Auditory and Vestibular System Flashcards
What makes up the outer ear?
- concha
- external auditory meatus
What makes up the middle ear?
- tympanic membrane
- ossicles
- tensor tympani
- stapedius
- oval window
What makes up the inner ear?
- the vestibule
- the semicircular canals
- the cochlea.
What is the function of the inner ear?
- transduction of sound into nervous impulses
- captures frequency and loudness of sound
What is the function of the outer ear?
- focuses sound on the tympanic membrane
- modest amplification
- protects ear from external threats
What is the function of the middle ear?
- mechanical amplification of sound
- can reduce amplitude of sound via contraction of the tensor tympani and stapedius muscles
What is the inner ear innervated by?
Vestibulocochlear nerve (CN VIII)
What is Transduction?
- conversion of one energy form to another
- mechanical sound waves to electrical signals
What is the auditory pathway in the ear?
- stapes vibrates the oval window
- perilymph in the scala vestibuli vibrates
- waves in the perilymph move around the cochlea
- the waves move into the perilymph in the scala tympani
- the wave reaches the end at the round window
What is the auditory pathways to the brain?
- cochlea
- vestibulocochlear nerve
- dorsal or ventral cochlear nucleus
- inferior colliculus
- medial geniculate nucleus
- auditory cortex
What is the function of the inferior colliculus?
Localises sound so you know where it’s coming from
What is the pathway for signals in the dorsal cochlear nucleus?
Decussate to join the inferior colliculus
What is the pathway for signals from the ventral cochlear nucleus?
- travels through superior olive on the same or opposite side
- fibres travel to the inferior colliculus on the same side as the superior olive
Why is it rare to have total hearing loss after a stroke?
All connections after the superior olive are bilateral
What are the three ossicles?
- malleus
- incus
- stapes
What are the three scala/compartments of the cochlea?
- scala vestibuli
- scala tympani
- scala media
What is the structure of the cochlea?
Spiral tunnel
Decribe the three scala
- scala vestibuli and scala tympani = bony sturctures containing perilymph
- scala media = membranous structure containing endolymph
What is the difference between endolymph and perilymph?
- perilymph = rich in Na+
- endolymph = rich in K+
Where is sound tranduced into a nervous signal?
Organ of corti, located in the scala media
What does the organ of corti contain?
- basilar membrane
- inner and outer hair cells
- tectorial membrane
What are the mechanical properties of the base of the basilar membrane?
- narrow
- short and stiff hair cells
- detects high frequencies
What are the mechanical properties of the apex of the basilar membrane?
- wide
- long and loose hair cells
- detects low frequencies
How are hair cells arranged in the organ of corti?
Three rows of outer hair cells to one row of inner hair cells
What is the difference between the outer and inner hair cells?
- outer = efferents, modulate sensitvity of the response to sound
- inner = afferents, transduce sound into nerve signals, most important for hearing
Describe the process of mechanotransduction
- vibration of the basilar membrane oscillates hair cells
- stereoscilia sway with the same frequency as the basilar membrane
- the oscillations towards the longest cilia change the structure of the membrane ion channels and cause changes in their permeability
- K+ inlfux into hair cells leading to depolarisation
- K+ channels close when stereocilia oscilate towards shortest cilia, leading to repolarisation
What are the longest cilia called?
Kinocilia
What are stereocilia?
- hairs on the hair cells cells which detect sound
- located in the endolymph
What is the primary auditory cortex and how is it arranged?
- cortically processed sound
- tonotopically organised like basilar membrane
What is the secondary auditory cortex?
Where association of sound occurs
Where are the auditory cortices located?
Temporal lobe
How can we categorise causes of hearing loss?
- sensorinueral
- conductive
What is conductive hearing loss?
- ear connot transmit sound to cochlea due to mechanical problem
- middle or outer ear affected
- hair cells are functional when stimulated within the inner ear
- 10% of hearing loss, often reversible
What is sensorineural hearing loss?
