CN VIII Flashcards
Branches of vestibulochoclear n
Cochlear - hearing and detection of sound waves
Vestibular - sensation, head position relative to space and head movements relative to the body
CN VIII path
Exits brainstem at pontomedullary junction
- subarachnoid space to enter internal auditory meatus
- w/ NC VII in auditory canal through temporal bone
- enter cochlear and vestibular organs
Hearing and vestibular sensations come from
Structures contained in inner ear
Inner ear
Semicircular canals
Vestibule
Cochlear
Bony labyrinth
Filled with perilymph
Membranous labyrinth
Filled with endolymph
- semicircular canals
- utricle and saccule
- cochlear duct
Cochlea
Contains
- Scalia vestibule (superior chamber of perilymph)
- Scalia tympani (inferior chamber)
- cochlear duct b/n them
- basilar membrane b/n cochlear duct and Scalia tympani (extends entire width of cochlea)
- organ of corti (in cochlear duct, residing on basilar membrane - hair cells, tectorial membrane, terminals of cochlear nerve)
Converting sounds to neural signals
- sound waves hip tympanic membrane —> vibration of ossicles
- vibration of oval window (Scalia vestibule) —> move perilymph
- vibration of basilar membrane —> hair cells move
- hair cell movement stimulates cochlear nerve
Hair cells and frequency
Higher frequency sound activate cells closer to oval window
Lower frequency activate closer to apex of cochlea
Axons of CN VIII travel to the dorsal and ventral cochlear nuclei
Fibers from both nuclei cross over via trapezioid body (ventral cochlear synapse bilaterally in superior oligarchs nuclei)
Ascend in lateral leminiscus to inferior colliculus
Fibers cross and ascend in brachium of inferior colliculus to MGN
Information from one ear ascends
Bilaterally which is why you get info on both sides of your brain from one ear
Small CNS lesions (CN VIII)
Rarely cause unilateral loss of hearing because auditory info projects bilaterally in brainstem and cerebrum
Primary auditory cortex
Conscious awareness of intensity of sound
Auditory association cortex
Compares sounds with memories of other sounds and classifies sounds (Area 42) (as language, music, noise, etc.)
Wernicke’s area (Area 22)- language comprehension
Unilateral hearing loss - damage to
Damage to external auditory canal middle ear cochlear organ of corti Cochlear nerve Cochlear nuclei
Unilateral hearing loss
Interferes w/ ability to locate sounds because timing of auditory info from each ear is combated to locate sounds
Conductive hearing loss
Caused by abnormalities in outer or middle ear
-wax buildup, otitis media, tympainic membrane tear
Sensorineural hearing loss
Disorders of cochlear or CN VIII
Prolonged exposure to loud noise, onto toxic drugs, Menderes disease, acoustic neuroma
Basic hearing can be tested with
Different frequencies
Conductive and sensorineural hearing loss can be tested
using tuning fork w/ rinne or Weber test
Acoustic neuroma
Compress CN III where it enteres auditory meatus
Tumor at cerebellopontine angle
Most common cause of acoustic neuroma -almost always unilateral -slow, progressive unilateral hearing loss w/ c/o tinnitus and balance problems -mean age of onset is 50yo Can include CN VII and CN V Removed surgically
Vestibular sensation
Structures in inner ear
Vestibular apparatus
Semicircular canals and vestibule
Bone labyrinth w/ perilymph and membranous labyrinth w/ endolymph
Vestibular -membranous labyrinth
Utricle (openings to semicircular canal)
And saccule
Semicircular canal - small openings at each end to utricle
Vestibular - semicircular canals
Detect angular rotation of head (speed up or slow down rotation of head)
Canals are perpendicular to each other
Three pairs of semicircular canals
Right and left anterior (45 deg ant to frontal plane)
Right and left posterior (45 deg pst to frontal plane)
Right and left horizontal (30 deg above horizontal plane)
Semicircular Calais work together as coplanar pairs
RP and LA
RA and LP
R and L horizontal
Ampulla
Bulge located at one of each semicircular canal
Hair cells in crystal ampullaris have cilia
Cilia of hair cells project up to cupula
Movement of endolymph causes cupula to bend
-results in deflection of cilia causing excitation or inhibitition of hair cells
Hair cells convert displacement
Due to angular head rotation into neural firing and send axons into the vestibular nerve
Each semicircular canal responds best
To motion in its own plane (w/ coplanar pairs responding to a shared plane)
brain detects direction of head movement by comparing
input from coplanar pairs
When angular motion occurs w/in shared plane, each of the pairs produces reciprocal signals - excitatory from one w/ inhibitory from other.
