L03: Vestibulocochlear Function & Vestibular Syndromes (Carrera) Flashcards
Vestibular function***
- maintenance of posture/balance (head and body)
- functions intimately w/ the cerebellum
- can be peripheral or central
CN 8 name and chars.
vestibulocochlear n.
- ONLY cn to exit skull
- ONLY sensory
2 portions of cn8
1) vestibular
2) auditory
only cn to exit skull
8 (vestibulocochlear)
vestibulocochlear neurons synapse where?
with specialized “hair” cells in receptor organs of the inner ear that transform mechanical deformation into neural signals
sensory receptors for hearing are located where
cochlea in the Organ of Corti
transduction of hearing takes place in the:
Organ of Corti (houses hair cells)
blood supply to cochlea
stria vascularis
3 main components of cochlea
scala vestibuli
scala tympani
scala media/cochlear duct
middle ear houses:
ossicles (amplifiers of sound waves). First ossicle attaches to tympanic membrane
sound waves come into bulla and hit tympanic membrane which then conducts to rest of inner ear
:)
cochlear projections
- leave the spiral ganglion and enter brainstem at medulla oblongata/pons
- synapse in the cochlear nucleus
basic auditory pathway
inner ear –> cochlear nuclei in medulla –> pons –> midbrain –> auditory complex of forebrain
precocial species
(born with fully formed brains and functional hearing) chicken sheep cattle horses
altricial species
(born without fully formed brains and functional hearing)
humans
monkeys
dogs, cats
conductive deafness
failure of conduction of sound waves from the ear canal to the inner ear due to occlusion of the ear canal
-congenital or acquired (ie. rupture of tympanic membrane, exudate, ossicles with age, etc.)
sensorineal deafness
alterations of the neural structures of the auditory pathway
-congenital (hair cells of OoC) or acquired (infection, intracranial dz)
which animals susceptible to congenital sensorineural deafness
American Paint horses
lethal white foals
dalmatians
white dogs and cats with blue eyes
vestibular labyrinth
- composed of utricle and saccule
- located in large vestibule
- detects static or kinetic position (gravity and linear acceleration)**
macula
covered by hair cells projecting into gelatinous otolithlic membrane that contains otoliths
semicircular ducts
detect angular movements of the head
- located in semicircular canal
- receptor: crista ampullaris
where is entry into the skull?
internal acoustic meatus
where is entry into the brain?
@ lvl of trapezoid body and caudal cerebellar peduncle
-axons synapse onto cn8 or enter cerebellum
components of vestibular system
cerebellum brainstem cn8 vertical and horizontal ear canals semicircular canals tympanic bulla
vestibular nuclei
4 on each side of brainstem
brainstem dz will almost always reslt in vestibular dz b/c these nuclei take up so much space on brainstem
axonal projections from vestibular nuclei
- vestibulospinal tracts from spinal cord for anti-gravity muscles
- Medial Longitudinal Fasciculus rostrally for ocular and head movements
lateral vestibular nucleus projects to contra or ipsilateral ventral funiculus?**
IPSILATERAL; mediates anti-gravity system on ipsilateral side
medial longitudinal fasciculus rostral projections
- nuclei of cn3,4,6
- responsible for oculocephalic reflex
medial longitudinal fasciculus caudal projections
- medial vestibulospinal tract
- maintains body and limb position relative to the head
extraocular muscles innervation
cn3 (oculomotor): dorsal, medial, ventral recti mm.
cn4 (trochlear): dorsal oblique m. (OPPOSITE SIDE)
cn6 (abducent): lateral rectus and retractor bulbi mm.
physiologic nystagmus
moves eyes to hold images during head rotation or target motion
MLF connects cn8 to which cn?
3,4,6
vomiting center
- in reticular formation of the medulla
- receives afferent input from the vestibular portion of the vestibulocochlear n.
cerebral projections
- synapse in thalamus
- provide conscious awareness of the body’s position in space
“if you excite on one side, you get ipsilateral facilitation of extensors and contralateral facilitation of flexors”
:)
when you are vestibular, you lean/tilt towards or away from lesion?
TOWARDS. If a lesion prevents activation of one side, the ipsilateral nuclei won’t be excited as much as the other side, so you get relative facilitation of extensors on the normal side and lack of facilitation on the affected side.
CS of vestibular lesion
head tilt
circling, leaning, falling TOWARDS lesion
unilateral or asymmetric ataxia TOWARDS lesion
abn. eye position or movements
nausea
+/- postural reaction deficits
nystagmus
involuntary movements of the eyes
-defined by direction of the fast phase
pathologic nystagmus moves which directions?
horizontal, rotary, or vertical
pendular nystagmus
- not actually nystagmus
- due to a defect in visual pathways
- common in Himalayans, Siamese
Central vs. Peripheral vestibular
central: involves brainstem or cerebellum
peripheral: involves ear canal, bulla, cn8
CS of CENTRAL vestibular lesion
proprioceptive deficits dullness, stupor head tilt other cn deficits strabismus nystagmus (especially VERTICAL) cerebellar and forebrain signs
CS of PERIPHERAL vestibular lesion
NO proprioceptive deficits normal mentation head tilt toward lesion only cn7 or 8 deficits strabismus nystagmus (any direction) *Can manifest as Horner's syndrome!*
If something affects bulla, will commonly affect which nerve?
cn7 (facial n.)
Paradoxical vestibular syndrome
head tilt AWAY from the lesion, while fast phase of nystagmus may be TOWARD the lesion
- ALWAYS CENTRAL (not peripheral)
- lesion is on same side as proprioceptive deficits
