Auditory & Vestibular system Flashcards
where is the ear in relation to the skull?
embedded in petrous portion of temporal bone
what is the function of the outer ear?
capture sound and focus it on tympanic membrane
amplify frequencies by resonance in canal
protect ear from external threats
what does the outer ear consist of?
Pinna
external auditory canal
where does the middle ear start?
at tympanic membrane
what is the function of the middle ear?
amplifies sound by focusing vibrations
using leverage from incus-stapes joint to increase force on oval window
how does the middle ear focus vibrations?
large surface area of tympanic membrane to small surface area of oval window, therefore increases pressure
what is the role of the cochlea?
hearing part of inner ear
transduces vibration to nervous impulses
produces frequency and intensity analysis of sound
what are the compartments of the cochlea?
scala vestibuli, scala media and scala tympani
what are the scala vestibuli and scala tympani?
bony compartments of cochlea, contain perilymph (high in Na+)
what is the scala media?
membranous compartment of cochlea, contains endolymph
is the hearing organ/organ of corti
what is the basilar membrane?
base of the organ of corti
contains sensory receptors for hearing along its length which are arranged tonotopically
what is the function of the external auditory meatus?
guides sound waves to tympanic membrane
ceruminous glands secrete ear wax (cerumen) to trap foreign objects
what is the tympanic membrane?
ear drum
thin connective tissue membrane covered by skin externally and a mucous membrane internally
what are auditory ossicles?
transmit vibration of tympanic membrane to oval window
Malleus, incus, stapes and stapedius, tensor trympani
what is the malleus?
bone connected to tympanic membrane and incus
what is the incus?
connects malleus and stapes
what is the stapes?
footplate inserts onto oval window, connects middle and inner ear
what are the stapedius and tensor tympani?
skeletal muscles attached to auditory ossicles
protect ear from prolonged, loud but not explosive sounds
what is the eustachian/pharyngotympanic tube?
canal linking middle ear and nasopharynx
equalises middle ear pressure
but pathogens may travel through tube
what is the organ of corti?
hearing organ
contains inner and outer hair cells
allows transduction and modulation of sensitivity of sound
what is the arrangement of inner and outer hair cells in the organ of corti?
one row of inner hair cell to 3 rows of outer hair cells
what is the role of inner hair cells?
carry afferent information to auditory nerve
therefore transduction of sound
what is the role of outer hair cells?
modulate sensitivity of response to sound
because they carry efferents to auditory nerve
in contact with tectorial membrane above hair cells unlike IHCs
contraction of OHCs moves tectorial membrane
what are hair cells?
cells within organ of corti with stereocilia (hairs) which are able to deflect
describe the process of transduction in the organ of corti?
deflection of stereocilia towards the longest cilium (kinocilium) opens K+ channels
depolarises the cell, neurotransmitter released (glutamate)
higher amplitudes cause greater deflection and more K+ channel opening
hyperpolarisation closes K+ channels
what is the auditory pathway?
spinal ganglia from cochlea go via VESTIBULOCOCHLEAR nerve CN VIII to ipsilateral cochlear nuclei
crosses at superior olive level, connections are now bilateral
travels to inferior colliculus in the brainstem
then medial geniculate body in thalamus
enters temporal lobe auditory cortex or goes to lanuage centre
how is the primary auditory complex arranged?
tonotopically - small to large frequencies (pitch) rostral to caudally
what is frequency of sound?
pitch in Hz
cycles per second
what is sound amplitude?
volume in dB
sound pressure
what are the frequency and volume ranges for humans?
20-20,000Hz
0dB-120dB
name every type of hearing assessment
tuning fork (weber, rinne test) pure tone audiometry central processing assessment tympanometry otoacoustic emissions auditory evoked potentials auditory brainstem response cortical potentials
what is the weber test?
tuning fork placed on top of head (should hear on both sides)
tests for unilateral hearing loss
conductive - heard louder in deaf ear
sensorineural - heard louder in normal ear
what is the rinne test?
tuning fork test
compares air conduction vs bone conduction in each ear
detects unilateral hearing loss
positive - normal, air louder than bone
negative - bone louder than ear, conductive
false positive - air louder than bone, sensorineural
what is pure tone audiometry?
measures hearing acuity for variations in sound intensity an frequency
produces an audiogram where hearing thresholds are plotted
what is a central processing assessment?
assessments of hearing abilities other than detection e.g localisationm, speech in noise, filtered speech
verbal and non verbal tests
what is tympanometry?
examination to test middle ear and mobility of tympanic membrane and conduction bones (ossicles)
what is otoacoustic emissions test?
newborn hearing screening and hearing loss monitoring test
OAEs produced by outer hair cells are detected
what are auditory evoked potentials?
records time it takes nerves in the auditory system to respond to sound and electrical stimulation
evoked by clicks or tone bursts
what is auditory brainstem response ABR?
measures electrical activity from 8th nerve and brainstem nuclei/tracts evoked by clicks
why are cortical potentials measured?
not widely used
neurological or processing disorders may affect them
what are the types of hearing loss?
conductive
sensorineural
mixed
what is conductive hearing loss?
