Mechanism of Hearing

Introduction

Hearing begins when sound vibrations are transformed into electrical signals by the cochlear hair cells. Specialized mechanical and neural mechanisms analyze sound frequency and intensity, enabling precise pitch discrimination and accurate transmission of auditory information to the brain.

Sound Waves

• Sound waves are mechanical vibrations consisting of alternating compressions and rarefactions that travel through a material medium.
• ·  They are produced by a vibrating source, such as the vocal cords, a tuning fork, or a loudspeaker.
• Sound waves require an elastic medium, such as air, water, or solids, for propagation and cannot travel through a vacuum.
• When sound waves reach the tympanic membrane, they initiate the process of hearing by producing mechanical vibrations.

Speed

• In dry air at 20°C and sea level, sound travels at approximately 340 meters per second.
• The speed of sound increases with higher temperature because molecules move more rapidly.
• Sound travels much faster in liquids and solids than in air.
• In fresh water, the speed is approximately 1,400–1,500 meters per second, and it is slightly higher in seawater because of greater density and elasticity.

Frequency

• Frequency is the number of sound wave cycles occurring each second and is measured in hertz.
• The normal human hearing range extends from 20 to 20,000 hertz.
• Human hearing is most sensitive to frequencies between 1,000 and 4,000 hertz, which include most speech sounds.
• A pure tone consists of a single frequency, whereas a complex sound contains multiple frequencies.
• Speech, music, and most environmental sounds are examples of complex sounds.

Pitch

• Pitch is the subjective perception of sound frequency.
• Higher-frequency sounds are perceived as higher-pitched, whereas lower-frequency sounds are perceived as lower-pitched.
• During normal conversation, the average fundamental frequency is approximately 120 hertz in adult males and 250 hertz in adult females.
• The human ear can distinguish nearly 2,000 different pitches, and this ability improves with auditory training.
• Pitch discrimination is greatest between 1,000 and 3,000 hertz, where frequency differences as small as 3 hertz can be detected under ideal conditions.

Wavelength

• Wavelength is the distance between two successive points of the same phase, such as adjacent compression peaks.
• Wavelength is inversely proportional to frequency; therefore, higher-frequency sounds have shorter wavelengths, whereas lower-frequency sounds have longer wavelengths.

Loudness

• Loudness is the subjective perception of sound intensity and depends primarily on the amplitude of sound waves.
• Greater amplitude produces a louder perceived sound.
• Loudness is also influenced by frequency, as the human ear is more sensitive to some frequencies than others.
• Sound intensity is expressed in decibels, a logarithmic unit that compares the measured sound intensity with a standard reference intensity.
• An increase of 10 decibels represents a tenfold increase in sound intensity, although the perceived increase in loudness is smaller.
• The normal hearing threshold is approximately 0 decibels in healthy young adults.
• Human hearing is most sensitive to sounds between 1,000 and 4,000 hertz, particularly at moderate intensities of around 60 decibels, which correspond to the frequency range of normal speech.

$$\mathrm{Number\ of\ decibel = 10\log\frac{Intensity\ of\ the\ sound\ heard}{Intensity\ of\ standard\ sound}}$$

Hearing Threshold

• The hearing threshold is the lowest sound intensity that can be detected by a healthy young adult under quiet conditions.
• At a frequency of 1,000 hertz, the normal hearing threshold is approximately 0 decibels.
• The threshold varies with frequency, and the ear is most sensitive between 1,000 and 4,000 hertz.
• Sounds around 20 decibels correspond to whispering, 60 decibels to normal conversation, 80 decibels to shouting, and 120 decibels are generally perceived as uncomfortable.
• Exposure to sounds of approximately 140 decibels may produce pain and can damage the auditory system.

Masking

Masking is the phenomenon in which background noise increases the hearing threshold for another sound, making it more difficult to detect.

Mechanism of Masking

• It occurs because ongoing auditory stimulation reduces the ability of the auditory system to detect weaker incoming sounds.
• The effectiveness of masking depends largely on the frequency of the masking sound.
• Low-frequency sounds mask higher-frequency sounds more effectively than the reverse.
• White noise, which contains a broad range of frequencies, is an effective masking stimulus and is commonly used in audiological testing.
• Masking explains why conversation becomes difficult in noisy environments such as factories or busy streets.
• Even apparently quiet surroundings contain low levels of environmental noise that produce a mild masking effect.
• Individuals with conductive hearing loss may perceive sounds more clearly in the affected ear when background noise is present because environmental sounds are attenuated while bone-conducted speech remains relatively unaffected.
• In a soundproof room, the absence of ambient noise may make faint internal body sounds more noticeable and can alter the subjective perception of hearing.