- both air and bone conduction are affected similarly
- problem with the inner ear or vestibulocochlear nerve
- sound is unable to be effectively transduced to the auditory cortex
- 90% of hearing loss, often irreversible
What are possible causes of conductive hearing loss?
outer ear: - wax and foreign bodies middle ear: - otitis externa/media - otosclerosis (stuck stapes)
What are causes of sensorineural hearing loss?
- drugs
- Meniere’s disease
- congenital infection
- trauma
- ageing
What are the main 2 tuning fork tests?
- Rinne’s test
- Weber test
What are the results for the Rinne’s test?
- normal = air conduction > bone conduction
- conductive HL = bone conduction > air conduction
- sensorineural HL = normal
What are the results for the Weber test?
- normal = sound equal in both ears
- conductive HL = sound louder in bad ear
- sensorineural HL = sound louder in good ear
What is the function of the vestibular system?
- detect and inform about head movements
- keep images fixed to the retina during head movements
- balance
What are the main inputs of the Vestibular system?
Movement and gravity
What are the outputs of the Vestibular system?
- ocular reflex (maintains fixed image upon movement)
- postural reflex (ensures individuals maintain composure)
What structure within the inner ear is anterior?
The cochlea concerned with auditory input.
Which nerve is involved in the vestibular system?
Vestibulocochlear nerve (CN VIII)
What structure within the inner ear is posterior?
- vestibular system labyrinth
- contains bony, membranous and vestibular labyrinth
Where does the bony labyrinth lie?
Cavities in the petrous temporal bone
What is part of the vestibular organ?
- vestibular labyrinth (utricle and saccule), joined by a conduit.
- Saccule is connected to the cochlea.
- Three semi-circular canals (Anterior, posterior and lateral) in each ear.
- ampulla on each side, connected to the utricle.
- Canals contain endolymph fluid.
What angles are the semi-circular canals at?
- the anterior and posterior semi-circular canal form a 90-degree angle
- lateral canals are horizontal to the other canals
- therfore they work in pairs
What is the role of the stereocilia on the hair cells?
- orientated to a particular side
- deflection induces cellular depolarisation in response to endolymph movement
What are the otolith organs?
- utricle
- saccule
What is the function of the utricle?
- detects changes in horizontal acceleration
- changes head position during neck flexion and extension
What is the function of the saccule?
- detects changes in vertical acceleration
- changes head position during lateral tilt
How are cells arranged in the otolith organs?
- located in the maculae
- horizontal in the utricle
- vertical in the saccule
What is the maculae made up of?
- hair cells
- gelatinous matrix (otolith crystals)
- otoliths on top
What is in the ampulla?
- crista (hair cells)
- cupula which facilitates hair cell movement
What does endolymph contain?
high concentration of K+
How does endolymph in the semilunar canals move upon rotation?
- moves in the opposite direction to rotation
- exerts force on the cupula, causing it to fill and bend cilia
- will either excite or inhibt cilia depending on the direction of rotation (towards or away from the cilia)
What should NOT be present in the semi-circular canals?
otoliths
Where does the vestibular nerve form nuclei?
- Oculomotor nucleus
- Medial longitudinal fasciculus
- Abducens nucleus
- Vestibular nuclei (Main generator of reflex.)
- Vestibular ganglion
- Vestibulospinal tract
What does the vestibular nuclei have projections to?
- spinal cord
- nuclei of the extraocular muscles
- cerebellum
- centres of cardiovascular and respiratory control
Where do the primary afferents of the vestibular nerve end?
vestibular nuclei in the brainstem
Where are the main processing centres of the vestibular cortex?
in the parieto-insular vestibular cortex (parietal lobe)
What is responsible for processing in the vestibular system?
- main: vestibular nuclear complex
- adaptive: cerebellum
What happens with the stereocilia move towards the kinocilium?
- depolarisation
- increasing nerve discharge
- excitation
What happens with the stereocilia move away from the kinocilium?
- hyperpolarisation
- reduced nerve discharge
What are the 2 different vestibular reflexes?
- vestibulo-ocular reflex
- vestibulo-spinal reflex
What happens in the vestibulo-ocular reflex?
