Vestibular system key words

Linear acceleration detectors

Maculae

• Utricle → horizontal

  • macula utriculi

• Saccule → vertical

  • macula sacculi

Angular acceleration detectors

Cristae ampullares (3x) within ampullary cupulae

• detection of angular acceleration in three dimensions

• each cupulae oriented perpendicular to each other

Primary vestibular inputs [3]

• Visual

• Tactile

• Proprioceptive

Peripheral vestibular apparatus connections

Semicircular canals → ampullary cupulae → utricle → saccule → cochlea

Bony labyrinth

Cavity within petrous temporal bone

Membranous labyrinth

Structure within bony labyrinth

Mechanism of receptor hair cells

• Head position movement

• Fluid (endolymph) moves in opposite direction (inertia) against hair cells

• Stereocilia deflected

  • towards kinocilium (rigid ‘hair’)

    • depolarisation → excitation

  • away from kinocilium

    • hyperpolarisation → inhibition

• Hair cell transduces mechanical movement into an electrical signal (action potential)

Maculae structure

1. Neuroepithelium

   • Hair cells (cilia)

   • Supporting cells

2. Gelatinous statoconial membrane

3. Statoconia (otoliths)

   • Movement of statoconial membrane displace cilia → cause excitation or inhibition

Movement upwards

causes endolymph to move downwards

(saccule → vertical linear acceleration)

Movement to the left

causes endolymph to move to the right

(utricle → horizontal linear acceleration)

Crista ampullaris components

1. Neuroepithelium

   • Hair cells

   • Supporting cells

2. Gelatinous cupula (NO OTOLITHS)

• Cupula project into endolymph in ampulla of semicircular canal

  • angular acceleration from head rotation

  • inertia of endolymph within canal → cupula displaced

Vestibular nuclei paired nuclei [4]

1. Rostral

2. Medial

3. Lateral

4. Caudal

• CNVIII - nuclei in medulla oblongata

Vestibular nuclei inputs [3]

1. Vestibular nerve

2. Cerebellum

3. Spinal cord

Vestibular projections [6]

1. Cerebral cortex

   • Conscious proprioception

2. Spinal cord

   • Automatic correction in posture (e.g. crossed extensor reflex)

   • autonomic reflex of body tone

     • Inhibit contralateral extensors when ipsilateral extensors engaged (ipsilateral flexors inhibited)

3. Contralateral vestibular nuclei

   • connecting left and right sides

4. Cerebellum

   • Archicerebellum (floccus + nodule) → balance

     • vestibulocerebellum = flocculonodular lobe

   • Paleocerebellum (vermis) → proprioception

5. Reticular formation

   • Vomiting and cardiovascular centres

   • nausea, increased sympathetic tone

6. Extra-ocular muscles → linked to nystagmus

Diseased side of the vestibular system

Animal falls/leads to diseased side

(ipsilateral side)

Cerebellum vestibular projection

• Archicerebellum (floccus + nodule) → balance

  • vestibulocerebellum = flocculonodular lobe

• Paleocerebellum (vermis) → proprioception

• inhibitory dampening down of signals to ipsilateral vestibular nuclei

Reticular formation vestibular projection

• Vomiting and cardiovascular centres

• nausea, increased sympathetic tone

Reticular projection extraocular muscles

• Medial longitudinal fasciculus (MLF) → white matter tract in brainstem

• Links vestibular nuclei of CNVIII to extraocular muscle nuclei

• Enables and coordinates synchronised eye movements

  • conjugate eye movements

Vestibular control of eye position → nystagmus

• Nystagmus → rhythmic, oscillatory, involuntary eye movements

Normal/physiological nystagmus

• Vestibulo-ocular reflex

• Test via moving head to side

• Eyes locked onto target until target leaves FOV

• Jerky movement of eyes → snaps back to resting position

Abnormal nystagmus

• Pathological/spontaneous

  • Standing still → nystagmus occurs

• Lack of vestibulo-ocular reflex

Vestibulo-ocular reflex

• When eyes focused on target → receptor cells constantly firing

  • Increased firing on side towards movement

  • Decreased firing on side away from movement

• Head still → equal firing (dynamic equilibrium)

Movement of head and eye during vestibulo-ocular reflex

• Animal test - constant target on the right

• Vet physically turns head left

• Head turns left → eyes move right

  • Focus on object on right

  • Constant gaze on moving target

• When focussed object exits FoV → eyes jerk left, back to the centre

Vestibular syndrome

• Collection of clinical signs caused by vestibular dysfunction

• Not a disease → syndrome

Bilateral vestibular syndrome

• Head moves weirdly (bobbles) since both sides affected

Peripheral vestibular syndrome- source of pathology

• Vestibular apparatus (inner ear)

• Vestibulocochlear nerve

Central vestibular syndrome- source of pathology

• Vestibular nuclei

• Central vestibular projections

Central vestibular projections [5]

1. Vestibulospinal tract

2. Palaeocerebellum (floccus and nodule, vestibulocerebellum)

   • Proprioception

3. Medial longitudinal fasciculus

4. Thalamus (through towards)→ somatosensory cortex

5. Reticular formation → cardiac and vomit centres

Central V peripheral vestibular syndrome using clinical signs → general points

1. Horner’s syndrome more commonly peripheral

2. All other clinical signs can happen with central

3. Head tilt and ataxia always happen with both

Vestibular syndrome clinical signs [9]

1. Head tilt

2. Ataxia

3. Proprioceptive deficits

4. Muscle weakness (paresis)

5. Change in mentation (brain function)

6. CN deficits

7. Horner’s

8. Strabismus

9. Nystagmus

Strabismus

• Deviated eye position WHEN EYE FIXED

  • Positional strabismus

  • Ventral strabismus (when head elevated)

  • Ipsilateral strabismus (if peripheral ipsilateral lesion)

Vertical nystagmus

Up and down beats of eye movement

Positional nystagmus

Changes nature according to head position

Dysconjugated nystagmus

Nystagmus between eyes not coordinated

Spontaneous vertical nystagmus

(can also be horizontal or rotatory)

Eyes involutarily jerk up and down by default

Inducible nystagmus

PATHOLOGICAL

• Nystagmus not noticeable at rest

  • animal has adapted to nystagmus in regular position

• When animal put in new position nystagmus appears

Pendular nystagmus

GENETIC/non pathological

• Eyeball movements same speed both directions (no jerky movments)

• Spontaneous, congenital

• Accompanies ocular albinism → cats with blue irises

Unilateral lesion pathological nystagmus

• Loss of constant stimulation from side of lesion

• No dynamic equilibrium:

  • Perceived as head rotating away from side with lesion

  • Lesion on left

  • Perceived a head rotating to the right

  • eyes look left and then snap to the right

Peripheral vestibular system summary

• Vestibular apparatus (labyrinth)

• Vestibular nerve

Central vestibular system summary

• 4 pairs vestibular nuclei

  • Rostral

  • Medial

  • Lateral

  • Caudal

• Flocculonodular lobe (archicerebellum)

• Projections to higher/lower centres

Vestibular syndrome summary

• Underlying syndrome → can have many disease causes

• Nystagmus may be physiological (normal → pendulous) or pathological

• Pathological nystagmus → characterised by direction

• Character of nystagmus etc → determine central V peripheral (to determine possible causes)

• nystagmus → look towards lesion location (for unilateral lesion)