10. Vestibular system Flashcards
Describe the relationship between components of the membranous labyrinth (utricle, saccule, semi-circular canals, cochlea) (slide 9)
- 3 SEMI-CIRCULAR CANALS: anterior, posterior, lateral. Each has an ampulla with a crista ampullaris.
- UTRICLE: connected with semi-circular canals. Has 1 macula
- SACCULE: connected with cochlear duct. Has 1 macula.
Two types sensory receptors of the vestibular systems
- 3 Crista ampullaris (in the ampulla) - ANGULAR acceledation
- 2 Macula (in the utricle and saccule) - LINEAR acceledation + gravity
Crista ampularis - structure
Slide 10
- 1 per ampulla/horizontal canal
- Columnar neuroepithelial cells (hair cells + supportive cells)
- HAIR CELLS have stereocilia + one kinocilium
- Embedded in CUPULA - gelatinous structure, which project in the ampulla
Macula - structure
Slide 11
- 1 per utricle (horizontal) and saccule (vertical)
- Columnar neuroepithelial cells (hair cells and supportive cells)
- HAIR CELLS have strereocilia + 1 kinocilium
- Embedded in STATOCONIORUM - gelatinous structure, covered by otoliths (statocconia)
In the hair cells of macula and crista…
* Bending of sterocilia TOWARDS kinocilium causes ENTRY of K+ –> ____ polarization
* Bending of sterocilia AWAY from kinocilium causes EXIT of K+ at the basolateral portion –> ____polaraization
- Towards kinocilium = DEpolarization
- AWAY from kinocilium = HYPERpolarization
Endolymph is K+ rich, Na poor
Describe steps of vestibular receptor transduction
Slide 16
- Stereocilia bent towards kinocilium
- stretches tip link
- opens K+ channels
- K+ entry -> DEPOlarization
- Activation of Ca+ channels
- NT release on vestibular nerve dendrites
Describe the orientation of semi-circular canals
Slide 17
- Anterior & posterior: vertical plane
- Lateral: horizontal plane
How are semi-circular canals paired
- R+L lateral
- R anterior + L posterior
- L anterior + R anterior
Crista (angular acceleration) - how does it work
Head still: equal firing R/L (vestibular nerve always fires)
Head movement to the L:
* L Cupula deflected towards utricle = increase firing
* R Cupula deflected away from utricle = decrease firing
* Causes jerk nystagmus to L (fast phase)
Macula (linear acceleration) - how does it work (slide 23)
(idem crista…)
Components of the peripheral vestibular pathway
- 5 receptors (2 macula, 3 crista) containing sensory hair cells
- Dendrites of bipolar neurons synapse with base of hair cells
- Cell bodies form vestibular ganglion
- Axons form vestibular nerve
- Leave bony labyringht through internal acoustic meatus, along with cochlear nerve
Describe the central vestibular pathways
Axons of the vestibular nerve….:
* Most synapse on VN (lateral to 4th ventricle)
* Others reach cerebellum via caudal c.p. (to flocculonodular lobe + fastigial nucoeus)
Name the 4 VN
Slide 27
- Rostral
- Medial
- Lateral
- Caudal
The macula and crista reach which VN?
Crista: R + M
Macula: C + L
How does the cerebellum influence VN
- Inhibitory purkinje fibers from flocculonodular lobe directly synapse on VN, which are facilitatory to ipsilateral extensors
- Lx to the flocculonodular lobe therefore frees VN from cerebellar inhibition = controlateral head tilt = paradoxical vestibular syndrome
Name the two DESCENDING VESTIBULAR TRACTS. Where do they travel? Impact on LMN?
- MEDIAL VESTIBULOSPINAL TRACT (to motor CNXI + cervical SC; involved in vestibulospinal reflex)
- LATERAL VESTIBULOSPINAL TRACT (to entire SC; limbs + trunk)
- Ventral funiculus
- Facilitatory to ipsilateral EXTENSORS (inhibits flexors)
See extrapyramidal lecture for more details
Name one ascending vestibular tract
Slide 35
Medial longitudinal fasciculus
* in dorsomedial tegmentum
* adjacent to motor nuclei of CN III, IV, VI
* Involved in vestibulo-ocular reflex
Pathway from vestibular nuclei to cortex not well understood
Which vestibular structures are responsible for
* Horizontal eye movement
* vertical eye movement
* torsional eye movement
- Horizontal: horizontal semi-circular ducts (lateral) + utricle
- Vertical: vertical semi-circular ducts (anterior & posterior) + saccule
- Torsiona: vertical ‘’ + utricle
2 phases of vestibulo-ocular reflex
- Slow in opposite direction of head (as head starts moving)
- Fast in same direction as head (as head reaches end of visual field)
Nystagmus
- Draw the elements of the physiologic nystagmus (horizontal vestibuloocular reflex)
Slide 40
On both sides (R/L)
* Crista ampullaris (always fire, and firing increase when head moves ipsilateral)
* Motor nucleus VI - innervates lateral rectus
* Adjacent PARAbducent nucleus
* Motor nucleus III - innervates medial rectus
* Eyes with medial and lateral rectus
* MLF interconnects VN, III, VI nuclei
Nystagmus
Describe the SLOW PHASE of nystagmus as head turns to the L
Slide 41
- L vestibular nerve fires more
- Activates R nucleus VI (R lateral rectus)
- Activates L nucleus III (L medial rectus)
- = conjugate eye movement to the R
Nystagmus
Describe the FAST PHASE of nystagmus as head turns to the L
- Excitatory burst cells of the L parabducen nucleus are eventually activated
- Activates L nucleus VI (L lateral rectus)
- Activates R nucleus III (R medial rectus)
- = jerk eye movement to the L
Nystagmus
Understand impact of a unilateral lx (pathologic nystagmus) to
* Peripheral vestibular system
* flocculonodular lobe
Peripheral vestibular sys :
* Fast phase AWAY from lx (as if opposite vestibular N is activated)
* Head tilt TOWARD lx (ipsilateral reduced extensor tone to CNXI and cervical SC)
Flocculonodular lobe
* Fast phase TOWARD lx (ipsi VN over-stimulated)
* Head tilt AWAY from lx (ipsi excessive extensor tone)