Results in neural firing that will increase in one vestibular nerve and decrease in another
During ipsilateral head rotations
Ipsilateral afferents are excited
Otolith organ
Vestibular pathway
Calcium carbonate crystals embedded in top of gelatinous layer to give mass (shearing force w/ movement)
Respond to linear motion and static head tilt w/ respect to the gravitational axis
Utricle
Vestibular pathways
Excitation occurs during horizontal linear motion and head tilt
Saccule
Excitation occurs during vertical linear motion
Macula
In utricle and saccule
Contain hair cells in gelatinous layer
Linear motion and head tilt cause otoconia to move and then gelatinous layer and hair cells move
Hair cells convert
Displacement into neural firing and send axons into the vestibular nerve
Vestibular nuclei
Have many connect w/ cerebrum, brainstem, motor systems, extraocular systems
Sends awareness of head position to parietal association cortex that is integrated w/ visual and tactile information to contribute to spatial awareness
Vestibular nuclei adjust
Posture
Muscle tone
Eye position in response to movement of head in space
Vestibular nuclei integrate
Information from multiple senses, acting as central processor
-primary processor of vestibular info
Assist w/ refining movement (numerous connection w/ cerebellum)
Vestibular nuclie also receive
Visual
Proprioceptive
Tactile
Auditory info
Vestibular nuclei lye
On lateral floor of 4th ventricle in pons and rostrum medulla
Four vestibular nuclei
Lateral
Medial
Superior
Inferior
Lateral vestibular nucleus
Gives rise to lateral vestibulospinal tract
Extends entire length of cord
*maintaining balance and extensor tone
Medial vestibular nucleus
Largest
Gives rise to medial vestibulospinal tract (medial motor systems)
Extends to C spine
*controlling head and neck position
Medial longitudinal fasciculus
Connect vestibular nuclei to oculomotor, trochlear and abducens nuclei
Fibers mainly from superior vestibular nucleus and inferior (ascends in MFL to CN III, IV, VI
MLF function
Match eye movements resulting in conjugate gaze in all directions
Pathway mediates the vestibulo-ocular reflex in which eye movements are adjusted for changes in head position
Vestibular info is sent mainly to
Flocculonodular lobes and vermis in cerebellum
Vestibulocerebellum
Vestibular nuclei also send info to
- Lateral temporal junction and posterior insula - conscious perception of head position and movement
- CN XI nucleus - influence head position
- Reticular formation - influence reticulospinal (posture and gait) tracts and autonomic centers for nausea and vomiting
Vertigo
Spinning sensation of movement
Suggestive of vestibular disease more than any other sensations
Most often from peripheral vestibular disorders (inner ear structures - CN VIII, SCCs, otolith)
Vertigo - lesions
Anywhere along the vestibular pathway from labyrinth, CN VIII, vestibular nuclei, cerebellum, cortex
Peripheral vestibular disorders w/ vertigo often accompanied by
Disequilibrium
Nystagmus
Nausea
Vomiting
Benign paroxysmal positional vertigo (BPPV)
Sudden changes in head position resulting in vertigo and nystagmus
Otoconia from nearby masculine dislodge and flat into a SCC
- trauma or virus
- at rest, otoconia settle in gravity dependent position in SCC
- head moved quickly - otoconia move into new position in SCC causing abnormal movement of endolymph - causing abnormal signals in CV III
*acute onset of vertigo and nystagmus
BPPV test and tax
Test w/ dix hallpike maneuver (PT rapidly but gently turns head so that one ear is down)
Treated w/ repositioning maneuvers
Vestibular neuritis
Inflammation of vestibular nerve
Virus
Several days of intense vertigo, disequilibrium, nystagmus, nausea, symptoms subsiding over a few weeks
Meds during acute phase my suppress vertigo and N/V
Ménière’s disease
Endolymphatic hydrops
- excessive fluid and pressure in endolymph
- recurrent intense vertigo along w/ fullness in ear w/ fluctuating hearing loss and tinnitus and N/V
- meds during acute phase may suppress vertigo and N/V, w/ extreme cases vestibular nerve may be severed to relieve symptoms
