Hearing & balance Flashcards
Otic capsule
in the petrous portion of the temporal bone
Components of vestibular sensory organs
3 semicurcular canals, oriented 90 degrees to each other
2 otoliths
auditory cochlea
endolymphatic sac
Semicircular canal function
detect angular acceleration due to head rotations
Utricle/saccule
2 sac-like organs between semicircular canals and cochlea
utricle in the horizontal plane
saccule in the saggital plane
sense body orientation relative to gravity and linear motion
Perilymph
fills space between bony and membranous labyrinths
composition similar to extracellular/cerebrospinal fluid (low potassium and high Na)
ultrafiltrate of CSF/blood
drains via venules and middle ear mucosa
Endolymph
inside
in contiguous open lumen of semicircular canals, utricles and saccules
unique extracellular fluid - similar to intracellular fluid (high K, low Na)
produced by dark cells of sensory epithelium
absorbed by endolymphatic sac
Sensory epithelia components
Cristae of semicircular canals (3)
Maculae of utricle and saccule (2)
Sensory epithelia contain hair cells which are receptor cells and detect movement of endolymph
Vestibular hair cell types
Stereocilia - 60-100/hair cell
Kinocilium - 1/hair cell
Release glutamate as neurotransmitter
Vestibular hair cell distribution
utricles/saccules: ~35000 hair cells
Ampulla of semicircular canals: ~8000
Vestibular hair cell function
Transforms endolymph motion into neuronal signal
Bends one way: excitation; the other way: inhibition
Semicircular canal sensory epithelia
Ampulla: swelling at the end
Cristae: hair cell sensory epithelium within ampula
Cupula: acellular, gelatinous mass hinged gate spanning ampulla lumen
All hair cells in ampulla re oriented in the same direction with kinocilium closest the utricle
Semicicular canal arrangement
Pairs of crista arranged as mirror opposites
e.g. Right anterior - Left posterior
Otoconia
calcium carbonate crystals sit on top of otolithic membrane
pressure of otoconia deflects hair cell cilium
Utricle plane
horizontal
Saccule plane
vertical
Vestibular afferent
bipolar neurons
Vestibular ganglion
Scarpa’s ganglion
CN VIII course
follows internal auditory meatus with facial nerve
enters brainstem at jxn of pons and medulla = cerebellopontine angle
projects to the vestibular nuclear complex, cerebellum
Vestibular nucleus complex location
dorsal pons and medulla beneath 4th ventricle
Vestibular nuclei (4)
Lateral
Medial
Superior
Inferior
Lateral vestibular nucleus fxn
Deiter’s nucleus
innervates gravity-opposing muscles of limbs to maintain posture
Medial vestibular nucleus fxn
reflex adjustments of neck and trunk muscles to restore head position after disturbance
Keep head upright
Superior/medial vestibular nucleus fxn
eye movements
vestibulo-ocular reflex
Inferior vestibular nucleus
integrates multi-sensory input, and cerebellum to regulate VOR gain in response to constant input
Lateral vestibulospinal tract
input: utricle,saccule, semicircular canals
target: ventral horn alpha/gamma motor neurons that innervate gravity-opposing muscles of limbs
Medial vestibulospinal tract
input: primarily semicircular canals
pathway: bilateral; descending MLF
Target: cervical and upper thoracic spinal cord, motor neurons innervating neck musculature
VOR tract
input: semicircular canals
pathway: 3 neuron arcs
1) bipolar neurons
2) medial and superior vestibular nuclei
3) motor neurons in abducens nucleus and oculomotor nuclei that innervate oculomotor muscles
Horizontal VOR
coordinates 4 muscles:
L, R lateral recti
L, R medial recti
Vestibular nystagmus
rhythmic alteration of slow and fast movements during VOR
VOR: slow
Saccade: fast
Caloric testing
Eye will deviate towards the cold water
and nystagmus towards the other side
Benign paroxysmal positional vertigo
displaced otoconia lodged in semicircular canals
acute violent sensation of dizziness;vomiting
Meniere’s disease
endolymphatic hydrops
increased endolymph in inner ear
can damage hair cells
Vestibular neuritis
Viral infection of vestibulocochlear nerve
rather common; unilateral damage
Perilymph fistula
breach in oval and/or round window
leaky endolymph –> decrease internal concentration
opposote of Meniere’s
Ototoxicity
toxicity induced death of hair cells by some antibiotics
e.g. aminoglycosides: gentamicin, streptomycin, kanamycin, neomycin, etc
Mal de Debarquement
failure of CNS plasticity to respond to prolonged movement
getting off boats/ships - people never regain land sensation (land sickness)
Aging/dizziness/balance
visual/motor deficits
BPPV
gradual hair cell loss
Bilateral vestibular dysfunction
occurs with ototoxicity
slow onset of loss of vestibular fxn
instability of eyes with head movements
instability when walking in the dark (no visual input)
Unilateral vestibular dysfunction
severe acute symptoms extreme dizziness, nausea, vomiting deviation towards side of lesion when walking abnormal nystagmus displaced otoconia, viral infection
Vestibular compensation
Gradual recovery from unilateral lesions
learning-induced changes to central circuits
vestibular inputs ignored in favour of vision/proprioception
