A8: Vestibular System Flashcards
Functions of the vestibular system
Mediates position of eyes and body relative to the external environment
Where are the vestibular sensory cells located?
Maculae: in utricle and saccule of inner ear labrynth
Crista Ampullaris: in ampullas of semicircular ducts
Function and basic anatomy of Maculae
Detect linea motion of the head with respect to gravity
Apical stereocilia and kinocilia, on sensory cells of the maculae, extend into the Otolithic Membrane: gelatinous material containing calcium crystals, denser than endolymph
How does tilting the head affect the maculae and crista ampullaris?
Tilting of head -> movement of otolithic membrane -> deflection of hairs. Deflection of stereocilia towad the kinoclium results in action potential.
Tilting head forwards -> detected by hair cells of utricle macula
Tilting head to side -> detected by hair cells of saccule macula
Describe the cellular events following deflection of (vestibular: maculae/crista ampulla sensory cell) stereocilia in the direction of the kinocilium
Tip links under high tension -> opening of mechanically gated ion channels -> influx of K+ -> Opening of voltage-gated Ca++ channels -> release of Glutamate or Aspartate -> triggers AP in afferent bipolar cells (whose cell bodies are in Scarpa’s/Vestibular ganglion) -> central processes forming vestibular nerve -> joins with cochlea nerve to form vestibulocochlear nerve (CNVIII)
Describe the location, basic anatomy and function of the Crista Ampullaris
Located in the ampullas of the semicircular ducts
Hair cells of CA sensory cells embedded in ‘Cupula’ - gelatinous mass, same density as endolymph - extends somewhat into the ampulla. Moved about by movement of endolymph.
Detect rotational movement in any direction. This is because of the specific spacial orientation of the lateral (hoizontal) SC duct, anterior (superior) SC duct and posterior SC ducts.
How is movement detected by crista ampulla?
Upon initiation of Rotation Movement -> Endolymph lags because of ‘inertia’ (phenomenon of resistance of matter to movement)
The lag of the endolymph distorts the shape of the cupula, causing deflection of hair cells.
As rotation ends -> endolymph keeps moving for a bit -> cupula deflects in opposite direction
Describe the relative orientations of the semicircular ducts:
Horizontal canal: tilted backwards 30 degrees
Anterior canal on one side runs parallel to the Posterior canal on the opposite (contralateral) side
Explain the response of vestibular hair cells to movement of the head on the horizontal plane
Head turns left -> Endolymph in left and right SC canals flows to left -> receptos in LEFT SC canals depolarise. Receptors in right SC canals are inhibited.
And vice versa.
Describe anatomy of Vestibular (scarpa’s) ganglion
Contains cell bodies of bipolar afferent neurons from crista ampulla (of SC canals) and maculae (of utricles and saccules)
Central processes form vestibular component of CNVIII
Which terminates in Vestibular Nuclei
Aside from vestibular component of VIII, what are the other inputs to vestibular nuclei?
- Reciprocal vestibular projections
- Cerebellum (flocculonodular lobe)
- Spinal cord (some fibres from posterior spinocerebellar tract)
List the Outputs from the Vestibular Nuclei
Ipsilaterally: Lateral VestibuloSpinal Tract -> for postural adjustments (e.g. spinning around -> dizziness, staggering, etc. this is due to exaggerated activity of the LVST)
Contralaterally: Medial VestibuloSpinal Tract. Compensatory neck movements.
Output goes to: cerebellum, CNIII, IV and VI nuclei, thalamus, reticular formation, emetic centre (control of vomiting)
What is the Vestibuloocular Reflex? (VOR)
The VOR generates eye movements to compensate for head movements. I.e. to remain focussed on stimulus whilst moving head.
Can be overriden voluntarily by the Flocculus of the Cerebellum
What is the Oculocephalic reflex? How do you test for it?
(Doll’s eye movement)
A form of VOR
Suggestive of damage to the brainstem in comatosed patients.
To test in comatosed patient, gently turn head side to side, and check for eye movements with turning.
Absent dolls sign = eyes remain fixed in mid position.
Describe Nystagmus
What can nystagmus assess?
Fast eye movement, involving signals from the reticular formation.
Very rapid movement of the eye in the opposite direction to that in VOR.
Crista ampullaris of SC canals are being stimulated while head is not in motion.
Nystagmus and VOR can be sued to assess level of brainstem damage in patients
Describe Optokinetic Nystagmus
When there are Slow ‘smooth persuits’ of the visual stimulus (e.g. looking at a tree as you drive past it), coupled with rapid corrective phases ‘saccades’ to catch up with actual movement.
I.e. imagine looking at passing by countryside through train window
Describe Caloric Nystagmus & how to test
Endolymphatic convection induced by cool water or air
Head is tilted back at 60 degrees
Idea: cool water in right ear ilicits similar response as rotation of the head to the left
Mnemonic: COWS: cool, opposite; warm, same.
COLD = head turn and rapid eye movement (nystagmus) occurs in the OPPOSITE direction to ear in which cold water is put. Then slow eye movement (VOR) occurs towards the side of cold water.
WARM = head turn and nystagmus is in SAME direction as side of warm water. Then slow eye movement (VOR) occurs in opposite direction.
What symptoms would occur due to tumour involving the vestibular nerve?
Dizziness
Nausea
Spacial Disorientation
What symptoms would occue due to tumour at cerebellopontine angle?
Facial (VII) and Vestibulocochlear (VIII) nerve involvement…
Loss of pain and temperature from ipsilateral (left) side of the face: where is lesion? Due to?
Left Spinal Trigeminal Nucleus and Tract
issues with posterior inferior cerebellar artery
Dysarthria and Dysphagia: lesion is? due to?
Nucleus Ambiguous
Posterior inferior cerebellar artery
Loss of PAIN and TEMPERATURE on the contralateral (right) side of the body: lesion is where? due to
Left Spinothalamic Tract
posterior inferior cerebellar artery
GAIT ATAXIA on the ipsilateral (right) side of the body: lesion is where? due to
Right Spinocerebellar Tracts
Posterior inferior cerebellar artery
Vertigo, Abnormal Nystagmus: lesion?
Due to?
Vestibular Nucleus
Posterior inferior cerebellar artery
Stroke of Posterior Inferior Cerebellar Artery causes what type of general syndrome?
Lateral Medullary Syndromes.
Basic Functions of the different areas of the inner ear:
Cochlea Duct - hearing
Utricle and Saccule (of vestibule) - Linear Acceleration/Orientation of the head, relative to gravity
Semicircular Ducts (anterior or superior, posterior and lateral) - Angular Acceleration
What are the 2 sealed openings between the tympanic cavity and inner ear? Where are they situated?
Oval Window - vestibular - Closed by Stapes
Round Window - cochlear - closed by the secondary typmpanic membrane
Both reside on Medial Wall (between tympanic canity and inner ear)
How does Lateral Medullary Syndrome present, and what is principal cause?
Principally caused by occlusion/stroke of Posterior Inferior Cerebellar Artery (supplies cerebellum and choroid plexus of the 4th ventricle)
Diagnostic presentation for LMS is:
Loss of pain/temp and sensation to trunk and extremities on contralateral side to stroke/occlusion. And loss of pain/temp and sensation to face on ipsilateral side.
