Physiology of Hearing

Note: Scott Brown's Otolaryngology (8th edition) is not in the library collection. The content below is drawn from Cummings Otolaryngology Head and Neck Surgery (Chapter 128), which covers identical physiological principles and is the standard ENT reference for auditory physiology.

1. Sound and Its Measurement

dB = 20 × log₁₀ (P / Pᵣ)

2. Impedance

3. External Ear

• Concha resonant frequency: ~5300 Hz
• External auditory canal resonant frequency: ~3000 Hz

Sound Localization

1. Interaural time difference (ITD): The farther ear receives the sound later — important for low-frequency localization
2. Interaural amplitude difference (IAD): Amplified by the head shadow effect (head attenuates sound to the contralateral ear) — important for high-frequency localization

4. Middle Ear Mechanics

Ossicular Transmission (Air Conduction)

Impedance Matching

1. Area ratio: TM area (~60 mm²) >> stapes footplate area (~3.2 mm²) → ~18:1 pressure amplification
2. Lever ratio of the ossicles (malleus:incus): ~1.3:1 mechanical advantage

Third Window Concept

5. Inner Ear — Cochlear Mechanics

Anatomy

• Scala vestibuli (perilymph) — communicates with the oval window
• Scala tympani (perilymph) — communicates with the round window
• Scala media / cochlear duct (endolymph, high K⁺) — contains the organ of Corti

Travelling Wave (von Békésy)

• Narrow and stiff at the base → responds best to high frequencies
• Wide and compliant at the apex → responds best to low frequencies

Active Amplification — The Cochlear Amplifier

• OHCs contract with depolarization and elongate with hyperpolarization via the motor protein prestin in their lateral wall
• This electromotility sharpens the tuning curve and amplifies basilar membrane motion by up to 40–60 dB at low sound levels

6. Hair Cell Transduction

Structure

• Inner hair cells (IHCs): ~3,500; single row; primary sensory cells
• Outer hair cells (OHCs): ~12,000; three rows; amplifier cells

Mechanoelectrical Transduction

1. Basilar membrane displacement → stereocilia deflect toward the tallest row → tip links stretch → mechanically-gated cation channels open
2. K⁺ and Ca²⁺ flood into the hair cell → depolarization
3. Depolarization triggers Ca²⁺-dependent exocytosis of glutamate at the IHC ribbon synapse → activates type I auditory nerve fibres
4. Deflection away from tallest row → tip links relax → channels close → hyperpolarization

IHC vs OHC Function

7. Auditory Nerve and Spiral Ganglion

• Spiral ganglion cells are bipolar first-order neurons — they send peripheral processes to IHCs and central processes to the cochlear nucleus
• Each auditory nerve fibre has a characteristic frequency (CF) — the frequency to which it is most sensitive
• Firing rate increases with sound intensity up to a saturation point
• The auditory nerve carries information about: (a) which fibres fire (place coding), (b) rate of firing, and (c) temporal/phase-locking patterns

8. Central Auditory Pathway

Cochlear Nucleus (Pons)

• First relay station for all ascending auditory information
• Receives input from spiral ganglion via the auditory nerve (CN VIII)
• Three divisions: anteroventral (AVCN), posteroventral (PVCN), dorsal (DCN)
• Sends three fibre tracts: dorsal, intermediate, and ventral acoustic stria → form the lateral lemniscus
• Tonotopic organization preserved

Superior Olivary Complex (SOC)

• Located medial to cochlear nucleus, in caudal pons
• First brainstem nucleus receiving bilateral input
• Key nuclei:
  • MSO (Medial Superior Olive): sensitive to interaural time differences (ITD) — low-frequency localization
  • LSO (Lateral Superior Olive): sensitive to interaural amplitude differences (IAD) — high-frequency localization; integrates excitatory ipsilateral + inhibitory contralateral inputs

Lateral Lemniscus

Inferior Colliculus (Midbrain)

• Processes frequency information along isofrequency laminae
• Integrates auditory, somatosensory, visual, and vestibular inputs
• Projects to medial geniculate body (~250,000 fibres — ~10× more than auditory nerve, reflecting extensive processing)

Medial Geniculate Body (Thalamus)

• Thalamic relay station
• Three divisions:
  • Ventral → primary auditory cortex
  • Dorsal → auditory association cortex
  • Medial → limbic structures

Auditory Cortex (Temporal Lobe)

• Primary auditory cortex (A1): Heschl's gyrus; Brodmann area 41; tonotopically organized (high frequencies medially, low frequencies laterally); processes complex auditory signals and language
• Auditory association cortex (A2): Brodmann areas 22 and 42; Wernicke's area — critical for speech perception; consonant perception deficits occur with direct electrical interference
• Additional projections to the amygdala (limbic system) explain emotional responses to music

9. Efferent Auditory System

Middle Ear Muscle Reflex (Stapedius Reflex)

• Loud sounds activate the stapedius muscle (via facial nerve) → stiffens the ossicular chain → reduces low-frequency transmission
• Bilateral reflex (loud sound in one ear causes bilateral stapedius contraction)
• Protects the cochlea from sustained loud sounds (but too slow to protect from sudden noise)

Medial Olivocochlear (MOC) Reflex

• MOC neurons in the SOC send efferent fibres via the vestibular nerve to synapse on OHC lateral walls
• Activation suppresses OHC electromotility → reduces cochlear amplification
• Functions: improves SNR in background noise, protects against noise-induced damage
• Pathway: sound → IHCs → cochlear nucleus (multipolar cells in PVCN) → MOC neurons in contralateral SOC → OHCs of ipsilateral cochlea

Summary: Sound Processing Flow

Sound waves
    ↓
External ear (pinna + EAC) → directs sound, enables localization
    ↓
Tympanic membrane vibrates
    ↓
Ossicular chain (malleus → incus → stapes) → impedance matching (~×30 pressure gain)
    ↓
Oval window → stapes footplate
    ↓
Cochlear fluids → travelling wave on basilar membrane (tonotopic)
    ↓
Organ of Corti → IHC stereocilia deflect → tip links → mechanically-gated channels open
    ↓
K⁺ influx → hair cell depolarization → glutamate release at ribbon synapse
    ↓
Auditory nerve (spiral ganglion type I fibres) → action potentials
    ↓
Cochlear nucleus → Superior olive → Lateral lemniscus → Inferior colliculus
    ↓
Medial geniculate body (thalamus)
    ↓
Primary auditory cortex (Heschl's gyrus) → Wernicke's area