Auditory and Vestibular System Flashcards
What makes up the outer ear?
- pina
- external auditory meatus
What makes up the middle ear?
- malleus
- incus
- stapes
- eustachian tube
What makes up the inner ear?
- the vestibule
- the semicircular canals
- the cochlea.
What is the function of the inner ear?
transduction
What is Transduction?
conversion of one energy form to another
mechanical sound waves to electrical signals
What term is used to describe the arrangement of the basilar membrane?
- tonotopic arrangement
- high to low frequency from base to apex
high frequency: tight and narrow
What is the auditory pathways to the brain?
- cochlear nucleus
- superior olive (bilateral)
- inferior colliculus
- medial geniculate body
auditory cortex
How does the middle ear amplify sound?
- focuses vibrations from large SA (tympanic membrane) to small SA (oval widow), increasing pressure
- leverage from the incus -stapes to increase the force on the oval window
- malleus, incus and stapes articulate to facilitate the transmission of sound into the inner ear
What are the three ossicles?
- malleus
- incus
- stapes
How does the inner ear transduce sound?
- deflection of the stereocilia towards the longest cilium opens potassium ion channels within the inner hair cell, as the tectorial membrane makes contact
- ionic interchange of potassium within the endolymph and inner hair ells causing cell depolarisation
- inducing the opening of voltage-gated calcium channels releasing excitatory neurotransmitters (glutamate) onto the afferent nerve
What are the three scala/compartments of the cochlear?
- scala vestibuli
- scala tympani
- scala media
How can we categorise causes of hearing loss?
- sensorinueral
- conductive
What is conductive hearing loss?
- dissociation between air and bone conduction
(bone is maintained, greater threshold required for air conduction) - middle or outer ear affected
- hair cells are functional when stimulated within the inner ear
What is sensorineural hearing loss?
- both air and bone conduction are affected similarly
- problem with the inner ear of vestibulocochlear nerve problem
- sound is unable to be effectively transduced to the auditory cortex
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?
inner ear: - presbycusis - ototoxicity (loss of hair cells) nerve: - VIII nerve tumour (acoustic neuroma)
What are the main 2 tuning fork tests?
- Rinne test
- Weber test
What are the 3 main inputs of the Vestibular system?
- visual
- proprioceptive
- vestibular information
What are the outputs of the Vestibular system?
reflexes to maintain a stable posture and stable gaze
- ocular (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.
What structure within the inner ear is posterior?
Labyrinth concerned with the vestibular system. The bony labyrinth resides within the petrous part of the temporal bone.
What is part of the vestibular organ?
- utricle and saccule (in the vestibule), joined by a conduit.
- Saccule is connected to the cochlea.
- Three semi-circular canals (Anterior, posterior and lateral) in each ear.
- with an 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
- the anterior canal is 45 degrees to the front
Describe the structure of vestibular hair cells?
- kinocilium (largest cilium)
- stereocilia
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 maculae made up of?
- hair cells
- gelatinous matrix (otolith crystals)
- hair cells are horizontal in the utricle
- hair cells are vertical in the saccule
What is in the ampulla?
- crista (hair cells)
- cupula which facilitates hair cell movement
What does endolymph contain?
high concentration of K+
What should NOT be present in the semi-circular canals?
otoliths
Where does the vestibular nerve form nuclei?
- Oculomotor nucleus
- Trochlear 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 does the primary afferents of the vestibular nerve end?
- vestibular nuclei
- cerebellum
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 is the function of the vestibular system?
- detect and inform about head movements
- postural control
- keep images fixed in the retina during head movements
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
Describe the relationship between the hair cell potentials in the left and right ear?
- antagonistic
- excitation and inhibition occur simultaneously at that specific velocity
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)
- involves the vestibular nuclei and oculomotor nuclei
Why does the vestibulo-ocular reflex occur?
to ensure images remain fixed in the retina
What happens in the vestibulo-spinal reflex?
- motor neurones to the neck and back muscles (medial tract)
- postural control, compensatory body movement according to head position
How do you assess the vestibular system?
- anamnesis (Hx)
- posture and gait (posturography)
- cerebellar function (CT and MRI)
- eye movements
What tests can be done to test the vestibular system?
- caloric test
- video head impulse test
- vestibular evoked myogenic potential
- rotational test
What happens in a caloric test?
