Vestibular System

Vestibular Anatomy - Inner Ear GPS

• Location: Inner ear (petrous temporal bone).
• Labyrinths:
  • Bony: Outer bony shell; contains perilymph.
    • Includes: 3 Semicircular Canals, Vestibule (houses Utricle, Saccule), Cochlea.
  • Membranous: Inner sacs/ducts within bony labyrinth; contains endolymph.
    • Includes: Semicircular Ducts, Utricle & Saccule (Otolith Organs), Cochlear Duct.
• Sensory Receptors:
  • Cristae Ampullares: In ampullae of semicircular ducts.
    • Sense angular acceleration (head rotation).
  • Maculae: In utricle & saccule.
    • Sense linear acceleration & gravity.
    • Contain otoconia ($CaCO_3$ crystals). 📌 "Ear stones"
• Innervation: Vestibular nerve (branch of CN VIII).

⭐ The vestibular system's primary sensory organs, the semicircular canals and otolith organs, are located within the bony labyrinth of the inner ear.

Balance Transduction - Hair Cell Hustle

• Sensory Receptors: Hair cells in ampullary cristae (angular acceleration) & utricular/saccular maculae (linear acceleration).
• Mechanism: Mechanical force (endolymph/otolith movement) bends stereocilia.
  • Towards Kinocilium: Stretches tip links → opens MET $K^+$ channels.
    • $K^+$ influx (from $K^+$-rich endolymph) → Depolarization.
    • $Ca^{2+}$ influx → ↑ Glutamate release → ↑ AP frequency in vestibular nerve.
  • Away from Kinocilium: Compresses tip links → closes MET $K^+$ channels.
    • ↓ $K^+$ influx → Hyperpolarization.
    • ↓ Glutamate release → ↓ AP frequency in vestibular nerve.
• Resting State: Tonic neurotransmitter release.

⭐ Deflection of stereocilia towards the kinocilium causes depolarization of the hair cell, while deflection away causes hyperpolarization.

Neural Connections - Reflex Relay Race

• Primary Afferents: Scarpa's ganglion → Vestibular Nuclei (VN) & Cerebellum (flocculonodular lobe).
• Vestibular Nuclei (VN): 4 nuclei (Superior, Lateral, Medial, Inferior) - central relay station.
  • Inputs: Vestibular organs, cerebellum, visual, proprioceptive signals.
  • Outputs: Key reflex pathways.
• Key Reflexes & Pathways:
  • VOR (Vestibulo-Ocular Reflex): Maintains gaze stability during head movements.

    ⭐ The Vestibulo-Ocular Reflex (VOR) stabilizes gaze during head movements, maintaining visual acuity. Its gain is typically close to 1.0.

    • Path: VN → Medial Longitudinal Fasciculus (MLF) → Oculomotor nuclei (III, IV, VI) → Eye muscles.
  • VSR (Vestibulospinal Reflexes): Critical for posture.
    • Lateral Vestibulospinal Tract (LVST): From Lateral VN → Ipsilateral spinal cord → Extensor motor neurons (anti-gravity).
    • Medial Vestibulospinal Tract (MVST): From Medial VN → Bilateral cervical spinal cord (via MLF) → Neck/axial muscle control (head stabilization).
  • VCR (Vestibulocollic Reflex): Stabilizes head position in space; acts on neck muscles.
• Cerebellar Loop: VN ↔ Flocculonodular lobe (vestibulocerebellum); crucial for modulating and adapting reflexes (e.g., VOR adaptation).
• Cortical Pathway: VN → Thalamus (VPI/VPLc nuclei) → Parieto-Insular Vestibular Cortex (PIVC) & other areas for conscious perception of motion and spatial orientation.

Clinical Clues - Dizzying Dilemmas

• Benign Paroxysmal Positional Vertigo (BPPV):
  • Brief, intense vertigo with specific head movements.
  • Dx: Positive Dix-Hallpike maneuver. Cause: Otoconia in semicircular canals (SCC).
• Meniere's Disease:
  • Episodic vertigo (minutes to hours), fluctuating sensorineural hearing loss (SNHL), tinnitus, aural fullness.
  • Cause: Endolymphatic hydrops.
• Vestibular Neuritis:
  • Sudden, severe vertigo (lasts days), nausea, vomiting.
  • No auditory symptoms. Cause: Inflammation of vestibular nerve (often viral).
• Labyrinthitis:
  • Similar to vestibular neuritis (sudden vertigo) but includes hearing loss and/or tinnitus.
  • Cause: Inflammation of the labyrinth.
• Key Diagnostic Tests:
  • Dix-Hallpike test: For BPPV.
  • Head Impulse Test (HIT): Differentiates peripheral vs. central causes.
  • Caloric Testing: Evaluates horizontal SCC function.

    ⭐ The COWS mnemonic (Cold Opposite, Warm Same) describes the direction of nystagmus fast phase during caloric testing of the horizontal semicircular canal.

High‑Yield Points - ⚡ Biggest Takeaways

• Otolith organs (utricle, saccule) sense linear acceleration and head position.
• Semicircular canals detect angular acceleration (head rotation).
• Sensory receptors: maculae in otoliths, cristae ampullares in semicircular canals.
• Vestibulo-Ocular Reflex (VOR) maintains stable vision during head movements.
• Nystagmus: involuntary eye movement; fast phase indicates direction.
• Caloric test: COWS (Cold-Opposite, Warm-Same) mnemonic for nystagmus.
• BPPV is caused by displaced otoconia in semicircular canals_._