The following CBME core competencies are covered in this chapter. EN1.1 Describe the anatomy and physiology of the ear, nose, throat, and head and neck. PY10.15 Describe and discuss the functional anatomy of the ear and auditory pathways and physiology of hearing.

Physiology of Hearing: From Sound Waves to Neural Perception

Introduction

Definition of hearing: Hearing is the process by which the ear detects sound waves and converts them into meaningful neural signals that are interpreted by the brain. It enables us to recognise the nature, intensity, pitch, and location of sounds, thereby facilitating communication and environmental awareness.

Functional divisions of the ear: The auditory system comprises three functional components: the external ear, middle ear, and inner ear (cochlea). The external and middle ears collect, conduct, and amplify sound waves, whereas the cochlea converts mechanical vibrations into electrical impulses. These impulses are then transmitted through the central auditory pathway to the auditory cortex, where they are perceived as sound.

Overview of the hearing process: Although the cochlea is responsible for hearing, the remaining structures of the inner ear – the semicircular canals, utricle, and saccule – form the vestibular apparatus, which maintains balance and spatial orientation. The mechanism of hearing can be understood as a sequence of three major events:

1. Mechanical Conduction of Sound (Conduction Apparatus): This initial stage involves the collection and transmission of sound waves through the external and middle ear.
2. Cochlear Transduction (Conversion of Mechanical Energy into Electrical Impulses): Here, the mechanical vibrations are converted into electrical signals by specialised hair cells within the cochlea.
3. Conduction of Electrical Impulses to the Brain (Neural Pathways): Finally, these electrical impulses travel along specific neural pathways to the brain’s auditory centres for interpretation.

Fundamentals of Sound

What is Sound?

Sound waves are produced by a vibrating object that creates alternating areas of compression (pushing together) and rarefaction (spreading apart) of air molecules.

Formation and Structure of Sound Waves:

• Structure of Sound Waves: Sound waves are composed of alternating areas of compressed and spread-out air molecules.
• Tuning Fork Example: A vibrating tuning fork compresses air molecules in front of it and spreads out molecules behind it, creating sound waves.
• Propagation of Sound Waves: Disturbed air molecules collide with further molecules, causing new areas of disturbance and allowing sound waves to travel progressively further from the source. Sound waves eventually fade (attenuate) when they become too weak to affect the surrounding region.

 

Speed of Sound

• In Air: Sound travels at 344 meters (1120 feet) per second in air at 20°C and at sea level.
• In Liquids and Solids: Sound moves faster in liquids and solids than in the air.
• Transition from Air to Liquid: Sound waves are poorly transmitted from air to liquid because of acoustic impedance mismatch – most energy is reflected at the interface, not absorbed or transmitted.

 

Mechanism of Hearing

1. Mechanical Conduction of Sound (Conduction Apparatus)

This phase ensures that sound energy efficiently reaches the inner ear despite changes in the transmitting medium.

A. Role of the External Ear in Mechanical Conduction

The external ear, comprising the pinna and the external auditory meatus, plays a vital role in capturing and directing sound.

• Pinna (Auricle): This visible, cartilaginous part of the ear plays a crucial role in localising sound, particularly in the vertical plane. It effectively captures sound waves and funnels them into the external auditory meatus. Therefore, its unique shape helps us discern the direction of a sound source.
• External Auditory Meatus (Ear Canal): The external auditory meatus conducts sound waves to the tympanic membrane and acts as a resonating tube, amplifying sound frequencies important for speech perception. It provides a 10–15 dB gain, with maximum amplification around 3000 Hz (2.5–4 kHz).
• Tympanic Membrane (Eardrum): This essential component functions as a sensitive pressure receiver, converting airborne sound waves into mechanical vibrations. The tympanic membrane, along with the ossicular ligaments and middle ear muscles, provides mechanical dampening that prevents excessive vibration and distortion of the signal.

B. Role of the Middle Ear in Mechanical Conduction

The middle ear, a small air-filled cavity, performs four critical functions that significantly enhance hearing. These mechanisms overcome the impedance mismatch between air and the inner ear fluids.

1. Impedance Matching Mechanism (Transformer Action):

The Challenge: The cochlear fluids have a much higher acoustic impedance than air. Therefore, if sound waves struck the oval window directly, approximately 99.9% of the sound energy would be reflected at the air–fluid interface, resulting in a conductive hearing loss of about 30 dB.

