Explain complete sound conduction through inners & middle ear
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Sound Conduction: Middle Ear to Inner Ear — Complete Pathway
1. External Ear (Sound Collection & Amplification)
- The pinna gathers sound from a 135° arc and increases sound pressure by ~6 dB
- The external auditory canal (EAC) acts as a closed cylinder (~2.5–3.5 cm long); its resonant frequency is ~3,500 Hz (lowered to ~2,700 Hz when the conchal component is included)
- Combined, external ear structures increase sound pressure at the tympanic membrane by 15–22 dB
— Cummings Otolaryngology, p. 299
2. Tympanic Membrane (TM) — Catenary Lever
- Sound waves strike the tympanic membrane (TM), which vibrates
- The TM is rigidly fixed at its periphery but mobile centrally — this geometry makes it act as a catenary lever, producing a ~2-fold gain in sound pressure at the malleus handle
- The thin pars flaccida has only a minor acoustic role in humans
3. Ossicular Chain — Mechanical Amplification (Middle Ear)
The three ossicles — malleus → incus → stapes — form a lever system:
| Lever System | Function | Gain |
|---|---|---|
| Catenary lever (TM shape) | Pressure concentration onto malleus handle | ~2× |
| Ossicular lever (malleus-incus unit) | Rotates around axis of anterior mallear ligament + incudal ligament | Small additional advantage |
| Combined catenary + ossicular lever ratio | 2.3:1 | ~7 dB |
| Hydraulic (areal) lever | TM area (~55 mm²) >> stapes footplate area (~3.2 mm²) | ~17:1 pressure gain |
Total middle ear impedance-matching gain: ~26–35 dB (centred around resonant frequency 0.9–1.0 kHz). Above 1 kHz, pressure gain at the stapes decreases at −8.6 dB/octave.
"The mean sound-pressure gain produced by the human middle ear is 26.6 dB and is centered around its resonant frequency."
— Cummings Otolaryngology, p. 301
4. Impedance Matching — Why It's Critical
Air has low acoustic impedance; cochlear fluid has high acoustic impedance. Without the middle ear's impedance-matching system, ~99.9% of sound energy would be reflected at the air-fluid interface (~30 dB loss). The combined mechanisms of:
- The large TM area → small oval window area ratio (hydraulic lever)
- The ossicular lever system
...enable efficient energy transfer from air to fluid.
— Costanzo Physiology, p. 98
5. Oval Window — Air/Fluid Interface
- The footplate of the stapes inserts into the oval window
- Inward movement of the stapes footplate displaces the perilymph of the scala vestibuli
- Because the cochlea is enclosed in bone, when the oval window pushes in, the round window must bulge outward (pressure relief)
6. The Cochlea — Structure
The cochlea is a fluid-filled spiral divided into three scalae:
| Compartment | Fluid | Location |
|---|---|---|
| Scala vestibuli | Perilymph (like ECF: high Na⁺, low K⁺) | Above Reissner's membrane |
| Scala media (cochlear duct) | Endolymph (like ICF: high K⁺, low Na⁺) | Between Reissner's and basilar membranes |
| Scala tympani | Perilymph | Below basilar membrane → round window |
The scala vestibuli and scala tympani are connected at the apex via the helicotrema.
7. Basilar Membrane — Traveling Wave & Frequency Analysis
- Stapes movement at the oval window generates a "traveling wave" along the basilar membrane
- The wave travels from base to apex but dies at the point of maximal resonance for that frequency
Tonotopic organisation:
| Basilar Membrane Location | Properties | Frequency Detected |
|---|---|---|
| Base (near oval window) | Narrow, stiff | High frequencies (e.g., 8,000+ Hz) |
| Apex (near helicotrema) | Wide, compliant | Low frequencies (e.g., 20–200 Hz) |
"High-frequency sound waves travel only a short distance along the basilar membrane before they reach their resonant point and die; very low-frequency sound waves travel the entire distance."
— Guyton & Hall Medical Physiology, p. 658
For frequencies below ~200 Hz, the volley (frequency) principle also applies — cochlear nerve fibers fire volleys synchronized to the sound frequency.
8. Organ of Corti — Sensory Transduction
The organ of Corti sits on the basilar membrane, bathed in endolymph of the scala media. It contains two types of hair cells:
- Inner hair cells (~3,500): arranged in a single row; the primary sensory receptors
- Outer hair cells (~12,000): arranged in 3 parallel rows; amplify and fine-tune basilar membrane motion
Hair cell mechanics:
- Hair cell bodies rest on the basilar membrane
- Stereocilia (cilia) are embedded in the tectorial membrane
- Vibration of the basilar membrane causes a shearing force between the basilar membrane (more elastic) and the tectorial membrane (stiffer), bending the cilia
9. Mechanotransduction — Hair Cell Depolarization
The transduction cascade (numbered steps):
- Traveling wave deforms the basilar membrane → shearing of stereocilia against tectorial membrane
- Bending cilia toward tallest stereocilium → opens mechanically gated K⁺ channels (tip links)
- K⁺ flows into the hair cell from endolymph (high K⁺) down its concentration gradient → depolarization
- (Bending away → closes K⁺ channels → hyperpolarization)
- Depolarization opens voltage-gated Ca²⁺ channels at the presynaptic terminal
- Ca²⁺ influx → glutamate release from hair cell → excites afferent cochlear nerve fibers
- Oscillating depolarization/hyperpolarization → intermittent glutamate release → action potentials in CN VIII
The oscillating receptor potential produced is called the cochlear microphonic potential.
— Costanzo Physiology, pp. 99–100
10. Encoding of Loudness
- Louder sounds → greater basilar membrane amplitude → higher firing rate in cochlear nerves
- More hair cells recruited at the fringes of the resonating zone → spatial summation
- Outer hair cells activated only at high intensities → signal very loud sounds
- The ear compresses a ~1 trillion-fold range of sound energy into a ~10,000-fold perceived range
11. Auditory Neural Pathway (Onward from Cochlea)
Hair cells → CN VIII (cochlear nerve) → cochlear nuclei (medulla) → some fibers cross via trapezoid body to contralateral side; others remain ipsilateral → lateral lemniscus → inferior colliculus (midbrain) → medial geniculate nucleus (thalamus) → primary auditory cortex (Heschl's gyri, temporal lobe)
The tonotopic map generated at the basilar membrane is preserved at every level of the CNS up to the auditory cortex.
— Costanzo Physiology, p. 101
Summary Flowchart
Sound waves → Pinna/EAC (collect + +15–22 dB)
↓
Tympanic membrane (catenary lever)
↓
Malleus → Incus → Stapes (ossicular lever)
↓
Oval window (hydraulic lever: large TM → small footplate)
[Total impedance match: ~26–35 dB gain]
↓
Perilymph of scala vestibuli displaced
↓
Traveling wave along basilar membrane
(tonotopic: high freq → base; low freq → apex)
↓
Organ of Corti: shearing of stereocilia against tectorial membrane
↓
K⁺ influx → depolarization → Ca²⁺ entry → glutamate release
↓
Action potentials in CN VIII → CNS auditory pathway
↓
Round window bulges outward (pressure release)
Sources: Cummings Otolaryngology Head & Neck Surgery, p. 271–303 | Costanzo Physiology 7th Ed., pp. 97–101 | Guyton & Hall Medical Physiology, pp. 655–661
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