Role of the External and Middle Ear in the Physiology of Hearing

Introduction

I. THE EXTERNAL EAR

1. Components

• The pinna (auricle)
• The external auditory meatus (EAM)
• The external auditory canal (EAC) - approximately 25-26 mm long in adults
• The outer surface of the tympanic membrane

2. The Pinna

• The unique morphology of the pinna and its position on the craniofacial skeleton transform incoming sounds with delay paths that depend on the direction of the sound source
• This creates head-related transfer functions (HRTFs) - frequency and direction-dependent modifications that the auditory cortex uses to localize sound in the vertical plane (elevation) and to distinguish front from back
• Interaural time differences (ITD) and interaural level differences (ILD) are both influenced by pinna morphology

• When coupled with the EAM and EAC, the pinna-canal unit provides frequency-specific resonance that helps compensate for the impedance mismatch at the air-fluid interface
• This is a preparatory gain mechanism before sound reaches the middle ear

• A dysmorphic pinna may reflect underlying EAC and middle ear dysmorphy (developmental co-origin in utero)
• Pinna deformities demand audiovestibular workup - in Down syndrome, 3.5 pinna defects per ear are found on average (vs. 2.5 in non-Down), all associated with conductive hearing loss
• Pinna landmarks (antihelix, concha) are anchors for behind-the-ear hearing aid retention; dysmorphic pinnae may preclude standard fitting

3. External Auditory Canal (EAC)

• An outer cartilaginous third (contains sebaceous and ceruminous glands)
• An inner bony two-thirds
• Two constrictions: at the cartilage-bone junction, and 5 mm before the tympanic membrane (the isthmus)

Resonant frequency ≈ 340 / (4 × 0.025) = ~3,400 Hz (~3.4 kHz)

• Complete occlusion of the EAC by impacted wax causes up to 40 dB conductive hearing loss, with an initial high-frequency shift starting around 2 kHz (loss of resonant gain)
• In occlusive hearing aid moulds, the lost resonant gain must be incorporated into the prescription
• Cerumen serves non-acoustic functions: lubrication, antimicrobial protection, lateral migration of dead skin

II. THE MIDDLE EAR

1. Components

• Tympanic membrane (TM)
• Ossicular chain (malleus, incus, stapes)
• Middle ear muscles (tensor tympani, stapedius)
• Eustachian tube
• Oval window and round window
• Middle ear ligaments and mucosal folds

2. The Air-Fluid Impedance Mismatch Problem

• Impedance of air: ~415 Pa·s/m
• Impedance of cochlear fluid (water-like): ~1.5 × 10⁶ Pa·s/m

3. The Middle Ear Transformer Mechanism

A. Hydraulic (Area) Ratio - The Principal Mechanism

• Effective vibrating area of TM: ~55 mm² (Bekesy calculated effective area ~55 mm²)
• Area of stapes footplate: ~3.2 mm²
• Area ratio = ~17:1 (Scott-Brown's gives 18:1; Shambaugh gives 17:1)

Pressure gain = 17-18× = ~24-25 dB

B. Ossicular Lever Ratio

• Handle of the malleus (manubrium) is longer than the long process of the incus
• Lever ratio = 1.3:1 (Bekesy's calculation)
• This provides an additional ~2 dB gain

C. Buckling (Curved Membrane) Effect of the Tympanic Membrane

Note: The buckling effect is debated in the literature and not uniformly quantified, but is accepted as a contributor.

4. Total Transformer Ratio

"Bekesy calculated the effective vibrating surface of TM area compared with stapes footplate area to be 17:1 and the lever effect of the ossicular chain 1.3:1. The 17:1 hydraulic ratio × the 1.3 lever ratio yields a total increase of pressure at the oval window of 22 times, termed the sound-pressure transformer ratio of the normal human ear and equates to approximately 27 dB gain."

• Shambaugh Surgery of the Ear

5. The Tympanic Membrane

• The TM has two parts: the pars tensa (tight, fibrous, the primary vibrating portion) and the pars flaccida (Shrapnell's membrane, less tight)
• The outer (canal) surface shows a uniphasic response with little pressure variation
• The inner (middle ear) surface shows a multiphasic response with significant pressure variation - due to the non-uniform structure providing multiple integrated multilever mechanical advantage
• A travelling wave is set up in the TM, mainly collected at the rim and conducted to the umbo, then coupled to the manubrium of the malleus
• At low frequencies (<1 kHz): membrane transfers energy by uniform movement to the malleus
• At high frequencies: movement is more complex, with partial shunting by the middle ear
• Reflectance of the membrane is high at low frequencies (<1 kHz) and lowest between 1-4 kHz - meaning maximum energy delivery to the cochlea occurs in this speech-critical range
• Above 15 kHz, reflectance again rises toward total, suggesting the middle ear limits sound propagation at very high frequencies

6. The Ossicular Chain

• Incudomalleal joint: saddle joint, rigid at speech frequencies, allowing malleus and incus to move as a unit
• Incudostapedial joint: lenticular process of incus + capitulum of stapes; allows some independent stapes movement

• The ossicles act as a series of coupled levers transmitting vibrations from the TM to the oval window
• The manubrium of the malleus and the long process of the incus have different lengths, constituting the lever system