The Solution: The tympanic membrane and ossicular chain function as an impedance-matching transformer, converting low-pressure, high-displacement vibrations in air into high-pressure, low-displacement vibrations suitable for transmission into the fluid-filled cochlea.

Mechanisms of Impedance Matching

• Hydraulic (Area Ratio) Effect: The effective vibrating area of the tympanic membrane (≈55 mm²) is approximately 17 times larger than the area of the stapes footplate (≈3.2 mm²). Consequently, the same force is concentrated onto a much smaller area, increasing the pressure at the oval window by approximately 17-fold.
• Ossicular Lever Action: The handle of the malleus is approximately 1.3 times longer than the long process of the incus, providing a mechanical advantage that increases the force transmitted to the stapes by approximately 1.3 times.
• Curved Tympanic Membrane (Catenary) Effect: Because the tympanic membrane is conical rather than flat, its peripheral movement is greater than its central movement. This produces a small additional pressure gain during sound transmission. Although traditionally described as contributing about 6 dB, its role is relatively minor compared with the area ratio and ossicular lever mechanisms.

Combined Amplification: The area ratio of the tympanic membrane to the stapes footplate and the lever action of the ossicles together provide an overall pressure gain of approximately 22-fold (about 26–27 dB). The curved (catenary) configuration of the tympanic membrane contributes only a minor additional gain, resulting in an overall middle-ear transformer gain of approximately 25–30 dB.

2. Acoustic Reflex:

Nature: The acoustic (stapedial) reflex is an involuntary brainstem reflex that reduces the transmission of loud sounds and provides limited protection against prolonged exposure to intense noise.

Mechanism: The acoustic reflex is elicited by loud sounds, typically at 70–100 dB HL (approximately 70–90 dB above the hearing threshold). It primarily causes contraction of the stapedius muscle, which pulls the neck of the stapes posteriorly and stiffens the ossicular chain, thereby reducing the transmission of low-frequency sound energy to the cochlea. The tensor tympani muscle contributes minimally to this reflex but may contract during activities such as chewing and swallowing.

• Stapedius Muscle: The stapedius is the smallest skeletal muscle in the human body and is supplied by the facial nerve (CN VII). It contracts reflexly in response to loud sounds, stiffening the ossicular chain and reducing the transmission of low-frequency sounds by approximately 10–20 dB. Because the acoustic reflex has a short latency, it provides little protection against sudden impulse noises such as gunshots or explosions.
• Tensor Tympani Muscle: The tensor tympani is supplied by the mandibular division of the trigeminal nerve (CN V3). It inserts into the handle of the malleus and pulls it medially, tightening the tympanic membrane. It may contract during chewing, swallowing, and tactile stimulation of the external auditory canal, but contributes minimally to the acoustic reflex.

Advantages:

• It prevents cochlear damage from prolonged exposure to loud music or occupational noise (e.g., jet aircraft).
• It selectively masks low-frequency environmental sounds, allowing us to concentrate on higher-frequency sounds (above 1000 Hz), which are crucial for speech understanding.
• It reduces the intensity of sounds generated intrinsically, such as during vocalisation or chewing, preventing these internal noises from overwhelming our hearing.

3. Phase Differential Between Cochlear Windows:

The Principle: The perilymph and endolymph of the inner ear are almost incompressible. Therefore, hearing depends on the creation of a pressure difference across the cochlear partition (basilar membrane) rather than on compression of the cochlear fluids. The oval window and round window create this pressure difference by moving in opposite phases.

Mechanism: When the stapes footplate moves inwards at the oval window, it generates a pressure wave in the perilymph of the scala vestibuli. As the cochlear fluids cannot be compressed, the displaced fluid pushes the round window membrane outwards into the middle ear cavity. When the stapes moves outwards, the round window moves inwards. These reciprocal movements create a pressure gradient between the scala vestibuli and scala tympani across the basilar membrane. This pressure gradient produces a travelling wave along the basilar membrane, which bends the stereocilia of the hair cells and initiates auditory transduction.

If the same sound pressure reaches both the oval and round windows simultaneously, the pressure difference across the basilar membrane decreases markedly. As a result, the basilar membrane shows little movement, the hair cells receive minimal stimulation, and hearing becomes significantly impaired.

Contribution to Hearing: The tympano-ossicular system maintains this phase difference by directing most sound energy to the oval window, while the round window acts as a flexible pressure-release membrane. Experimental studies suggest that this mechanism improves sound transmission by approximately 4 dB in a normal middle ear.