outer/middle ear problems
what is sensorineural hearing loss?
problem located in inner ear or auditory nerve
what is mixed hearing loss?
conduction and transduction of sound affected
more than one part of ear affected
how is hearing loss quantified?
mild
moderate
severe
profound
what are the causes of conductive hearing loss?
outer ear - wax, foreign body
middle ear - otitis, otosclerosis
what are the causes of sensorineural hearing loss?
inner ear - presbycusis, ototoxicity
nerve - VIII nerve tumour
how is hearing loss treated?
based on underlying cause
- hearing aids
- brainstem implants
- cochlear implants
what are hearing aids?
amplify sound without replacing any structure
are made to measure for certain frequencies and are dependent on cell preservation
what are cochlear implants?
replaces hair cell function
receives, transmits, analyses and transforms sound to electrical impulses
needs functioning auditory nerve to work
what are brainstem implants?
suitable when auditory nerves are damaged
electrical signals sent to set of electrodes placed in brainstem, aiming for cochlear nucleus
what inputs does the vestibular system compose of?
visual (eye), proprioceptive (pressure) and vestibular information
what outputs does the vestibular system have?
mainly reflexes - postural control, ocular reflex
nausea
where is the vestibule located?
in inner ear, connected to cochlea
deeper into head than cochlea
what is the structure of the vestibule?
utricle and saccule
what does the vestibular organ consist of?
utricle, saccule and semicircular canals
what is the structure of a vestibular hair cell?
stereocilia and a kinocilia (largest cilia)
what are the otolith organs?
utricle and saccule
cells are located on the maculae, placed horizontally in the utricle and vertically in the saccule
what is the maculae?
part of utricle and saccule that detects linear acceleration
contain hair cells, gelatinous matrix and otolith crystals on top to aid hair deflection
orientation - vertical = saccule horizontal = utricle
what connects the utricle and the semicircular canals?
ampullae
what is the structure of the ampulla?
crista where hair cells are located, surrounded by cupula which helps hair cell movements
how do hair cells detect head movement?
gravity deflects calcium carbonate crystals, causing stereocilia to bend towards kinocilia, depolarisation occurs, increasing hair cell firing rate from basal level
OR
gravity deflects calcium carbonate crystals, causing stereocilia to bend away from kinocilia, hyperpolarisation occurs, decreasing hair cell firing rate from basal level
what sort of movement/acceleration does the utricle detect?
horizontal movement
what sort of movement/acceleration does the saccule detect?
vertical movement
what sort of movement/acceleration do the semicircular canals detect?
angular, canals work in pairs
cupulla moves and displaces hair cells
output signal on VIII nerve is velocity
what are the vestibular output/reflexes?
vestibulo-ocular reflex
vestibulo-spinal reflex
what is the vestibuloocular reflex?
keeps images fixed in retina
connection between vestibular nuclei and oculomotor nuclei - eye movement in opposite direction to head but with same velocity and direction
what is the vestibulospinal reflex?
maintains postural control by compensatory body movement according to head position
motor neurons to limb muscles (lateral tract)
or neck/back muscles (medial tract)
how is the vestibular system function assessed?
posture and gait
cerebellar function
eye movements
all via vestibular tests, imaging and history taking
what are the vestibular tests?
caloric test
video head impulse test
vestibular evoked myogenic potential
rotational test
what are peripheral vestibular disorders?
labyrinth or VIII nerve dysfunction
aka menieres, BPPV, vestibular neuritis, unilateral/bilateral vestibular hypofunction
what are central vestibular disorders?
CNS brainstem/cerebellum dysfunction
aka stroke, MS, tumour
which vestibular diseases have acute presentation?
vestibular neuritis
stroke
which vestibular diseases have intermittent presentations?
benign paroxysmal positional vertigo
which vestibular diseases have recurrent presentations?
menieres disease
migraines
which vestibular diseases have progressive presentations?
schwannoma of VIII
degenerative conditions e.g MS
what is BPPV?
crystals displaced from utricle into semi circular canals causing nystagmus and nausea upon certain head movements
what is meniere’s disease?
hearing and balance issues due to malabsorption of endolymph
describe the vestibular ocular reflex when turning head to the right
right lateral canal stimulated (otolith movement) and left lateral canal inhibited (hyperpolarised)
stimulates right vestibular nucleus, inhibits left vestibular nucleus
left abducens nucleus stimulated, right abducens inhibited
right oculomotor nucleus stimulated, left oculomotor nucleus inhibited
therefore right medial rectus will contract
and left lateral rectus will contract
eye moves in opposite direction to head tilt
simply - vestib and oculomotor on side turning towards are stimulated, abducens inhibited and opposite on other side
what area in the brain may be associated with cerebral control of vestibular system?
parieto-insular vestibular cortex
interpretation of Webers test
If a patient has a unilateral CONDUCTIVE hearing loss, the tuning fork sound will be heard louder in the deaf ear.
If a patient has a unilateral SENSORINEURAL hearing loss, the tuning fork sound will be heard louder in the normal ear.
In bilateral and symmetrical hearing loss of either type Weber’s test will be normal.
