Vestibular Sensation – Comprehensive Study Notes

Vestibular System Overview

• Vestibular organs = "vestibular labyrinth" = 3 semicircular canals + 2 otolith organs in each inner ear.
  • Provide a “sixth sense” for spatial orientation and balance.
  • Evolutionarily ancient; analogues exist even in plants (e.g., trees’ upward growth) and invertebrates (jellyfish statoliths).
• Core functional outputs
  • Sense linear motion, angular motion, tilt.
  • Drive vestibulo-ocular reflex (VOR) → counter-rotate eyes to stabilize vision.
  • Contribute to balance, autonomic regulation, self-motion perception, multisensory orientation.
• Common dysfunction symptoms: spatial disorientation, dizziness, vertigo, imbalance, blurred vision, illusory self-motion.

Vestibular Contributions

• Balance = neural control keeping body’s center of mass over base of support.
• Visual stability = eye rotations that compensate for head rotations via VOR.
• Active sensing paradigm
  • Vestibular system continuously integrates
  • Afferent signals (sensory → brain)
  • Efferent commands (brain → muscles)
  • Sense of equilibrium is therefore active, not passive.

Evolutionary Development & Graviception

• Graviception = processes sensing gravity’s direction.
  • Plants: shoots grow against gravity.
  • Jellyfish: statolith crystals analogous to human otoconia.

Modalities & Qualities of Spatial Orientation

• Three modalities (distinct receptor sets & energies)
  1. Angular motion (rotation) – semicircular canals.
  2. Linear motion (translation) – otolith organs.
  3. Tilt (static orientation to gravity) – otolith organs.
• Coordinate system (head-centric)
  • x: forward (naso-occipital)
  • y: left ear to right ear (interaural)
  • z: up (dorsoventral)
• Rotations
  • Roll: around x
  • Pitch: around y
  • Yaw: around z
• Amplitude = size of motion (e.g., \Delta \text{angular velocity}).
  Direction = line faced/moved along.

Vestibular Organs

General Hair-Cell Transduction

• Hair cell = mechanoreceptor with stereocilia + kinocilium.
• Baseline neurotransmitter release even at rest (~constant spike rate \approx 100\;\text{spikes/s}).
• Deflection toward tallest stereocilia → depolarization & increased firing; opposite deflection → hyperpolarization & decreased firing.

Semicircular Canals (SCC)

• Geometry: \tfrac{3}{4} toroid, length 15\,\text{mm}, outer diameter 1.5\,\text{mm}; inner membrane tube 0.3\,\text{mm}.
• Fluids: Perilymph (outer) & Endolymph (inner).
• Ampulla: swelling where transduction occurs; contains crista (≈ 7000 hair cells) capped by gelatinous cupula.
• Physics: Head rotation → inertia of endolymph → cupula deflects → hair-cell activation.
• Direction coding
  • Three SCC planes ≈ orthogonal; each is maximally sensitive to rotations perpendicular to its own plane.
  • Push–pull symmetry: e.g., left horizontal canal depolarizes while right hyperpolarizes for the same yaw.
• Amplitude coding
  • Spike-rate changes \propto angular velocity.
  • High resting rate permits bidirectional coding (increase & decrease).
• Dynamics
  • Respond to acceleration, not constant velocity.
  • During constant rotation, firing adapts back toward baseline within ~5–6\,\text{s}; perceptual velocity decays more slowly (velocity storage).
  • Greatest sensitivity at frequencies \le 1\,\text{Hz}.
  • Complex trajectories decomposed via Fourier analysis → responses to sine waves predict responses to any motion.

Otolith Organs

• Two per ear
  • Utricle (≈ 30{,}000 hair cells) – horizontal macula.
  • Saccule (≈ 16{,}000 hair cells) – vertical macula.
• Macula = planar sheet of hair cells topped by gelatinous membrane with calcium-carbonate crystals (otoconia).
• Shear forces from linear acceleration or gravity shift otoconia, deflecting hair bundles.
• Amplitude coding: receptor potential \propto magnitude of linear acceleration/gravitational shear.
• Direction coding
  • Utricle: sensitive to horizontal acceleration & gravity components.
  • Saccule: sensitive to vertical acceleration & gravity components.