Mechanisms That Maintain the Phase Differential

• Ossicular Coupling Effect: The ossicular chain transmits sound mainly to the oval window, thereby minimising direct stimulation of the round window.
• Acoustic Coupling Effect: The air-filled middle ear cavity acoustically isolates the round window from direct sound waves, helping maintain the pressure difference between the oval and round windows.

4. Natural Resonance of External and Middle Ear:

Concept: The external and middle ear, owing to their inherent anatomical and physiological properties, preferentially transmit certain frequencies of sound to the inner ear. This phenomenon is known as natural resonance.

Resonant Frequencies:

• External Auditory Canal: Resonates optimally around 3000 Hz.
• Tympanic Membrane: Resonates efficiently between 800–1600 Hz.
• Middle Ear Cavity: Has a natural resonance of approximately 800 -1200 Hz.
• Ossicular Chain: Resonates optimally between 500–2000 Hz. 

Overall Impact: Due to these combined resonant characteristics, the human auditory system exhibits its greatest sensitivity and efficient sound transmission between 500 and 3000 Hz. This frequency range is crucial as it encompasses most speech and daily conversation sounds.

Minimum Audibility Curve (Audiogram): The minimum audibility curve, or audiogram, graphically depicts the lowest sound intensity (in dB HL, Hearing Level) that a person can hear at various frequencies. It illustrates that:

• The amplification of sound intensity is highest between 1000 and 3000 Hz, reflecting the combined resonance of the ear’s conductive components.
• Sounds below 20 Hz (infrasound) or above 20,000 Hz (ultrasound) are typically not amplified or perceived by the human ear. This is why the human ear can generally perceive the pitch of sound only between 20 Hz and 20,000 Hz, with maximum sensitivity occurring between 1000 and 3000 Hz.

Travelling Wave Theory and Tonotopic Organisation

The inward movement of the stapes footplate at the oval window generates a travelling wave within the cochlear fluids. This wave propagates along the basilar membrane from the base of the cochlea towards the apex.

The basilar membrane is not uniform throughout its length. The basal turn is narrow, thick, and stiff, whereas the apical turn is broader, thinner, and more flexible. Consequently, different sound frequencies produce maximum vibration at different locations along the basilar membrane.

• High-frequency sounds produce maximal vibration at the base of the cochlea.
• Low-frequency sounds produce maximal vibration near the apex of the cochlea.

This orderly arrangement of frequencies is known as tonotopic organisation and forms the physiological basis for frequency discrimination.

Clinical Importance

• Noise-induced hearing loss initially affects the basal turn, producing a characteristic 4000 Hz notch on pure-tone audiometry.
• Presbycusis commonly begins with degeneration of hair cells in the basal turn, resulting in high-frequency hearing loss.
• Cochlear implants preserve tonotopic organisation by stimulating different regions of the cochlea according to sound frequency.

2. Cochlear Transduction (Conversion of Mechanical Energy into Electrical Impulses)

The process of converting mechanical energy (vibrations) into electrical nerve impulses occurs within the hair cells of the organ of Corti, located in the cochlea. This critical step is known as auditory transduction.

The process occurs in the following sequence:

• Step 1. Movement of the Stapes Footplate: The stapes footplate is intimately connected to the oval window, which is in direct contact with the scala vestibuli, filled with perilymph. Movements of the stapes footplate create pressure changes in the perilymph of the scala vestibuli.
• Step 2. Movement of the Basilar Membrane: These pressure waves travel through the cochlear fluids and produce a travelling wave along the basilar membrane. As the basilar membrane moves, it slides against the relatively stationary tectorial membrane, creating a shearing force.
• Step 3. Bending of the Stereocilia: This differential movement generates shearing forces that distort and bend the stereocilia (hairs) of the hair cells. Bending towards the tallest stereocilium opens mechanically gated ion channels, and bending away from the tallest stereocilium closes these channels.
• Step 4. Hair Cell Depolarisation: When the stereocilia bend towards the tallest stereocilium, mechanically gated ion channels open. Potassium (K⁺) and a small amount of calcium (Ca²⁺) enter the hair cell from the potassium-rich endolymph. The stria vascularis maintains the high potassium concentration of the endolymph (approximately 150 mEq/L) and the positive endocochlear potential. The influx of potassium depolarises the hair cell, producing a receptor potential. During depolarisation, the resting membrane potential changes from approximately −60 mV to −50 mV. When the stereocilia bend away from the tallest stereocilium, the ion channels close, potassium entry decreases, and the hair cell hyperpolarises. Bending the stereocilia towards the tallest stereocilium causes depolarisation, whereas bending them away causes hyperpolarisation.
• Step 5. Neurotransmitter Release: Depolarisation opens voltage-gated calcium channels at the base of the hair cell. Calcium influx triggers the release of glutamate onto the afferent fibres of the cochlear nerve.
• Step 6. Generation of Action Potentials:Glutamate stimulates the afferent fibres of the cochlear nerve, generating action potentials. These impulses travel through the central auditory pathway to the auditory cortex, where the brain perceives them as sound.

