Auditory & Vestibular system Flashcards
where is the ear in relation to the skull?
embedded in petrous portion of temporal bone
What frequency of motion do the vestibular and hearing organs capture respectively?
Vestibular organ: capture low frequency motion (movements)
Hearing organ: capture high frequency motion (sound)
What is frequency? What does it measure?
Cycles per second, measures pitch (perceived tone) in Hz
What is amplitude/ loudness?
Sound pressure (dB), subjective attribute correlated with physical strength.
Human range of hearing (frequency and amplitude)
Frequency: 20–20,000Hz
Loudness: 0 dB to 120 dB sound pressure level (SPL)
what is the function of the outer ear?
Capture sound and focus it on tympanic membrane
Modest amplification (10DB) of upper range of speech frequencies by resonance in the 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?
The main function of the middle ear is mechanical amplification (can provide an additional 20-30dB)
(by using leverage from incus-stapes joint to increase force on oval window)
how does the middle ear focus vibrations using pressure?
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
It does so in a way that captures the frequency (or pitch) and intensity (or loudness) of the 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
This is where the hearing organ/organ of corti is located
what is the basilar membrane?
How is it arranged?
base of the organ of corti
Basilar membrane is arranged tonotopically, using the same principle as a xylophone
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 is the function of auditory ossicles?
transmit vibration of tympanic membrane to oval window
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?
What does it do?
What is the disadvantage of it?
canal linking middle ear and nasopharynx
equalises middle ear pressure
but pathogens may travel through tube
what is the organ of corti?
What 2 types of hair cells does it contain
hearing organ
contains inner and outer hair cells
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
Where is the tectorial membrane located and what is its function?
The tectorial membrane is above the hair cells and allows hair deflection, which in turn will depolarise the cell.
what is the role of inner hair cells?
IHC carry 95% of the afferent information of the auditory nerve. Their function is the transduction of the sound into nerve impulses.
what is the role of outer hair cells?
OHC carry 95% of efferents of the auditory nerve. Their function is modulation of the sensitivity of the response.
what are hairs of hair cells known as?
stereocilia
describe the process of transduction in the organ of corti?
What closes K+ channels?
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?
Spiral ganglions via the vestibulo-cochlear nerve (8th Cranial nerve) travel to the ipsilateral cochlear nuclei (monoaural neurons) in the brainstem (pons)
Auditory information crosses at the superior olive level
After this point all connections are bilateral
travels to inferior colliculus in the brainstem
then medial geniculate body in thalamus
enters temporal lobe auditory cortex or goes to lanuage centre
Types of hearing loss based on anatomy of hearing loss
Conductive hearing loss: Problem is located in outer or middle ear.
Sensorineural hearing loss: The sensory organ (cochlear) or the nerve (auditory nerve). (90% of all hearing loss!)
Central hearing loss: Very rare and originates in the brain and brainstem
Types of hearing loss based on timing
Sudden hearing loss minutes to days
Progressive hearing loss months to years
Causes of conductive hearing loss
Outer ear- cerumen impaction, foreign body
Middle ear- otitis, otosclerosis
Causes of sensorineural hearing loss
Inner ear- Noise, presbycusis, ototoxicity
Nerve- vestibular schwannoma (acoustic neuroma)- unilateral
Bedside hearing tests
Whisper in ipsilateral ear whilst rubbing fingers in contralateral ear
Tuning Fork – Weber and Rinne tests assess the presence of gross hearing loss.
Bedside tests are quick but not accurate
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 an audiogram?
The audiogram is where the hearing thresholds are plotted to define if there is a hearing loss or not. A normal hearing threshold is located between 0 – 20dB
How can sensorineural and conductive hearing loss be distinguished using audiometry
what is otoacoustic emissions test?
The normal cochlea produces low-intensity sounds called OAEs
These sounds are produced specifically by the outer hair cells as they expand and contract
This test is often part of the newborn hearing screening and hearing loss monitoring.
how is hearing loss quantified?
mild
moderate
severe
profound
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?
What organs are repsonsible for these inputs?
WHat happens next?
Movement and gravity
Mechanical sensors (canals and otoliths)
The CNS integrates this information and generates responses.
The outputs are perception and reflexes that maintain posture and gaze. (ocular reflex and postural control)
where is the vestibule located?
in inner ear, connected to cochlea
what otolith organs make up the vestibule?
utricle and saccule
Is the utricle or saccule connected to the cochlea?
Saccule
what does the vestibular organ consist of?
utricle, saccule and semicircular canals
What are the three semicircular canals? What are they connected to?
Anterior, posterior, lateral
Connected to utricle
what is the structure of a vestibular hair cell?
stereocilia and a kinocilia (largest cilia)
what are the otolith organs?
utricle and saccule
Where are the cells of the utricle and saccule located? How are these oriented?
cells are located on the maculae, placed horizontally in the utricle and vertically in the saccule
what are the maculae made up of?
The maculae contain the hair cells, a gelatinous matrix and the otoliths on top. These otholiths are carbonate crystals that help the deflection of the hairs.
What types of movements can otolith organs detect?
what connects the utricle and the semicircular canals?
ampullae
Where in the semicircular canals are hair cells located and what is found in the rest of the semicircular canals?
The hair cells in the canals are located in the ampulla. The rest of the canal only has a liquid high in potassium called endolymph
what is the structure of the ampulla?
crista where hair cells are located, surrounded by cupula which helps hair cell movements
What does the resting potential in vestibular hair cell do?
Generates basal discharge to the nerve
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 do the semicircular canals detect? Why?
How
angular, canals work in pairs
Where do the vestibular nerve afferents project to?
Primary afferents end in vestibular nuclei in the brainstem (pons)
Vestibular system functions?
To detect and inform about head movements
To keep images fixed in the retina during head movements
BALANCE
what are the vestibular output/reflexes?
vestibulo-ocular reflex
vestibulo-spinal reflex
what is the vestibuloocular reflex?
How is this achieved?
What cranial nerve nuclei are connected?
keeps images fixed in retina- eye movement in opposite direction to head but with same velocity and direction
connection between vestibular nuclei and oculomotor nuclei
Difference in symptoms between vestibular disorders causing acute+unilateral VS slow+ unilateral/ any bilateral loss
what structures are affected by peripheral vestibular disorders?
Give examples
Vestibular organ and/or VIII nerve
e.g. Vestibular neuritis, Benign Paroxysmal Positional Vertigo (BPPV), Meniere’s disease
what structures are affected by central vestibular disorders?
CNS (brainstem/cerebellum)
e.g. stroke, MS, tumour
Red flags for vestibular disease
Headache
Gait problems
Hyper-acute onset ((think stroke))
Hearing loss
Prolonged symptoms (>4 days)
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?
migraines
menieres disease
which vestibular diseases have progressive presentations?
Schwannoma vestibular (VIIIth nerve)
Degenerative conditions (MS)
Test used for BPPV
Dix-Hallpike test
What test can be used to distinguish vestibular neuritis from stroke?
What does this include
HINTS exam
HeadImpulseTest
Horizontal rotational VOR
Nystagmus (Slow movement in one direction and fast compensatory movement)
Vestibular organ Vs Cerebellar/brainstem nystagmus
TestofSkewDeviation
Verticalmisalignment - usually absent in peripheral pathology
HINTS exam differences peripheral vs central vestibular disease
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.