Transmission of Sound Waves

Transmission through Outer and Middle Ear

• Sound waves enter the external auditory canal and strike the tympanic membrane, causing it to vibrate.
• These vibrations are transmitted through the auditory ossicles—the malleus, incus, and stapes—to the oval window.
• During transmission, the ossicular chain amplifies sound vibrations before they enter the inner ear.

Ossicular Conduction

• Ossicular conduction is the principal pathway for normal hearing.
• Vibrations pass from the tympanic membrane through the ossicular chain to the footplate of the stapes, which transmits them to the cochlear fluids through the oval window.

Air Conduction within the Middle Ear

• A small portion of sound also travels through the air within the middle ear cavity.
• These vibrations reach the round window and the perilymph of the scala tympani.
• Because the transmitted energy is minimal compared with ossicular conduction, this pathway contributes very little to normal hearing.

Bone Conduction

• Bone conduction transmits sound vibrations directly through the bones of the skull to the cochlea, bypassing the external and middle ear.
• It becomes evident when a vibrating tuning fork is placed on the mastoid process or forehead.
• Bone conduction is clinically useful for distinguishing conductive from sensorineural hearing loss during bedside hearing tests.

H4: Transmission through the Inner Ear

• Movement of the stapes at the oval window generates pressure waves in the perilymph of the scala vestibuli.
• Because the cochlea is enclosed within rigid bone, the incompressible perilymph transmits these pressure waves efficiently.
• Increased pressure in the scala vestibuli displaces the vestibular membrane and subsequently moves the basilar membrane.
• The pressure wave then passes through the cochlea into the scala tympani and is relieved by outward movement of the round window.
• When the stapes moves outward, the pressure within the cochlea decreases, producing movement of the basilar membrane in the opposite direction and inward displacement of the round window.
• Oscillatory movement of the basilar membrane causes corresponding movement of the organ of Corti.
• This relative movement between the basilar membrane and the tectorial membrane bends the stereocilia of the hair cells, initiating mechanical-to-electrical transduction, the essential first step in auditory signal generation.

H3: Transduction of Sound Waves

Auditory transduction is the process by which mechanical sound vibrations are converted into electrical receptor potentials in cochlear hair cells and subsequently into action potentials in the auditory nerve.

H4: Resting State of Hair Cell

• In the resting state, two important electrical potentials are present: the hair cell resting membrane potential and the endocochlear potential.
• The resting membrane potential of the hair cell is approximately −60 millivolts.
• This negative potential is maintained primarily by continuous outward movement of potassium ions across the basolateral membrane.
• The basolateral surface of the hair cell is surrounded by perilymph, which contains a low potassium concentration and has an electrical potential close to 0 millivolts.
• The stereocilia project into endolymph, a potassium-rich fluid with a positive endocochlear potential of about +80 millivolts.
• Tight junctions between supporting cells maintain separation between endolymph and perilymph, preserving the ionic gradients.
• Consequently, the electrical gradient across the apical membrane is approximately 140 millivolts, providing a strong driving force for potassium entry.
• Even without sound stimulation, approximately 15% of the mechanically gated ion channels remain open, producing a small resting influx of potassium ions.

H4: Genesis of Receptor Potential in Hair Cells

H5: Process of Depolarization

• Hair cells function as highly sensitive mechanoreceptors, responding to minute deflections of their stereocilia.
• Deflection of the stereocilia by as little as 0.5 nanometers can initiate a measurable response.
• When the stereocilia bend toward the tallest stereocilia, mechanically gated ion channels open.
• Potassium ions, and to a lesser extent calcium ions, enter the hair cell from the endolymph, producing depolarization.
• Depolarization opens voltage-gated calcium channels at the basal end of the hair cell.
• Calcium influx triggers release of the neurotransmitter glutamate from synaptic vesicles.
• Glutamate stimulates adjacent afferent nerve terminals, generating action potentials in cochlear nerve fibers.
• Greater stereociliary deflection produces greater neurotransmitter release and a higher frequency of action potentials.
• Following activation, molecular mechanisms involving myosin adjust tension in the tip links, restoring the stereocilia toward their resting position.
• Potassium then leaves the hair cell through basolateral potassium channels into the perilymph, restoring the resting membrane potential.