7. The Middle Ear Space and Pressure Regulation

• Stiffness of the middle ear system (determined by TM, ossicular ligaments, air column compression): limits low-frequency transmission (stiffness-dominated below resonance)
• Mass of the system (dominated by ossicles): limits high-frequency transmission (mass-dominated above resonance)
• Maximum energy transfer between 1-3 kHz (resonant frequency range)

• Maintains air pressure equalization between middle ear and nasopharynx
• Tubal opening is best demonstrated at 6-8 kHz (high-frequency sounds), evolutionarily appropriate since consonant-containing speech sounds are in the high-frequency range
• Eustachian tube dysfunction (e.g., early OME): deprivation of air increases stiffness → low-frequency conductive hearing loss
• Progression to fluid (glue ear): increased mass → high-frequency hearing loss

8. The Middle Ear Muscles

A. Tensor Tympani

• Innervation: Trigeminal nerve (V₃ - medial pterygoid nerve)
• Attachment: To the handle (manubrium) of the malleus
• Action: Pulls malleus anteriorly, tensing the TM
• Activation: Low electrical activity in response to sound; primarily active with tactile EAC/facial stimulation, swallowing, anticipation of loud sounds, startle (all involve bony skull movement that can stimulate the cochlea)
• Disorder: "Tensor tympani syndrome" - audible thump in ear (rare)

B. Stapedius

• Innervation: Facial nerve (VII)
• Attachment: To the neck of the stapes
• Action: Pulls annular ligament of the footplate; stiffens the ossicular chain
• Activation: Primarily responds to high-intensity, low-frequency sounds (~0.8 kHz); bilateral reflex even with unilateral stimulation

• Reflex arc: auditory stimulus → cochlear nerve → cochlear nuclei → superior olivary complex → facial motor nucleus → stapedius → stiffens ossicular chain
• Moves stapes ~50 micrometers
• Increases impedance of the middle ear system
• Attenuates sound reaching the cochlea, especially at low frequencies, by 10-15 dB
• Effectively protects the cochlea from loud, low-frequency sounds and preserves perception of high-frequency consonant-containing speech in a noisy environment
• Pre-vocalization (efferent) reflex: both muscles contract before speech to protect the ear from one's own voice (bone-conducted stimulation via the skull)
• Latency is ~25-150 ms; therefore the reflex cannot protect against sudden impulse noise (e.g., gunshot)
• The reflex is abolished in: middle ear disease, facial nerve palsy (ipsilateral), auditory neuropathy

• Presence/absence indicates integrity of the reflex arc
• Useful to distinguish middle ear disease from sensorineural/neural pathology
• Preserved in cochlear SNHL (reflex usually present, may be at lower SL - "reflex decay" in retrocochlear pathology)

9. The Middle Ear Windows

A. Oval Window

• Sealed by the stapes footplate and annular ligament
• The primary site of preferential sound energy delivery to the cochlea
• At higher frequencies, the middle ear cavity occasionally shunts this preference

B. Round Window

• Covered by the secondary tympanic membrane (flexible)
• Functions as a pressure relief valve: when the stapes footplate moves inward (via the oval window), the round window membrane moves outward
• This out-of-phase movement is essential for the cochlear travelling wave - cochlear fluid is incompressible, so fluid displacement at the oval window must be accommodated by an equal displacement at the round window
• The pressure difference between oval and round windows drives cochlear fluid movement and is the essential stimulus for inner ear transduction
• The intact tympanic membrane protects the round window from sound reaching it directly in phase with the oval window (which would cancel the pressure differential and abolish hearing)

C. "Third Window" Concept

• Virtual third windows (vestibular aqueduct, bony skull, ossicular inertia) can shunt some energy from the middle ear
• Pathological third windows (e.g., superior semicircular canal dehiscence, perilymph fistula, dilated vestibular aqueduct): absorb or shunt preferential oval window energy, producing:
  • Decreased air-conduction thresholds
  • Improved (paradoxically enhanced) bone-conduction thresholds
  • A "false" or spurious air-bone gap on audiometry

III. SUMMARY - THE COMPLETE SOUND PATHWAY

Airborne sound wave
       ↓
Pinna (localization, frequency-specific resonance, HRTF)
       ↓
External Auditory Canal (resonance at ~3 kHz, +10-15 dB gain)
       ↓
Tympanic membrane (vibrates; multilever buckling; area 55 mm²)
       ↓
Ossicular chain: Malleus → Incus → Stapes (lever ratio 1.3:1)
       ↓
Stapes footplate (area 3.2 mm²) → Oval window
    [Transformer ratio ~22:1 = ~27 dB gain]
       ↓
Cochlear fluid: pressure wave (oval window in, round window out)
       ↓
Basilar membrane travelling wave → Hair cell transduction
       ↓
VIII nerve → Auditory cortex

IV. APPLIED POINTS FOR PG ENT EXAM

• Scott-Brown's Otorhinolaryngology Head & Neck Surgery, 8th Ed. - "Applied Physiology of Hearing" (Soumit Dasgupta), pp. 619-625
• Shambaugh Surgery of the Ear - "Physiology of Middle Ear" (Block 1, pp. 78-80; Block 5, p. 492)
• Cummings Otolaryngology Head and Neck Surgery - Middle Ear Physiology section