 

Electrical Responses of Hair Cells (Cochlear Potentials)

Four distinct electrical potentials can be recorded from the ear:

Potential	Nature	Location Recorded	Characteristics	Clinical Significance
Endocochlear Potential	DC	Scala Media	+80 mV at rest; maintained by stria vascularis (K+ pump)	Provides energy for transduction
Cochlear Microphonic (CM)	AC	Extracellularly (e.g., Scala Media vs. Tympani)	Mirrors sound waveform; sum of hair cell receptor potentials; graded, non-propagated	Assesses hair cell function (ECoG)
Summating Potential (SP)	DC	Superimposed on AP	Sustained DC shift; hair cell receptor potential; graded, non-propagated	Elevated SP/AP ratio in Meniere’s disease (ECoG)
Compound Action Potential (CAP)	All-or-none	Auditory Nerve Fibres	Synchronous firing of 1st order neurons; propagated	Used in electrocochleography (ECoG) and contributes to Wave I of auditory brainstem response (ABR).

Key Differences from Action Potentials: Both Cochlear Microphonics and Summating Potentials are receptor potentials. They are graded (not all-or-none), not propagated along nerve fibres, have no refractory period, and have no latency (occur instantaneously with stimulus).

3. Conduction of Electrical Impulses to the Brain (Neural Pathways)

Once the hair cells convert mechanical vibrations into electrical signals, these impulses are transmitted through the auditory pathway to the cerebral cortex, where sound is perceived and interpreted.

Auditory Pathway

Hair cells → Spiral ganglion → Cochlear nerve (CN VIII) → Cochlear nuclei → Superior olivary complex → Lateral lemniscus → Inferior colliculus → Medial geniculate body (thalamus) → Primary auditory cortex (Heschl’s gyrus, Brodmann areas 41 & 42)

Mnemonics for Auditory Pathway: “E COLI-MA”

• Eighth nerve (Auditory/Vestibulocochlear Nerve)
• Cochlear Nucleus (in medulla)
• Olivary complex (Superior Olivary Nucleus in pons)
• Lateral Lemniscus (tract in pons/midbrain)
• Inferior Colliculus (in midbrain)
• Medial Geniculate Body (in thalamus)
• Auditory Cortex (in temporal lobe)

Detailed Neural Transmission within the Auditory Pathways:

1. Bipolar cells of Spiral Ganglion (First-order Neurons): The bipolar neurons of the spiral ganglion receive impulses from the hair cells. Their axons form the cochlear division of the vestibulocochlear nerve (CN VIII), which terminates in the dorsal and ventral cochlear nuclei at the pontomedullary junction.
2. Cochlear Nuclei: From the cochlear nuclei, many fibres cross to the opposite side through the trapezoid body, while others remain uncrossed. Consequently, auditory information is represented bilaterally in the central auditory pathway above the level of the cochlear nuclei.
3. Superior Olivary Complex: The superior olivary complex, located in the pons, is the first site receiving input from both ears and plays a key role in sound localisation by analysing differences in the timing and intensity of sounds reaching the two ears.
4. Lateral Lemniscus: Fibres ascend through the lateral lemniscus, the principal ascending auditory tract of the brainstem. Some fibres synapse in the nucleus of the lateral lemniscus.
5. Inferior Colliculus: The fibres terminate in the inferior colliculus of the midbrain, an important auditory relay and reflex centre involved in integrating auditory information.
6. Medial Geniculate Body: Neurons from the inferior colliculus project to the medial geniculate body (MGB) of the thalamus, the final subcortical relay station.
7. Auditory Cortex: From the MGB, auditory radiations project to the primary auditory cortex (Brodmann areas 41 and 42) in Heschl’s gyrus of the superior temporal lobe, where sound is consciously perceived. The surrounding auditory association cortex (mainly Brodmann area 22) interprets complex sounds such as speech and music.

