what is the normal pathway for auditory stimuli to cause conscious perception of a sound?

Chapter 12: Auditory System: Construction and Function

12.ane The Vertebrate Hair Jail cell: Mechanoreceptor Machinery, Tip Links, Grand⁺ and Ca²⁺ Channels

Figure 12.1
Mechanical Transduction in Hair Cells.

The cardinal structure in the vertebrate auditory and vestibular systems is the pilus cell. The hair cell first appeared in fish as part of a long, thin assortment along the side of the body, sensing movements in the water. In higher vertebrates the internal fluid of the inner ear (not external fluid as in fish) bathes the pilus cells, but these cells nevertheless sense movements in the surrounding fluid. Several specializations make homo hair cells responsive to diverse forms of mechanical stimulation. Pilus cells in the Organ of Corti in the cochlea of the ear answer to audio. Pilus cells in the cristae ampullares in the semicircular ducts respond to athwart acceleration (rotation of the caput). Hair cells in the maculae of the saccule and the utricle respond to linear acceleration (gravity). (See the chapter on Vestibular System: Construction and Part). The fluid, termed endolymph, which surrounds the hair 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 form a barrier betwixt endolymph and perilymph that maintains the ionic imbalance.

Figure 12.ane illustrates the process of mechanical transduction at the tips of the pilus prison cell cilia. Cilia sally from the apical surface of pilus cells. These cilia increase in length along a consistent axis. There are tiny thread-similar 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 ane, 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 K⁺ current depolarizes the cell, and opens voltage-dependent calcium channels. This in turn causes neurotransmitter release at the basal stop of the hair cell, eliciting an activeness potential in the dendrites of the VIIIth cranial nerve.

Press the "play" button to see the mechanical-to-electrical transduction. Hair cells normally have a minor influx of Chiliad⁺ at balance, then there is some baseline activity in the afferent neurons. Angle the cilia toward the tallest one opens the potassium channels and increases afferent activity. Angle the cilia in the opposite direction closes the channels and decreases afferent activeness. Bending the cilia to the side has no effect on spontaneous neural action.

12.2 Audio: Intensity, Frequency, Outer and Centre Ear Mechanisms, Impedance Matching by Area and Lever Ratios

The auditory system changes a wide range of weak mechanical signals into a complex series of electrical signals in the central nervous system. Audio is a series of pressure changes in the air. Sounds often vary in frequency and intensity over fourth dimension. Humans can observe sounds that cause movements but slightly greater than those of Brownian movement. Apparently, if we heard that ceaseless (except at accented zero) motility of air molecules we would have no silence.

Figure 12.ii
Air-conducted sounds eventually movement the inner-ear fluid.

Figure 12.2 depicts these alternating pinch and rarefaction (pressure) 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 three eye-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, equally the stapes moves in and out in that location needs to exist a compensatory movement in the opposite direction. Notice that the round window membrane, located below the oval window, moves in the opposite direction.

Because the tympanic membrane has a larger surface area than the stapes footplate there is a hydraulic amplification 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 fastened, there is a slight amplification of the sound pressure past a lever action. These ii impedance matching mechanisms effectively transmit air-born audio into the fluid of the inner ear. If the centre-ear apparatus (ear drum and ossicles) were absent, and so sound reaching the oval and round windows would be largely reflected.

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

Figure 12.three
Cantankerous-section of the coiled Cochlea.

The cochlea is a long coiled tube, with iii channels divided by two thin membranes. The superlative tube is the scala vestibuli, which is connected to the oval window. The bottom tube is the scala tympani, which is continued to the circular 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 segmentation between the scalae media and tympani.

