Auditory System

Ear Anatomy - Sound's Grand Entrance

• Outer Ear: Collects & directs sound.
  • Pinna (Auricle): Cartilage; sound localization.
  • External Auditory Canal (Meatus): ~2.5 cm; S-shaped; outer 1/3 cartilage (cerumen), inner 2/3 bone.
  • Tympanic Membrane (Eardrum): Vibrates; 3 layers. Pars tensa & flaccida.
• Middle Ear (Tympanic Cavity): Air-filled; transmits & amplifies vibrations.
  • Ossicles (📌 MIS: Malleus, Incus, Stapes): Smallest bones.
    • Stapes footplate on oval window.
  • Muscles: Tensor Tympani (CN V3), Stapedius (CN VII - acoustic reflex).
  • Eustachian Tube: To nasopharynx; pressure equalization.
• Inner Ear (Labyrinth): Sensory transduction.
  • Bony Labyrinth: Perilymph (Cochlea, Vestibule, Semicircular Canals).
  • Membranous Labyrinth: Endolymph; contains sensory organs.

⭐ Stapedius muscle (CN VII) dampens loud sounds (acoustic reflex); paralysis causes hyperacusis. Smallest skeletal muscle in the body.

Sound Transmission - Wave to Nerve Impulse

• Air to Cochlea: Sound → TM → Ossicles (Malleus, Incus, Stapes) amplify pressure (~22x) → Oval Window.

  • Impedance matching: Ossicular lever action & TM/Oval Window area ratio (~17:1).

• Cochlear Mechanics:

  • Stapes → Perilymph wave (Scala Vestibuli → Helicotrema → Scala Tympani).
  • Basilar Membrane (BM) vibrates; Round window dampens.
  • Tonotopy: High freq. at BM base, low freq. at apex.

• Transduction (Organ of Corti):

  • BM & Tectorial Membrane shear → Stereocilia bend (Hair Cells).
  • Towards kinocilium: Tip links open MET K+ channels.
  • K+ influx (from endolymph) → Depolarization → Ca2+ influx → Glutamate release → Auditory nerve AP.

> ⭐ Endolymph's high K+ (~**150** mEq/L) & +**80**mV endocochlear potential are vital for hair cell excitation.

Auditory Pathways - Brain's Sound Map

• Sound's Journey: Cochlea → Auditory Cortex.
• Bilateral Pathway Relays (Ascending):
  • Spiral Ganglion (CN VIII) → Cochlear Nuclei (CN).
  • CN → Superior Olivary Complex (SOC) - binaural interaction starts.
  • SOC → Lateral Lemniscus (LL) → Inferior Colliculus (IC) - major auditory center.
  • IC → Medial Geniculate Body (MGB) of Thalamus.
  • MGB → Auditory Cortex (Temporal Lobe; Areas 41, 42).
• 📌 Mnemonic (Key Stops): "Cold Soup Left In My Area" (CN, SOC, LL, IC, MGB, Auditory Cortex).
• Tonotopy: Maintained throughout; specific frequencies map to specific areas.

⭐ The Superior Olivary Complex is the first major site for processing binaural cues (interaural time/intensity differences), vital for sound localization.

Hearing & Tests - Deciphering Decibels

• Decibel (dB): Logarithmic unit. Ref: $0 \text{ dB} = 20 \text{ µPa}$ (threshold).
  • ↑ 20 dB: $10 \times$ pressure increase.
  • ↑ 10 dB: $10 \times$ intensity increase.
• Hearing:
  • Range: 20-20,000 Hz.
  • Speech: 500-2,000 Hz.
  • Pain Threshold: approx. 120-130 dB.
• Tuning Fork Tests (512 Hz):
  • Rinne: (AC vs BC)
    • Normal/SNHL: AC > BC (+ve).
    • CHL: BC > AC (-ve).
  • Weber: (Lateralization)
    • CHL: To AFFECTED ear.
    • SNHL: To UNAFFECTED ear.
    • 📌 WEBER: CHL → Comes to affected ear.
• PTA (Pure Tone Audiometry):
  • Plots thresholds (dB HL vs Hz).
  • ABG > 10 dB indicates CHL.

⭐ In conductive hearing loss, Weber test lateralizes to the diseased ear because the masking effect of ambient noise is reduced on that side, making bone-conducted sound appear louder.

High‑Yield Points - ⚡ Biggest Takeaways

• Organ of Corti (scala media) has hair cells, the auditory receptors.
• Outer hair cells amplify; inner hair cells are main sensory receptors.
• Sound: Ossicles → oval window → cochlear fluids → basilar membrane vibration.
• Basilar membrane tonotopy: High frequencies at base, low frequencies at apex.
• Auditory pathway: Cochlear nuclei → SOC → IC → MGB (Thalamus) → Auditory cortex.
• Endolymph (scala media) is K+ rich, vital for hair cell excitation.
• Ototoxic drugs (aminoglycosides, cisplatin) risk sensorineural hearing loss.