Table: Auditory Pathway Relay Stations (E COLI-MA)

Neuron Order	Relay Station	Location	Key Function
1st Order	Spiral Ganglion Cells	Modiolus (Cochlea)	Innervate hair cells; Axons form Cochlear Nerve (part of CN VIII); Terminate in Cochlear Nuclei.
2nd Order	Cochlear Nuclei	Medulla Oblongata	Receive input from 1st order neurons; Begin processing of sound features.
2nd Order	Superior Olivary Complex	Pons	Crucial for sound localization (interaural time/intensity differences); Receives input from both cochlear nuclei.
3rd Order	Nucleus of Lateral Lemniscus	Pons/Midbrain	Ascends through Lateral Lemniscus; Some fibers from Cochlear Nuclei may bypass SOC and go directly here.
3rd Order	Inferior Colliculus	Midbrain	Major integrative center; Involved in auditory reflexes, sound localization, and frequency processing.
4th Order	Medial Geniculate Body (MGB)	Thalamus (Ventral part)	Final subcortical relay; Projects to auditory cortex; Involved in attention to sound.
5th Order	Auditory Cortex	Temporal Lobe (Brodmann’s areas 41, 42, 22, 21, 20)	Conscious perception of sound (pitch, loudness); Interpretation of complex sounds (speech, music, Wernicke’s area).

Neural Processing of Auditory Information

The human auditory system is remarkably sophisticated, capable of detecting sounds within the frequency range of 20-20,000 Hz. Furthermore, it can perform complex analyses of sound attributes:

Sound Intensity Encoding: Sound intensity (loudness) is encoded at the level of the cochlear nerve fibres. This involves various mechanisms, including the rate of firing of individual nerve fibres and the number of active fibres, allowing higher auditory centres to accurately perceive loudness.

Sound Localisation: The auditory system possesses an impressive ability to distinguish sounds from sources separated by as little as 1 degree. It primarily achieves this by utilising two key cues:

• Interaural Time Difference (ITD): For low-frequency sounds (below 3000 Hz), the brain detects minute time lags between the sound arriving at each ear. If a sound originates from the right, it reaches the right ear slightly before the left ear.
• Interaural Intensity Difference (IID): For high-frequency sounds (above 3000 Hz), the head acts as an acoustic shadow, causing the sound to be slightly louder in the ear closer to the source.
• Clinical Correlation: Lesions in the auditory cortex can significantly disrupt a person’s ability to localise sounds, even if their basic hearing thresholds remain relatively intact.

Key Terminologies

• Amplitude: Amplitude is the magnitude of pressure change in a sound wave. 
• Sound intensity is the amount of sound energy transmitted per unit area per second and is measured in watts per square metre (W/m²). It is the principal physical determinant of the perceived loudness of sound. Sound intensity level is expressed in decibels (dB), a logarithmic unit that represents the ratio of the measured sound intensity (I) to a standard reference intensity (I₀ = 10⁻¹² W/m²): L = 10 log₁₀ (I / I₀) dB​

  Because the decibel scale is logarithmic:

  • A 10-fold increase in sound intensity equals 10 dB.
  • A 100-fold increase equals 20 dB.
  • A 1000-fold increase equals 30 dB.

  0 dB HL represents the average threshold of hearing in young adults with normal hearing and does not indicate the absence of sound.

  Typical sound levels include:

  • Whisper: 20–30 dB
  • Normal conversation: 55–65 dB
  • Shouting: 85–95 dB
  • Threshold of discomfort: 110–120 dB
  • Threshold of pain: 120–130 dB