Figure 12.iii illustrates a cantankerous section through the cochlea. The three scalae (vestibuli, media, tympani) are cut in several places every bit they spiral effectually a central cadre. The cochlea makes 2-ane/2 turns in the man (hence the 5 cuts in midline cross section). The tightly coiled shape gives the cochlea its proper noun, which means snail in Greek (as in conch shell). Equally explained in Tonotopic System, high frequency sounds stimulate the base of the cochlea, whereas low frequency sounds stimulate the apex. This feature is depicted in the animation of Figure 12.3 with neural impulses (having colors from red to blue representing low to high frequencies, respectively) emerging from different turns of the cochlea. The activeness in Figure 12.3 would exist generated past white noise that has all frequencies at equal amplitudes. The moving dots are meant to indicate afferent action potentials. Depression 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 consequence of this arrangement is that low frequencies are institute in the central core of the cochlear nervus, with high frequencies on the outside.

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

Figure 12.4 illustrates one cross section of the cochlea. Sound waves crusade the oval and circular windows at the base of the cochlea to move in opposite directions (See Figure 12.2). This causes the basilar membrane to be displaced and starts a traveling wave that sweeps from the base of operations toward the apex of the cochlea (See Figure 12.7). The traveling wave increases in amplitude as it moves, and reaches a summit at a place that is straight related to the frequency of the sound. The illustration shows a section of the cochlea that is moving in response to sound.

Figure 12.5 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 downwards. The organ of Corti has a central stiffening buttress formed past paired pillar cells. Pilus cells protrude from the pinnacle of the Organ of Corti. A tectorial (roof) membrane is held in place by a hinge-like machinery on the side of the Organ of Corti and floats above the pilus cells. As the basilar and tectorial membranes move upwardly 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 (Come across Figure 12.1). The influx of potassium and then calcium causes neurotransmitter release, which in turn causes an EPSP that initiates action potentials in the afferents of the VIIIth cranial nerve. Almost of the afferent dendrites brand synaptic contacts with the inner hair cells.

Figure 12.6 looks down on the Organ of Corti. There are two types of hair cells, inner and outer. There is one row of inner hair cells and 3 rows of outer hair cells. Near of the afferent dendrites synapse on inner hair cells. Most of efferent axons synapse on the outer hair cells. The outer hair cells are active. They move in response to sound and amplify the traveling wave. The outer hair cells likewise produce sounds that tin be detected in the external auditory meatus with sensitive microphones. These internally generated sounds, termed otoacoustic emissions, are at present used to screen newborns for hearing loss. Effigy 12.6 shows an immunofluorescent whole mountain image of a neonatal mouse cochlea showing the iii rows of outer hair cells and the single row of inner hair cells. The mature human cochlea would look approximately the same. Superimposed schematically-depicted neurons show the typical blueprint of afferent connections. Ninety-five percent of the VIIIth nerve afferents synapse on inner hair cells. Each inner hair cell makes synaptic connections with many afferents. Each afferent connects to simply ane inner hair jail cell. About five percent of the afferents synapse on outer hair cells. These afferents travel a considerable altitude along the basilar membrane away from their ganglion cells to synapse on multiple outer pilus cells. Less than 1 percent (~0.v%) of the afferents synapse on multiple inner hair cells. The beneath micrograph is courtesy of Dr. Douglas Cotanche, Section of Otolaryngology, Children's Hospital of Boston, Harvard Medical School. Reprinted with permission.

Figure 12.vi
Hair cells on the mammalian basilar membrane.

12.four Tonotopic Organization

Figure 12.7
Tonotopic system of the mature human Cochlea.

Physical characteristics of the basilar membrane cause different frequencies to reach maximum amplitudes at different positions. Much as on a piano, high frequencies are at ane finish and low frequencies at the other. Loftier frequencies are transduced at the base of the cochlea whereas low frequencies are transduced at the apex. Effigy 12.7 illustrates the way in which the cochlea acts every bit a frequency analyzer. The cochlea codes the pitch of a sound by the place of maximal vibration. Notation the position of the traveling moving ridge at different frequencies. (Beware! Information technology may initially seem backwards that depression frequencies are not associated with the base.) Select different frequencies by turning the dial. If audio on your reckoner is enabled, you will hear the sound you selected. Hearing loss at high frequencies is common. The average loss of hearing in American males is almost a bicycle per 2nd per day (starting at about age 20, then a 50-yr quondam would likely take difficulty hearing over 10 kHz). If you tin can't hear the high frequencies, it may be due to the speakers on your computer, but information technology is always worth thinking about hearing preservation.

