17.4 Audition & the Vestibular System

17.4 Audition & the Vestibular System

Audition: Acoustic Impedance Matching & Basilar Tonotopy

Sound waves are cyclical compressions and rarefactions of air pressure, characterized by frequency (measured in Hertz, perceived as pitch) and amplitude (measured in decibels, perceived as loudness).

The Three Auditory Compartments

1. Outer Ear: The cartilaginous pinna (auricle) funnels acoustic pressure waves into the external auditory canal, directing them against the thin tympanic membrane (eardrum).
2. Middle Ear (Air-Filled Cavity): Contains the three ossicles: malleus (hammer), incus (anvil), and stapes (stirrup).
   • Impedance Matching Problem: Sound waves travel from low-impedance air into the high-impedance fluid of the cochlea. Without an amplification mechanism, 99.9% of acoustic sound energy would reflect off the fluid boundary. The middle ear overcomes this through two mechanical advantages: the lever action of the ossicles and the surface area ratio between the large tympanic membrane (~55 $mm^2$) and the tiny stapes footplate at the oval window (~3.2 $mm^2$), amplifying effective acoustic pressure ~20-fold.
   • Acoustic Reflex: In response to sustained loud sounds (>80 dB), the tensor tympani (cranial nerve $V_3$) and stapedius (cranial nerve VII) muscles contract reflexively, stiffening the ossicular chain and dampening vibrational transfer to protect the delicate inner ear.
3. Inner Ear (Bony and Membranous Labyrinth): The stapes footplate pushes into the oval window, displacing fluid within the snail-shaped cochlea.
   • Cochlear Fluids & Compartments: The upper scala vestibuli and lower scala tympani contain perilymph (resembles extracellular fluid: high $Na^+$, low $K^+$). The central scala media (cochlear duct) contains endolymph, an extraordinary biological fluid actively secreted by the stria vascularis with a high potassium ($K^+$) concentration (~150 mM) and a positive electrical potential of +80 mV (the endocochlear potential).

Hair Cell Mechanotransduction in the Organ of Corti

The sensory organ of hearing, the Organ of Corti, sits atop the basilar membrane within the scala media:

1. Fluid displacement from the oval window travels down the scala vestibuli, crosses the helicotrema, and exits via outward bulging of the membrane-covered round window.
2. This fluid wave displaces the basilar membrane vertically, shearing the stereocilia of auditory hair cells against the overlying, gelatinous tectorial membrane.
3. Stereocilia are organized in ascending height rows and interconnected at their tips by protein filaments called tip links (cadherin-23 and protocadherin-15 complexes).
4. Bending stereocilia toward the tallest stereocilium (the kinocilium in development) pulls the tip links taut, mechanically yanking open mechanically gated cation channels (TMC1/TMC2).
5. Electrochemical Gradient: Because the surrounding endolymph is rich in potassium and possesses an endocochlear potential of +80 mV, while hair cells have an internal resting potential of -45 to -70 mV, an immense electrical gradient (+150 mV driving force) drives potassium ($K^+$) rushing INTO the hair cell.
6. Inward $K^+$ current depolarizes the hair cell, opening voltage-gated $Ca^{2+}$ channels at the basolateral membrane. Calcium influx triggers vesicle fusion and release of glutamate onto spiral ganglion afferents of the cochlear nerve (CN VIII).

Fluid displacement in scala vestibuli/tympani
     ↓
Basilar membrane moves upward
     ↓
Stereocilia shear against tectorial membrane
     ↓
Tip links pulled taut ──> Mechanically gated cation channels open
     ↓
K+ rushes IN from high-K+ endolymph (+150 mV electrical driving force)
     ↓
Hair cell DEPOLARIZES
     ↓
Voltage-gated Ca2+ channels open ──> Intracellular [Ca2+] rises
     ↓
Glutamate exocytosis onto cochlear nerve fibers (CN VIII)

Tonotopic Organization (Place Theory)

The physical resonance properties of the basilar membrane dictate frequency selectivity along its entire length:

• Base (near oval window): The basilar membrane is narrow, stiff, and taut. It resonates maximally to high-frequency sounds (up to 20,000 Hz).
• Apex (near helicotrema): The basilar membrane is wide, flexible, and compliant. It resonates maximally to low-frequency sounds (down to 20 Hz).

This spatial map of sound frequencies—tonotopy—is preserved with strict point-to-point fidelity through the spiral ganglion, cochlear nuclei, and thalamus directly into the primary auditory cortex.

Central Auditory Pathway

From the spiral ganglion, action potentials propagate along the vestibulocochlear nerve (CN VIII) through an ascending brainstem pathway remembered by the mnemonic SLIM: Superior olive, Lateral lemniscus, Inferior colliculus, Medial geniculate nucleus:

1. Cochlear Nuclei (medulla): First synaptic station.
2. Superior Olivary Complex (pons): Crucial for bilateral sound localization. Computes Interaural Time Differences (ITDs) for low frequencies and Interaural Level Differences (ILDs) for high frequencies.
3. Lateral Lemniscus (pons to midbrain): Ascending fiber tract.
4. Inferior Colliculus (midbrain): Integrates auditory spatial information and coordinates acoustic startle reflexes.
5. Medial Geniculate Nucleus (MGN) of Thalamus: Thalamic auditory relay nucleus (MGN = Music/Audition; LGN = Light/Vision).
6. Primary Auditory Cortex (A1): Located on the transverse temporal gyri of Heschl in the superior temporal gyrus (Brodmann areas 41 and 42).

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The Vestibular System: Dynamic vs. Static Equilibrium

The vestibular apparatus, located adjacent to the cochlea in the temporal bone, monitors head position and motion:

• Semicircular Canals (Horizontal, Superior, Posterior): Oriented in three mutually orthogonal planes to sense rotational (angular) acceleration (e.g., shaking or nodding the head). Each canal expands at its base into an ampulla containing a sensory ridge, the crista ampullaris. Hair cell stereocilia project into a gelatinous, sail-like membrane called the cupula. When the head rotates, inertia causes endolymph to lag behind, deflecting the cupula and bending hair cells.
• Otolith Organs (Utricle and Saccule): Sense linear acceleration and head tilt relative to gravity. The sensory epithelium (the macula) contains hair cells embedded in an otolithic membrane studded with dense calcium carbonate crystals called otoconia (otoliths).
  • Utricle: Macula is horizontally oriented; monitors horizontal linear motion (e.g., accelerating in a car).
  • Saccule: Macula is vertically oriented; monitors vertical linear acceleration (e.g., riding an elevator).

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The Three Divisions of the Ear

Auditory questions typically localise a lesion, so keep each division tied to the physical operation it performs.