HNS40 Vestibular System Flashcards
***Vestibular system
Innervated by CN8 (via Vestibular ganglion / Scarpa’s ganglion)
-
**Semicircular canal (2 vertical, 1 horizontal)
—> Endolymph + **Cupula
—> Head rotation in 3-D space / Angular acceleration (3-dimentional) -
**Otolith organ
- **Utricle (horizontal plane) + ***Saccule (vertical plane)
—> Gravity / Linear acceleration (translational head movement)
—> Static head positional changes (head tilt) / Gravity
***Vestibular pathways
- Sensation (Perception):
- Vestibulo-thalamo-cortical projection / Vestibulo-cortical pathway - Sensori-motor interaction (Compensatory reflex):
- Vestibulo-ocular pathway (extraocular muscles to maintain visual acuity)
- Vestibulo-spinal pathway (maintain posture)
(~ Vermis (Spinocerebellum) / Flocculonodular lobe (Vestibulocerebellum))
Pathway: End organ —> ***Vestibular nerve (CN8) —> ***Scarpa’s ganglion —> ***Vestibular nucleus (medulla) —> Go up / down —> 1. Up: via Thalamus to Cortex (Vestibulo-thalamo-cortical projection) —> 2. Down: Vestibulo-ocular pathway + Vestibulo-spinal pathway
End organ of Vestibular system
Vestibular hair cell
- Stereocilia (矮) + Kinocilium (高) —> arranged from short to tall
- ALL hair cells in a semicircular canal have same structural orientation / unified (short to tall) —> same ***polarisation axis
Functional polarisation:
- Stereocilia pushed towards Kinocilium (矮推向高) —> ***Depolarisation in Hair cell —> ↑ Firing of CN8
- Kinocilium pushed towards Stereocilia (高推向矮) —> ***Hyperpolaristion —> ↓ Firing of CN8
- only triggered by forward/backward motion, lateral motion NO firing action (cannot move cilia)
***Semicircular canals
- Detect Head rotation in 3-D space / Angular acceleration (3-dimentional) (頭郁, 身唔郁)
- Small diameter
- Membranous inside, Bony outside
Contents: 1. Three canals on each side - Perpendicular to each other - Canals on both sides operate as complementary pairs (***One side ↑ firing, Other side ↓ firing) —> ***Synchronous ↑/↓ excitability
—> L/ R horizontal pairs (inclined 30o up) —> detect左右motion
—> L anterior / R posterior pair —> detect左前右後motion (flexion/extension)
—> R anterior / L posterior pair —> detect右前左後motion (flexion/extension)
- Endolymph (thick and sticky)
- move when head rotates (a bit lag behind due to moment of inertia)
- opposite movement to head movement (頭向右轉, endolymph向左轉) - Cupula
- within Ampulla (bulging part of semicircular canal)
- jelly-like substance covering cilia
- moved by Endolymph movement
—> Endolymph move
—> Cupula move
—> Cilia move
Movement of Semicircular canals hair cells (NOT examined)
ALL hair cells in a semicircular canal have same structural orientation / unified (short to tall):
**Right semicircular horizontal canal
—> acceleration of head to **Right / Clockwise
—> Endolymph to **Left / Anti-clockwise (viscous fluid lags behind due to inertia)
—> movement of Cupula
—> movement of Cilia of hair cells towards Kinocilium
—> **Depolarisation
—> ↑ Firing of CN8
—> ***Sensing of initiation of movement
(Vice versa for opposite side)
Constant speed of head rotation: Cilia revert back to normal —> CN8 activity back to normal —> subject cannot detect any rotational signal without other sensory cues
When turning stopped:
—> Endolymph move to **Right / Clockwise (due to inertia)
—> movement of Kinocilium of hair cells towards Cilia
—> **Hyperpolarisation
—> ↓ Firing of CN8
—> ***Sensing of termination of movement
(Vice versa for opposite side)
Complex head movement
Combination of activation of 3 pairs of semi-circular canals
—> Activated to different extent
***Otolith organs
- Utricle (horizontal plane)
- triggered by ***Horizontal motion of whole head (前後左右) - Saccule (vertical plane)
- activated by ***Vertical motion of whole head (上下)
Walking up stairs —> activate both Utricle + Saccule
Detect (頭唔郁, 身郁):
- Linear acceleration (translational head movement)