• Loudness: Loudness is the subjective perception of sound intensity. It depends primarily on sound intensity but is also influenced by frequency and the listener’s hearing sensitivity.
• Frequency: Frequency is the number of sound wave cycles produced per second and is measured in hertz (Hz). The normal human hearing range is approximately 20–20,000 Hz. The frequency range most important for speech understanding is 250–8000 Hz, with maximum intelligibility between 500 and 4000 Hz.
• Pitch: Pitch is the subjective perception of the frequency of a sound. Higher frequencies produce a higher pitch. The average fundamental frequency is approximately 120 Hz in adult males and 220–250 Hz in adult females.
• Acoustic impedance (Z = ρ × c): It is the opposition of a medium to sound propagation, determined by its density and sound speed. Liquids have higher impedance than air due to greater density and incompressibility. When sound encounters an air-fluid boundary, most energy is reflected due to impedance mismatch. In the ear, this mismatch is overcome by the middle ear’s impedance-matching mechanism—the tympanic membrane and ossicles transform low-pressure air vibrations into high-pressure fluid vibrations, enabling efficient transmission into the cochlea.
• Pure Tone: A pure tone consists of a single frequency (e.g., 250, 500, or 1000 Hz). Pure tone audiometry (PTA) measures hearing thresholds at frequencies ranging from 125 to 8000 Hz.
• Complex Sound: A complex sound consists of multiple frequencies. Human speech and most environmental sounds are complex sounds.
• Sound Pressure: Sound pressure is the variation in air pressure produced by a sound wave and is usually expressed as sound pressure level (SPL) in decibels (dB SPL).
• Overtones (Harmonics): The fundamental frequency is the lowest frequency produced by a vibrating source. Higher frequencies that are integer multiples of the fundamental frequency are called harmonics (overtones). Harmonics determine the quality (timbre) of a sound, allowing us to distinguish different voices or musical instruments producing the same pitch.

———— End of the chapter ————

Mechanism of Hearing in One Minute

1. Sound waves enter the external auditory canal.
2. The tympanic membrane vibrates.
3. The ossicles amplify sound by impedance matching.
4. The stapes moves the oval window.
5. A travelling wave forms in the cochlea.
6. Hair cell stereocilia bend.
7. Potassium enters the hair cells, causing depolarisation.
8. Glutamate is released.
9. The cochlear nerve generates action potentials.
10. The auditory cortex perceives sound.

High-Yield Points for Quick Revision

• Hearing Mechanism Stages: Mechanical conduction (external/middle ear), Transduction (cochlea), Neural conduction (auditory pathway).
• Sound Transmission Loss: ~30 dB lost when sound moves from air to liquid, overcome by impedance matching.
• Inner hair cells (≈3,500): Primary sensory receptors responsible for hearing.
• Outer hair cells (≈12,000): Cochlear amplifier that enhances sensitivity and frequency selectivity.
• Impedance Matching Components: Lever action of ossicles (1.3x), Hydraulic action of tympanic membrane (17x effective ratio).
• Acoustic Reflex (Acoustic Reflex): Protective, triggered by loud sounds (>70-80 dB HT), involves tensor tympani and stapedius.
• Acoustic Reflex Latency: ~40 ms, provides protection against prolonged loud sounds, not sudden impulses.
• Phase Differential: Oval and round windows vibrate out of phase, crucial for cochlear fluid movement (adds ~4 dB).
• Resonant Frequencies: EAC (~3000 Hz), Tympanic membrane (~800-1600 Hz), Middle ear (~800 Hz), Ossicular chain (~500-2000 Hz).
• Human Hearing Range: 20 Hz – 20,000 Hz; maximum sensitivity 1000-3000 Hz.
• Auditory Transduction: Mechanical bending of stereocilia opens K+ channels, leading to depolarisation.
• Endocochlear Potential: +80mV DC potential in scala media, maintained by stria vascularis (K+ pump), provides driving force for transduction.
• Cochlear Microphonic (CM): AC potential, mirrors sound waveform, sum of hair cell receptor potentials.
• Summating Potential (SP): DC potential, hair cell receptor potential, elevated SP/AP ratio is diagnostic for Meniere’s disease.
• Compound Action Potential (CAP): All-or-none response of auditory nerve fibres (first-order neurons).
• Auditory Pathway Mnemonic: E COLI-MA (Eighth nerve, Cochlear nucleus, Olivary complex, Lateral lemniscus, Inferior colliculus, Medial geniculate body, Auditory cortex).
• Sound Localisation Cues: Interaural Time Difference (ITD) for low frequencies, Interaural Intensity Difference (IID) for high frequencies.
• Superior Olivary Complex: Key brainstem nucleus for sound localisation.
• Medial Geniculate Body (MGB): Thalamic relay for auditory information.
• Primary Auditory Cortex: Brodmann’s areas 41, 42 in the temporal lobe (Heschl’s gyri).