H5: Process of Repolarization

• When stereocilia bend away from the tallest stereocilia, mechanically gated channels close.
• Potassium entry decreases, causing hyperpolarization of the hair cell.
• Hyperpolarization reduces calcium entry and decreases glutamate release, lowering the firing rate of auditory nerve fibers.
• Hair cells encode both the direction and magnitude of stereociliary displacement.
• Maximum electrical responses occur when stereocilia are deflected in their preferred direction, whereas deflection at right angles produces little or no response.
• This bidirectional signaling allows cochlear hair cells to function as precise biological transducers, accurately converting mechanical vibrations into graded electrical signals that form the basis of hearing.

H4: Role of Cochlea

• The cochlea converts mechanical sound vibrations into electrical signals that are transmitted through the auditory nerve.
• Coordinated movement of the basilar membrane, hair cells, and tectorial membrane enables efficient auditory transduction.
• The cochlea also encodes the frequency and intensity of sound, providing the neural information required for accurate sound perception and further processing within the central auditory pathway.

H4: Special Features of Basilar Membrane

• The basilar membrane shows marked structural and mechanical variation along its length, enabling precise analysis of sound frequencies.
• Although the cochlea gradually narrows toward its apex, the basilar membrane is narrow and stiff at the base and becomes wider and more flexible toward the apex.
• Its stiffness decreases by nearly 100-fold from the base to the apex, producing region-specific mechanical responses.
• Because of these differences, each part of the basilar membrane responds maximally to a particular sound frequency.
• A sound of a given frequency produces maximum vibration at a specific location, selectively activating the corresponding hair cells and auditory nerve fibers.
• This spatial representation of sound frequency is known as place coding or the place principle of hearing.
• The basal turn of the cochlea is most sensitive to high-frequency sounds.
• The apical turn responds preferentially to low-frequency sounds.
• Thus, the basilar membrane serves as the first site for pitch discrimination within the auditory system.
• The orderly frequency arrangement established in the cochlea is preserved throughout the auditory pathway, a property known as tonotopic organization.
• The basilar membrane also contributes to the perception of sound intensity.
• Louder sounds produce greater displacement of the basilar membrane, resulting in increased stimulation of hair cells.
• Greater hair cell activation leads to the generation of a higher frequency of action potentials in cochlear nerve fibers, allowing the brain to distinguish differences in sound intensity.
• This combination of frequency selectivity and intensity coding enables accurate analysis of complex auditory signals, including speech and music.

H3: Traveling Wave

• Movement of the stapes at the oval window generates pressure waves in the perilymph of the scala vestibuli.
• These pressure waves produce a traveling wave that moves along the basilar membrane from the base toward the apex of the cochlea.
• As the wave progresses, its amplitude gradually increases until it reaches a maximum at a location determined by the frequency of the sound.
• Beyond this point, the wave amplitude rapidly decreases and dissipates.
• High-frequency sounds produce maximum displacement near the base of the cochlea, close to the oval window.
• Low-frequency sounds produce maximum displacement near the apex, close to the helicotrema.
• Thus, each sound frequency activates a specific region of the basilar membrane, enabling precise pitch discrimination.
• This frequency-dependent movement forms the basis of the traveling wave principle and explains the place-specific activation of cochlear hair cells.
• The orderly frequency mapping established in the cochlea is preserved throughout the auditory pathway as tonotopic organization.

H4: The Shearing Force Bends Cilia

• The reticular lamina firmly supports the apical portions of the hair cells.
• The stereocilia of outer hair cells are embedded in the tectorial membrane, mechanically linking the two structures.
• During sound stimulation, the basilar membrane and tectorial membrane move with slightly different patterns because they are attached at different locations within the cochlea.
• This difference in movement generates a shearing force between the two membranes.
• The shearing force bends the stereocilia of the outer hair cells, initiating mechanical-to-electrical transduction.
• The stereocilia of inner hair cells are not embedded in the tectorial membrane.
• Instead, they are deflected by movement of the endolymph between the tectorial membrane and the hair cells.
• Upward movement of the basilar membrane bends the stereocilia toward their tallest row, producing depolarization of the hair cells.
• Downward movement bends the stereocilia in the opposite direction, resulting in hyperpolarization.
• Alternating depolarization and hyperpolarization generate graded receptor potentials that regulate neurotransmitter release to auditory nerve fibers.
• This highly coordinated mechanical system enables the cochlea to detect subtle differences in sound frequency and sound intensity, providing the basis for accurate hearing.