- Gravity (head tilt)
Movement of Otolith organ hair cells
Cilia / Kinocilium of Hair cells
- covered by **Gelatinous otolithic membrane with **Otoconia crystal on top (砸住) —> holding Gelatinous membrane in place
Structural orientation
- Hair cells in a single Utricle / Saccule are separated into ***2 subgroups with opposite Cilia direction (NO paired organs like Semicircular canal)
- Utricle hair cells: ***Multi-directional
- Saccule hair cells: ***Up-down orientation
- separated by Striola
Translational head movement / Head tilt:
—> **Depolarise a subgroup of hair cells
—> **Hyperpolarise another subgroup
Sensory transduction (same as Auditory pathway)
Movement of cilia —> Electrical signals
Electrical signals:
—> Pressure-sensitive K channels (on tip of cilia) open
—> K enter hair cell (high conc of K in Endolymph / Gelatinous otolithic membrane)
—> Depolarisation
—> Voltage-gated Ca channels open
—> Ca rush in
—> Release of synaptic vesicles containing Glutamate
—> Excitatory action at CN8
—> Generator potential —> Action potential along CN8
—> Vestibular afferent to Vestibular nucleus
***Central processing
Outputs from the vestibular nucleus to:
- Vestibulo-spinal pathway (neck + limb) —> Vestibulospinal tract + Reticulospinal tract
- Vestibulo-ocular pathway —> Otolith-related ocular counter-rolling + Canal-related vestibular nystagmus
- Cerebellum
- Autonomic nervous system - Motion sickness
- Cerebral cortex - Subjective orientation, Integration of Somatosensory + Labyrinth information
Compensatory Vestibular reflex:
Hair cell —> Vestibular ganglion —> Vestibular nucleus —> Spinal / Ocular motor neuron
- Vestibular reflexes - Vestibulo-spinal reflexes
- Maintenance of **posture + **equilibrium
- Compensatory actions of antigravity muscles (***Extensors of neck, trunk, limbs)
Pathway: Vestibular nuclei —> ***Vestibulospinal tract (Medial / Lateral) OR —> ***Reticulospinal tract
(Vestibular nuclei —> 4 subnuclei: Superior / Inferior / Medial / Lateral
Lateral vestibular nucleus —> Lateral vestibulospinal tract
Medial vestibular nucleus —> Medial vestibulospinal tract)
- Vestibular reflexes - Vestibulo-ocular reflex
- Maintain ***visual acuity (yellow spot focus on object of interest)
- Compensatory actions of extraocular muscles
- Open-loop reflex
- ***Otolith-related ocular counter-rolling
- Static head tilt
- Linear acceleration - ***Canal-related vestibular nystagmus
- Per-rotatory
- Post-rotatory
Pathway:
End organ —> Vestibular nuclei —> Spinal cord —> Pons (location of CN3, 4, 6 nuclei) —> CN3, 4, 6 —> Effector organ
Tri-neuronal reflex arc
- involves 3 neurons
1. End organ —> Vestibular nucleus
2. Vestibular nucleus —> Cranial nerve nuclei (CN3, 6)
3. Cranial nerve nuclei (CN3, 6) —> Extraocular muscle (Medial rectus + Lateral rectus)
Otolith-related ocular counter-rolling
- Static head tilt
- e.g. 50o lateral tilt —> 5o counter rolling (doll’s eye reflex)
- e.g. forward pitch (頭向前傾) —> eye elevation - Linear acceleration
- e.g. vehicle along horizontal, parallel swing in A-P direction
1 m/s^2 —> 4o eye deviation (elevation)
100 m/s^2 —> 40o eye deviation
Canal-related vestibular nystagmus
In the dark
Per-rotatory / Vestibular nystagmus (at onset of rotation)
- Slow phase (2s): equal and ***opposite to head motion, focus on moving object
- Fast phase (0.2s): anticipatory eye movement, fast reset the eyeball within orbit
Post-rotatory nystagmus (end of rotation: endolymph in Semicircular canal move to opposite direction)
- opposite movement to pre-rotator (also have Slow + Fast phase)
***Cerebellar-Vestibular interaction
End organ —> Vestibulocerebellum (Flocculus) —> Output of Purkinje cell —> ***inhibit Vestibular nucleus —> Vestibular-ocular / Vestibulo-spinal / Reticulo-spinal tract
∴ Vestibulo-ocular / Vestibulo-spinal / Reticulo-spinal reflex can be inhibited /
adjusted by output of cerebellum
