Physiology of Hearing

Functional Anatomy - Ear's Blueprint

• Outer Ear:
  • Pinna: Sound localization & collection.
  • External Auditory Canal (EAC): Conducts sound waves to TM; outer 1/3 cartilage, inner 2/3 bone.
• Middle Ear (Tympanic Cavity): Air-filled.
  • Tympanic Membrane (TM): Vibrates.
  • Ossicles (Malleus, Incus, Stapes): Transmit & amplify vibrations. 📌 MIS
  • Eustachian Tube: Pressure equalization.
• Inner Ear (Labyrinth): Fluid-filled.
  • Cochlea: Hearing; contains Organ of Corti (hair cells).

⭐ The middle ear provides an impedance matching mechanism, amplifying sound pressure by about 22 times from air to cochlear fluid.

Sound Conduction - Journey to Cochlea

• Outer Ear:
  • Pinna & External Auditory Canal (EAC) collect & channel sound.
  • EAC resonance boosts frequencies around 3 kHz.
• Middle Ear: (Air-filled)
  • Tympanic Membrane (TM) vibrates.
  • Ossicles (Malleus, Incus, Stapes - 📌 MIS) amplify & transmit vibrations.
  • Impedance matching: Area ratio (TM:Oval Window ≈ 17:1) & lever ratio (ossicles ≈ 1.3:1) → total amplification ≈ 22x.
  • Stapes vibrates against oval window.
• Inner Ear:
  • Oval window vibrations generate perilymphatic waves (scala vestibuli).

⭐ The middle ear's key role is impedance matching, efficiently transferring sound from air (low impedance) to cochlear fluid (high impedance), minimizing energy loss.

Cochlear Physiology - Dance of Hair Cells

• Basilar Membrane (BM) Vibration: Sound waves displace BM, creating a traveling wave.
  • Tonotopic organization: High frequencies stimulate BM base; low frequencies stimulate BM apex.
• Hair Cell (HC) Mechanotransduction:
  • Shearing force between BM & Tectorial Membrane bends stereocilia of HCs.
  • Bending towards kinocilium (tallest stereocilium) opens MET (mechanoelectrical transduction) channels via tip links.
  • K+ influx from K+-rich endolymph (main_tained by Stria Vascularis at +80 mV, the endocochlear potential). 📌 K+ INflux for excitation.
  • HC depolarization → Voltage-gated Ca2+ channels open → Ca2+ influx → Glutamate (neurotransmitter) release → Auditory nerve fiber firing.
• Outer Hair Cell (OHC) Amplification:
  • OHCs exhibit electromotility (change length via Prestin protein) in response to depolarization.
  • This actively amplifies BM motion, enhancing hearing sensitivity (by ~40-50 dB) and frequency selectivity (cochlear amplifier).

⭐ OHCs are responsible for the cochlea's remarkable sensitivity and sharp frequency tuning; their damage leads to significant hearing loss and poor frequency discrimination (e.g., in noise-induced hearing loss).

Auditory Neural Pathway - Brain's Sound Booth

• 1st Order Neuron: Spiral ganglion cells (cochlea) → Cochlear Nuclei (CN).
• CN (Pons/Medulla): Ventral & Dorsal nuclei; tonotopic organization maintained.
• Superior Olivary Complex (SOC - Pons): Binaural processing (sound localization); 1st site of bilateral input.
• Lateral Lemniscus (LL - Midbrain): Main ascending auditory tract.
• Inferior Colliculus (IC - Midbrain): Auditory reflex integration; sound orientation.
• Medial Geniculate Body (MGB - Thalamus): Auditory relay station to cortex.
• Primary Auditory Cortex (Temporal Lobe): Heschl's gyrus (Brodmann areas 41, 42); conscious sound perception.

📌 Mnemonic: E COLI MA (Ear/Spiral Ganglion → Cochlear Nucleus → Superior Olivary Complex → Lateral Lemniscus → Inferior Colliculus → Medial Geniculate Body → Auditory Cortex)

⭐ The Superior Olivary Complex is the first major site for binaural interaction, crucial for sound localization by comparing interaural time and intensity differences.

High‑Yield Points - ⚡ Biggest Takeaways

• Middle ear performs impedance matching via ossicles, transferring sound from air to fluid.
• Eustachian tube equalizes middle ear pressure with atmospheric pressure.
• Inner hair cells (IHCs) are primary auditory sensory receptors; Outer hair cells (OHCs) are cochlear amplifiers.
• Basilar membrane exhibits tonotopic organization: base for high frequencies, apex for low frequencies.
• Key auditory pathway: Cochlear N. → Cochlear Nuclei → SOC (localization) → IC → MGB (thalamus) → Auditory Cortex.