As you mind to these sounds, annotation that the high frequencies seem strangely similar. Think about cochlear-implant patients. These patients have lost hair-jail cell function. Their auditory nervus is stimulated by a series of implanted electrodes. The implant tin only be placed in the base of the cochlea, because it is surgically impossible to thread the fine wires more than about 2/3 of a turn. Thus, cochlear implant patients probably feel something like loftier frequency sounds.

12.5 The Range of Sounds to Which We Respond; Neural Tuning Curves

Figure 12.viii shows the range of frequencies and intensities of sound to which the homo auditory system responds. Our accented threshold, the minimum level of sound that we tin detect, is strongly dependent on frequency. At the level of hurting, audio levels are well-nigh six orders of magnitude above the minimal audible threshold. Acoustic level (SPL) is measured in decibels (dB). Decibels are a logarithmic scale, with each half dozen dB increment indicating a doubling of intensity. The perceived loudness of a sound is related to its intensity. Audio frequencies are measured in Hertz (Hz), or cycles per second. Normally, we hear sounds as low as xx Hz and as loftier as 20,000 Hz. The frequency of a sound is associated with its pitch. Hearing is best at about 3-4 kHz. Hearing sensitivity decreases at higher and lower frequencies, but more so at higher than lower frequencies. Loftier-frequency hearing is typically lost as we age.

Figure 12.8
Audiometric curve for a normal hearing subject and some neural tuning curves.

The neural code in the central auditory organization is circuitous. Tonotopic arrangement is maintained throughout the auditory system. Tonotopic organization ways that cells responsive to different frequencies are plant in unlike places at each level of the primal auditory system, and that at that place is a standard (logarithmic) human relationship between this position and frequency. Each cell has a characteristic frequency (CF). The CF is the frequency to which the cell is maximally responsive. A prison cell will usually respond to other frequencies, but only at greater intensities. The neural tuning curve is a plot of the aamplitude 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 Effigy 12.viii. The depicted neurons have CFs that vary from low to high frequencies (and are shown with red to bluish colors, respectively). If nosotros recorded from all auditory neurons, we would basically fill the area within the audibility curves. When sounds are soft they volition stimulate just those few neurons with that CF, and thus neural activity volition be confined to one prepare of fibers or cells at one particular place. As sounds get louder they stimulate other neurons, and the expanse 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 explain the structures, functions and pathways of the auditory system in an animated video "The Journey of Audio".

Test Your Knowledge

Question 1
A
B
C
D
E

Loftier frequencies are transduced

A. at the apex 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 respond is INCORRECT.

It may seem "backwards" but although the Cochlear duct seems to become smaller toward the noon, the basilar membrane actually gets wider.

B. at the base of operations of the cochlea

C. throughout the cochlea

D. by vibrations of the stapes

East. 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 CORRECT!

C. throughout the cochlea

D. past 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

C. throughout the cochlea This reply is Incorrect.

Loftier frequencies do not travel far along the basilar membrane. (As an aside, low frequencies traverse the length of the Cochlea, and hence cause the most impairment if they are sufficiently loud.)

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 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 change from mechanical energy to neural impulses, takes place in the hair cells, specifically through potassium channels at the tips of the stereocilia.

Due east. at the superior temporal gyrus

Loftier frequencies are transduced

A. at the apex of the cochlea

B. at the base of operations of the cochlea

C. throughout the cochlea

D. by vibrations of the stapes

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

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

Question 2
A
B
C
D
E