H3: Cochlear Potentials

• In response to sound stimulation, the cochlea generates three dynamic electrical responses: cochlear microphonic, summation potential, and auditory nerve action potential.
• These are recorded in addition to the resting endocochlear potential and hair cell resting membrane potential.
• Recording these electrical responses using electrodes placed near the cochlea or on the round window is called electrocochleography.
• Electrocochleography is useful for assessing cochlear and auditory nerve function.

Table 151.1: Difference between cochlear microphonics and action potentials.

Feature	Cochlear Microphonic (CM)	Auditory Nerve Action Potential (AP)
Origin	Generated mainly by cochlear hair cells	Generated by auditory nerve fibers
Latency	Absent	Present
Refractory period	Absent	Present
Sequence	Appears before the action potential	Follows the cochlear microphonic
Response at high stimulus rates	Follows rapid stimuli faithfully	Limited by neuronal firing properties
Fatigue	Does not fatigue	May show reduced responsiveness with repetitive stimulation
Polarity	Reverses with stimulus polarity	Remains a positive deflection
Masking by noise	Not significantly affected	Reduced or masked by broad-spectrum noise

H3: Cochlear Microphonics

• Cochlear microphonic is an electrical response produced by the combined receptor potentials of numerous cochlear hair cells.
• It is generated almost instantaneously when sound-induced movement of the stereocilia causes ion flow across the apical membrane of hair cells.
• The waveform closely follows the vibration of the basilar membrane and has the same frequency as the incoming sound stimulus.
• Experimental evidence indicates that the response arises predominantly from the outer hair cells.
• Damage to outer hair cells markedly reduces or abolishes the cochlear microphonic, whereas preservation of inner hair cells has little effect on this potential.

Summation of potentials:

• Summation potential is another cochlear response that appears as either a positive or negative direct-current shift during sound stimulation.
• It reflects sustained displacement of the hair cells and contributes to assessment of cochlear function during electrocochleography.

H3: Localization of the Source of Sound

• Sound localization is the ability to identify the direction from which a sound originates.
• This function helps individuals orient toward a sound source and respond appropriately to the surrounding environment.
• Accurate localization depends on comparison of auditory signals received by both ears and processing within central auditory pathways.

H4: Difference in Time of Arrival of Stimulus in Two Ears

• For low-frequency sounds (below approximately 3,000 hertz), the difference in the arrival time of sound at the two ears is the primary cue for localization in the horizontal plane.
• A sound originating directly in front of or behind the listener reaches both ears simultaneously.
• When the sound arises from one side, it reaches the nearer ear slightly earlier than the farther ear.
• The auditory system can detect interaural time differences as small as 20 microseconds, allowing precise localization.
• The high temporal sensitivity of cochlear hair cells enables detection of these minute timing differences.

H4: Difference in Intensity that Reaches the Two Ears

• For high-frequency sounds (above approximately 3,000 hertz), localization depends mainly on differences in sound intensity between the two ears.
• The head acts as an acoustic barrier, reducing the intensity of sound reaching the ear farther from the source.
• Consequently, the ear closer to the sound receives a stronger stimulus.
• The brain compares these intensity differences to determine the direction of the sound.

H4: Role of Pinna

• The pinna modifies incoming sound waves by reflecting and filtering different frequencies.
• These modifications vary according to the direction from which the sound approaches.
• In humans, the pinna plays a major role in vertical sound localization, helping distinguish sounds originating from above or below.
• Because the pinna is directed slightly forward, sounds arriving from the front and back produce different spectral patterns, aiding front–back discrimination.
• Unlike many animals, humans cannot actively rotate the pinna toward a sound source.
• Interaural differences in timing and intensity generate distinct patterns of neural activity in the auditory pathways.
• The superior olivary complex is the first auditory relay that receives bilateral input and performs initial comparison of these signals.
• Higher auditory centers, including the auditory cortex, integrate this information to determine the precise direction of the sound source.
• Lesions involving the auditory cortex do not usually cause deafness because of bilateral auditory pathways, but they significantly impair sound localization and the ability to interpret the spatial origin of sounds.