MCQs: Physiology of Hearing 

1. What is the primary function of the external auditory meatus in hearing? A. Amplify sound intensity B. Conduct sound waves to the tympanic membrane C. Localise sound source D. Protect the inner ear
2. What is the speed of sound in air at 20°C and sea level? A. 300 m/s B. 344 m/s C. 400 m/s D. 500 m/s
3. Which mechanism primarily addresses the 99.9% sound energy reflection at the air-liquid interface? A. Acoustic reflex B. Impedance matching C. Phase differential D. Natural resonance
4. What is the total amplification factor of the middle ear due to lever and hydraulic actions, as per the primary text? A. 14:1 B. 17:1 C. 18:1 D. 22
5. Which muscle contracts during the acoustic reflex to restrict ossicular movement? A. Tensor veli palatini B. Stapedius C. Levator tympani D. Mylohyoid
6. What is the human auditory system’s frequency detection range? A. 10–10,000 Hz B. 20–20,000 Hz C. 50–50,000 Hz D. 100–100,000 Hz
7. In auditory transduction, what causes depolarisation of hair cells? A. Oval window vibration B. Stereocilia bending C. Round window movement D. Perilymph flow
8. Which potential is used diagnostically for Meniere’s disease? A. Endo-cochlear potential B. Cochlear microphonic C. Summating potential D. Compound action potential
9. According to the E COLI-MA mnemonic, which structure follows the cochlear nucleus in the auditory pathway? A. Eighth nerve B. Olivary complex C. Inferior colliculus D. Medial geniculate body
10. What is the natural resonance frequency of the ossicular chain? A. 800 Hz B. 1600 Hz C. 3000 Hz D. 500–2000 Hz
11. Clinical Scenario: A 28-year-old male presents with sudden sensorineural hearing loss after exposure to a loud explosion at a construction site. Which mechanism’s failure likely contributed to his cochlear damage? A. Impedance matching B. Acoustic reflex C. Phase differential D. Auditory transduction
12. Clinical Scenario: A 45-year-old female reports episodic vertigo, tinnitus, and hearing loss. Electrocochleography reveals an elevated summating potential. What is the most likely diagnosis? A. Otosclerosis B. Meniere’s disease C. Acoustic neuroma D. Presbycusis
13. Clinical Scenario: A 35-year-old male with chronic otitis media presents with conductive hearing loss. Which middle ear component is most likely affected, disrupting sound conduction? A. Pinna B. Ossicular chain C. Cochlea D. Semicircular canals
14. Clinical Scenario: A patient’s otoscopy shows a non-vibrating tympanic membrane during loud sound exposure, suggesting acoustic reflex dysfunction. Which muscle is primarily implicated? A. Tensor tympani B. Sternocleidomastoid C. Masseter D. Buccinator
15. Clinical Scenario: A 30-year-old female reports difficulty localising sounds in crowded places. An MRI reveals a temporal lobe lesion. Which cortical area is most likely affected? A. Brodmann’s area 41 B. Brodmann’s area 17 C. Brodmann’s area 4 D. Brodmann’s area 8
16. Clinical Scenario: A 40-year-old male undergoes pure tone audiometry, revealing a hearing threshold elevation at 4000 Hz. What is the most likely underlying cause? A. External auditory canal obstruction B. Ossicular chain fixation C. Cochlear hair cell damage D. Auditory cortex lesion
17. Clinical Scenario: A 55-year-old factory worker presents with difficulty understanding speech in noisy environments. Which frequency range, critical for conversation, is most likely affected? A. 100–500 Hz B. 1000–3000 Hz C. 5000–8000 Hz D. 10,000–15,000 Hz
18. Clinical Scenario: A patient with suspected ossicular discontinuity is evaluated. What is the expected effect on sound transmission to the cochlea? A. Increased sound pressure B. Reduced oval window pressure C. Enhanced phase differential D. Unaltered transduction
19. Clinical Scenario: A 32-year-old female with tinnitus and unilateral hearing loss shows an abnormal compound action potential on testing. Which structure is most likely affected? A. Tympanic membrane B. Auditory nerve C. External auditory meatus D. Tectorial membrane
20. Clinical Scenario: During stapedectomy, an ENT surgeon notes a fixed stapes footplate. Which hearing mechanism is primarily disrupted? A. Acoustic reflex B. Impedance matching C. Neural conduction D. Natural resonance
21. Which of the following contributes to the phase differential between the oval and round windows? A. Tensor tympani contraction B. Ossicular coupling C. Cochlear microphonic potential D. Endo-cochlear potential
22. A patient with noise-induced hearing loss shows a dip at 4000 Hz on audiometry. Which structure is primarily damaged? A. Tympanic membrane B. Ossicular chain C. Organ of Corti D. Auditory cortex
23. The auditory cortex is located in which Brodmann’s area? A. Area 17 B. Area 41 C. Area 4 D. Area 8
24. Which of the following potentials is a graded, non-propagated response used in diagnosing Meniere’s disease? A. Compound action potential B. Endo-cochlear potential C. Summating potential D. Action potential
25. A patient with conductive hearing loss has a disrupted ossicular chain. What is the primary mechanism affected? A. Auditory transduction B. Impedance matching C. Neural conduction D. Sound localisation