- eye movement in opposite direction to head movement (same velocity and amplitude)
- impulse stimulates ipsilateral vestibular nucleus then the contralateral abducens nucleus and then the ipsilateral oculomotor nucleus
- contracts ipsilateral medial rectus and contralateral lateral rectus
- keeps images fixed on the retina
How do acute unilateral vestibular disorders usually present?
- imbalance
- dizziness
- vertigo
- nausea
How do slow unilateral and both bilateral vestibular disorders present?
- imbalance and nausea
- no vertigo
How can vestibular disorders be characterised by location?
- peripheral (affects vestibular organ and/or vestibulocochlear nerve)
- central (affects CNS, specifically brainstem/cerebellum)
What are some examples of peripheral vestibular disorders?
- Vestibular neuritis
- Benign Paroxysmal Positional Vertigo (BPPV)
- Meniere’s disease
What are some examples of central vestibular disorders?
- stroke
- multiple sclerosis
- tumours
What are the red flags for vestibular disorders?
- Headache
- Gait problems
- Hyper-acute onset
- Hearing loss
- Prolonged symptoms (>4 days)
How do you assess the vestibular system?
Assess eyes, ears and legs
What is the clinical exam for acute dizziness?
HINTS exam
What does the HINTS exam assess?
- Head Impulse test (horizontal rotation VOR)
- Nystagmus (vestibular vs brainstem)
- Test of Skew deviation (vertical misalignment)
How does Benign Paroxyxmal Positional Vertigo present?
intermittent vertigo when standing up which is resolved when sitting down, the duration about 30 seconds
How do you distinguish between BPPV and Meniere’s disease?
in BPPV vertigo episodes are around 30 seconds long
What is the pathophysiology of BPPV?
Dislodging of carbonate crystals in the semi circular canals
What impacts the extent of the deflection of stereocilia and potassium ion channel opening?
amplitudes
What is frequency/pitch?
- Hz
- cycles per second
- perceived tone
What is amplitude/loudness?
- dB
- sound pressure
What frequency is within the human range of hearing?
20-20,000Hz
What amplitude/loudness is within the human range of hearing?
0-120 dB
What decreases with age?
hearing acuity (especially at higher frequencies)
What is the role of a cochlear implant?
replaces the function of the hair cells by receiving sound, analysing it and transforming it into electrical impulses to the auditory nerve
What is required for a cochlear implant?
a functional auditory nerve
What can be used if the auditory nerve is not functional?
a brainstem implant
What is a brainstem implant?
electric signals from the cochlea can be relayed to a set of electrodes implanted directly into the brainstem
When is a brainstem implant advised?
patients with bilateral auditory nerve damage (high risk)
What do hearing aids do?
amplify sound (beyond hearing threshold of hearing loss)
What does the type of hearing aid depend on?
type, degree and characteristic of hearing loss
What are the possible treatments of hearing loss?
- hearing aids
- cochlear implants
- brainstem implants
- underlying cause
What is the threshold for normal hearing?
0-20dB
What is the threshold for mild hearing loss?
20-40dB
What is the threshold for moderate hearing loss?
40-70dB
What is the threshold for severe hearing loss?
70-90dB
What is the threshold for profound hearing loss?
90+ dB
What is otitis media?
- inflammation of the middle ear
- associated with infection
What is a characteristic of otitis media?
bubbles seen through the tympanic membrane
What is presbycusis?
progressive hearing loss due to age
What could affect cortical potentials?
- neurological conditions
- processing problems
Why is a tuning fork used?
to establish the presence or absence of a hearing loss with a significant conductive component
What can the Rinne and Weber test do?
distinguish between conductive hearing loss and sensorineural hearing loss
What does the Rinne test do?
compares bone and air conduction
What does Otoacoustic emissions (OAEs) test?
- OAEs are produced by the outer hair cells as the expand and contract
- none means the outer hair cells are NOT functional
What test is part of the new-born hearing screening and hearing loss monitoring?
Otoacoustic emissions (OAEs)
What is an audiogram?
where the hearing thresholds are plotted to define if there is a hearing loss or not
What is the difference between dizziness and vertigo?
- dizziness = room is still, you feel unstable
- vertigo = room is spinning, you feel stable
