55-101 Lecture Notes - Perilymph, Saccule, Benign Paroxysmal Positional Vertigo

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Published on 1 Feb 2013
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Outline of Lecture 41 (03-06 D; Minor)
Vestibular System
I. Function and properties of vestibular system
- The function of the vestibular system is to sense and control motion
- Vestibular afferents from the bony labyrinth project to vestibular nuclei (brainstem)
- Vestibular nerve has superior division (to horiz., sup. semicircular canals and utricule) and
inferior division (post. semicircular canal, saccule)
- The semicircular canals respond to angular acceleration whereas the otoliths (saccule and utricle)
respond to linear acceleration and translational movement of the head
- The membranous canal is filled with endolymph (high K) and the other vestibular space is filled
with perilymph (high Na), which sets up electrochemical gradient needed for signaling
- Mechanical deformation of stereocilia to or away from kinocilium excites and inhibits hair cell
activity, respectively
- In semicircular canals, physical movement of endolymph against cupula causes deformation; all
of the hair cells are oriented identically
- In otolith, hair cells project into mass containing CaCO3 crystals (otoconia), which transduces
linear acceleration into shear force across hair cells; the hair cells are oriented in opposite
directions
II. Vestibular reflexes
- Vestibulocollic (VCR) reflex stabilizes the head; vestibuloocular (VOR) reflex stabilizes the eye
- Reflex pathway is a three neuron arc: vestibular nerve afferent central vestibular neurons
extraocular neurons (III, VI) eye muscles; cerebellum provides conscious control at level of
central vestibular neurons
- Reflex pathway has appropriate excitatory and inhibitory synapses to ensure proper eye
coordination
- VOR reflex gain is dependent on specific context (e.g. eye position, bifocal glasses) in order to
prevent oscillopsia and blurred vision
- Ewald’s “laws”
1) Reflex eye and head movements are in plane of affected canal
2) Larger movements are induced by excitatory stimuli than inhibitory stimuli (due to larger
dynamic range)
III. Vestibular disorders
- Vertigo: illusion of motion, nauseating
- Oscillopsia: visualization of motion of stationary objects (e.g. in loss of VOR)
- Nystagmus: rapid beating of eyes due to imbalance of vestibular sensation
- Has a slow component directed toward weaker labyrinth and a resetting fast component
- Caloric test stimulates nystagmus by putting warm water in ear, which causes temperature
gradient and fluid flow in horizontal canal
- Benign paroxysmal positional vertigo: dislodged otoconia, reset by manipulation and gravity
- Vestibular neuritis: sudden onset of long-lasting vertigo, affects superior vestibular nerve
- Meniere’s syndrome: hearing loss, ringing, aural fullness, vertigo; caused by distended
endolymph spaces
- Superior canal dehiscence syndrome: vertigo due to extra opening into superior semicircular
canal; can be diagnosed with careful observation of eye nystagmus and imaging; corrected by
surgically sealing hole
- Bilateral vestibular hypofunction: oscillopsia symptoms; often caused by gentamicin antibiotic
Summary of major ideas
- Understand structures of and differences between semicircular canals and otoliths
- Understand reflex pathway and properties
- Of the disorders, pay attention to SCDS and effect of gentamicin
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