Answers

1: B. 2: B. 3: B. 4: D. 5: B. 6: B. 7: B. 8: C. 9: B. 10: D. 11: B. 12: B. 13: B. 14: A. 15: A. 16: C. 17: B. 18: B. 19: B. 20: B. 21: B. 22: C. 23: B. 24: C. 25: B.

FAQ’s in Viva

1. Q: What is impedance matching in hearing? A: Impedance matching is the process by which the middle ear, through the hydraulic action of the tympanic membrane and the lever action of the ossicles, amplifies sound pressure to overcome the energy loss that occurs when sound travels from air to the fluid-filled inner ear.
2. Q: What is the function of the acoustic reflex? A: The acoustic reflex is a protective reflex involving the contraction of the middle ear muscles (tensor tympani and stapedius) in response to loud sounds, thereby stiffening the ossicular chain and reducing sound transmission to the inner ear, thus protecting the cochlea.
3. Q: Which structures are involved in sound localisation? A: Sound localisation primarily involves the superior olivary complex in the brainstem, which analyses interaural time differences (for low frequencies) and interaural intensity differences (for high frequencies) between the two ears.
4. Q: What is the endocochlear potential? A: The endocochlear potential is a stable positive direct current (+80 mV) within the endolymph of the scala media, maintained by the stria vascularis, which provides the electrochemical gradient necessary for auditory transduction.
5. Q: What is the significance of an elevated SP/AP ratio in ECoG? A: An elevated Summating Potential (SP) to Compound Action Potential (AP) ratio in electrocochleography (ECoG) is a key diagnostic indicator for Meniere’s disease, reflecting endolymphatic hydrops.
6. Q: Name the main relay stations in the auditory pathway to the brain. A: The main relay stations in the auditory pathway are the cochlear nuclei, superior olivary complex, inferior colliculus, medial geniculate body (thalamus), and finally the auditory cortex in the temporal lobe.
7. Q: Why do sound waves not travel efficiently from air to liquid? A: Sound waves are largely reflected when moving from air to liquid because liquids have a much higher acoustic impedance (resistance to sound propagation) compared to air, leading to significant energy loss at the interface.

———— End ————

Reference Textbooks.

• Scott-Brown, Textbook of Otorhinolaryngology-Head and Neck Surgery.
• Glasscock-Shambaugh, Textbook of Surgery of the Ear.
• P L Dhingra, Textbook of Diseases of Ear, Nose and Throat.
• Hazarika P, Textbook of Ear Nose Throat And Head Neck Surgery Clinical Practical.
• Mohan Bansal, Textbook of Diseases of Ear, Nose and Throat Head and Neck Surgery
• Hans Behrbohm, Textbook of Ear, Nose, and Throat Diseases With Head and Neck Surgery.
• Salah Mansour, Middle Ear Diseases – Advances in Diagnosis and Management.
• Logan Turner, Textbook of Diseases of The Nose, Throat and Ear Head And Neck Surgery.
• Rob and smith, Textbook of Operative surgery.
• Anirban Biswas, Textbook of Clinical Audio-vestibulometry.
• Arnold, U. Ganzer, Textbook of  Otorhinolaryngology, Head and Neck Surgery.

Author:

Dr. Rahul Bagla
MBBS (MAMC, Delhi) MS ENT (UCMS, Delhi)
Fellow Rhinoplasty & Facial Plastic Surgery.
Renowned Teaching Faculty
Mail: msrahulbagla@gmail.com
India

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Keywords: PPT Free Download, Mechanism, Flowchart, Diagram, Notes, Auditory physiology, Cochlear function, Basilar membrane movement, Sound transduction, Hair cell depolarization, Auditory neural pathway, Endolymphatic potential, Cochlear microphonics, Tonotopic organization, Middle ear amplification, Oval window vibration, Inner ear hair cells, Otolithic membrane, Stereocilia deflection, Auditory nerve action potential, Brainstem auditory processing, Temporal coding in hearing, Physiology of Hearing