Spatial Orientation Perception

• Experimental paradigms
  1. Threshold estimation – minimal detectable motion direction.
  2. Magnitude estimation – subjective degrees of motion.
  3. Matching – align visible line with gravity in darkness.

Rotation Perception

• Accurate initially; perception decays over \approx 30\,\text{s} despite continued rotation.
• Stop rotation → illusionary after-rotation in opposite direction.
• Yaw thresholds: detect <1^{\circ}/\text{s} at \approx 1\,\text{Hz}; lower frequency ⇒ larger velocity required.

Translation Perception

• In darkness, participants reproduce both displacement and velocity profile of passive translations → brain integrates otolith-coded acceleration to velocity.

Tilt Perception

• Accurate for 0^{\circ} \le \text{tilt} \le 90^{\circ}.
• Aubert effect: Roll-tilting head while viewing vertical light streak → streak appears tilted oppositely.

Multisensory Integration

• Sensory integration merges multiple modality cues → increased accuracy.

Visual–Vestibular Vection

• Vection = illusory self-motion from visual input (IMAX, moving bus scenario).
• Large-field rotating visuals evoke rotational vection; sense of inverting is vetoed by gravity cues from otoliths.
  • Astronauts in microgravity do feel complete tumbling → gravity cue absent.

Beyond Integration: Active Sensing

• Brain distinguishes
  • Sensory reafference: afferent change due to self-generated actions.
  • Sensory exafference: afferent change due to external forces.
• Internal model subtracts efference copy of motor commands from afferent vestibular signals → estimates external motion.

Reflexive Vestibular Responses

Vestibulo-Ocular Reflex (VOR)

• Angular VOR: head yaw left → eyes yaw right; maintains gaze on target.
• Torsional VOR: roll head → eyes counter-roll a few degrees.
• Neural arc: SCC afferents → vestibular nuclei → ocular motor nuclei (III, IV, VI) → six extra-ocular muscles.
• Gain = \frac{\text{eye velocity}}{\text{head velocity}} ≈ 0.7–1.0 in dark across 0.01–1\,\text{Hz}.

Vestibulo-Autonomic Reflexes

• Vestibular input to autonomic centers regulates blood pressure when posture changes.
• Motion sickness arises from conflict between SCC, otolith, and visual cues; hypothesized evolutionary defense vs. neurotoxins.

Vestibulo-Spinal Reflexes & Balance

• Family of reflexes using vestibular, visual, kinesthetic inputs to stabilize posture.
• Loss of vestibular function → over-compensation sway, inability to stand in dark.
• Pathways: Vestibular nuclei → lateral & medial vestibulo-spinal tracts → spinal motor neurons controlling neck, trunk, limb muscles.

Multisensory Spatial Orientation Cortex

• No dedicated “vestibular cortex”.
  • Vestibular signals reach multiple cortical areas (parietal insula, somatosensory cortex, temporo-parietal regions) via thalamo-cortical pathways that also process vision & somatosensation.
• Top-down projections modulate vestibular nuclei ⇒ expectations influence tilt/motion perception.

Vestibular Pathologies

• Mal de debarquement: lingering rocking sensation post-sea travel; usually transient, can become chronic.
• Ménière’s disease: unpredictable attacks of vertigo, imbalance, nausea; treatments range from medication to surgical removal of labyrinth.
• Vestibular failure → spatial disorientation, oscillopsia (blurred vision with head moves), balance loss, motion sickness, cognitive issues.

Vestibular Aging

• Study of n=105 participants (ages 18\text{–}80) – direction-discrimination threshold along z axis.
  • Thresholds stable 18\text{–}40 yrs; increase rapidly thereafter.
  • Implication: vestibular decline starts ~4th decade.

Real-World & Recreational Relevance

• Roller coasters & amusement rides exploit vestibular sensations: high angular velocities (SCC) + rapid linear accelerations (otoliths) ⇒ thrill.
• Design of virtual reality, flight simulators, vehicle dynamics must consider visual-vestibular congruency to prevent motion sickness.