with red to blue colors, respectively). If we recorded from all auditory neurons, we would basically fill up 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 to one set of fibers or cells at one particular place. As sounds become louder they stimulate other neurons, and the surface area of activity will increase.

Graduate Students Sarah Baum, Heather Turner, Nadeeka Dias, Deepna Thakkar, Natalie Sirisaengtaksin and Jonathan Flynn of the Neuroscience Graduate Program at UTHealth Houston further explicate the structures, functions and pathways of the auditory arrangement in an animated video "The Journey of Audio".

Test Your Knowledge

Question 1
A
B
C
D
E

High frequencies are transduced

A. at the noon of the cochlea

B. at the base of operations of the cochlea

C. throughout the cochlea

D. by vibrations of the stapes

E. at the superior temporal gyrus

High frequencies are transduced

A. at the apex of the cochlea This answer is Incorrect.

Information technology may seem "backwards" but although the Cochlear duct seems to become smaller toward the apex, the basilar membrane really gets wider.

B. at the base of operations of the cochlea

C. throughout the cochlea

D. by vibrations of the stapes

E. at the superior temporal gyrus

High frequencies are transduced

A. at the apex of the cochlea

B. at the base of operations of the cochlea This answer is Right!

C. throughout the cochlea

D. past vibrations of the stapes

E. at the superior temporal gyrus

High frequencies are transduced

A. at the noon of the cochlea

B. at the base of operations of the cochlea

C. throughout the cochlea This reply is Incorrect.

High frequencies practise non travel far forth the basilar membrane. (As an aside, low frequencies traverse the length of the Cochlea, and hence cause the most damage if they are sufficiently loud.)

D. past vibrations of the stapes

Eastward. at the superior temporal gyrus

High frequencies are transduced

A. at the noon of the cochlea

B. at the base of the cochlea

C. throughout the cochlea

D. by vibrations of the stapes This answer is INCORRECT.

Sound is transmitted to the fluid of the inner ear through vibrations of the tympanic membrane, malleus, incus and stapes. Transduction, the modify from mechanical energy to neural impulses, takes identify in the hair cells, specifically through potassium channels at the tips of the stereocilia.

Eastward. at the superior temporal gyrus

High frequencies are transduced

A. at the apex of the cochlea

B. at the base of the cochlea

C. throughout the cochlea

D. past vibrations of the stapes

Due east. at the superior temporal gyrus This respond is Wrong.

Auditory afferents eventually accomplish the primary auditory cortex in Heschel's gyrus within insular cortex, and this area is tonotopically organized. Stimulation of this surface area leads to witting awareness of the sound, just the transduction from mechanical vibrations to neural activity occurs in the inner ear.

Question 2
A
B
C
D
E