- tests the semi-circular canals
- stimulates the inner ear with different temperature to identify how symmetrical the responses are)
What happens in a Video head impulse test (vHIT)?
measures the vestibulo-ocular reflex
What is Benign Paroxysmal Positional Vertigo (BPPV)?
- peripheral vestibular disorder
- labyrinth and/or vestibulocochlear nerve
How does Benign Paroxyxmal Positional Vertigo?
intermittant 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?
- otoliths from the utricle are detached from the maculae and dislodged into the semi-circular canals
- otoliths stimulate a larger endolymph flow, head movement is processed by the brain as bigger and faster than actually perceived
- otoliths settle int he posterior semi-circular canal (gravity)
- posterior semi-circular canal has an impermeable barrier to trap otoliths
What will stop BPPV?
- continues util otolith crystal returns to the utricle with repositioning manoeuvres (epley, sermont)
What is Meniere’s disease?
- increased amount of endolymph liquid in the inner ear
- degrades the membrane
- recurrrent
What characterises Meniere’s disease?
an episode of sudden onset vertigo
How does Meniere’s disease present?
- tinnitus
- episodes of vertigo
What are other examples of peripheral vestibular disorders?
- vestibular neuritis (acute)
- unilateral/bilateral vestibular hypofunction
- schwannoma vestibular tumour
What is schwannoma vestibular disorder?
- 8th nerve (vestibulocochlear) tumour
- progressive
What do central vestibular disorders affect?
CNS (brainstem/cerebellum)
What can cause central vestibular disorders?
- stroke
- MS
- tumours
What are possible differentials for dizziness?
- heart disorders
- presyncopal episodes
- orthostatic hypotension
- anaemia
- hypoglycaemia
- psychological
- gait disorders
What is the function of the outer ear?
- capture sound
- focus sound on the tympanic membrane
- amplify frequencies by resonance in the canal
- protects the ear from external threats
- cilia - prevents external entrance
- wax - covers inner ear, unfavourable pH for pathogens
What is the function of the middle ear?
the amplify sound
Describe the structure to the scala vestibuli and scala tympani?
bony structures with sodium-rich perilymph
Describe the structure of the scala media?
- membranous structure
- potassium-rich perilymph
- organ of corti
What do vibrations of the basilar membrane trigger?
- the movement of outer cell stereocilia
- moving the tectorial membrane into contact with the inner hair cells
- causing the opening of potassium ion channels
- depolarisation of cell membrane and transmission of an electrical impulse via the auditory vestibular nerve to the ipsilateral cochlear nerve
Describe the structure of the Organ of Corti?
- contains inner and outer hair cells
- inner hair cells are in ingle columns
- outer hair cells are in columns of 3
- OHCs are in continuous contact with the tectorial membrane (on vibrations, stereocilia movement facilitates the contact of the tectorial membrane with the IHCs
What is the role of the inner hair cells?
- transmit 95% of the afferent information of the auditory nerve
- transduction of sound into nerve impulses
What is the role of the outer hair cells?
- modulate the sensitivity of the response
- act as an active amplifier
- stereocilia length can be altered to allow the tectorial membrane to be brought closer to the inner hair cells
What impact the extent of the deflection of stereocilia and potassium ion channel opening?
amplitudes
What happens in the depolarisation phase of the inner hair cells?
opens K+ channels (upwards phase)
What happens in the repolarisation phase of the inner hair cells?
closes K+ channels
downwards phase
Describe the auditory pathways?
BRAINSTEM - spiral ganglions from each cochlea project by the auditory vestibular nerve into the ipsilateral cochlear nuclei - auditory information crosses the superior olive level, and now all connections are bilateral - inferior colliculus THALAMUS - medial geniculate body CEREBRAL HEMISPHERE - auditory cortex (temporal lobe)
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 the patient
What are the possible treatments of hearing loss?
- hearing aids
- cochlear implants
- brainstem implants
What is mixed hearing loss?
- both air and bone conduction is affected
- greater thresholds are required to discriminate a specific frequency
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 is the Weber test?
- hit tuning fork on knee or elbow
- place base on the patients forehead
- ask patient if it is louder in a particular ear
- unilateral conductive: louder in deaf ear
- unilateral sensorineural: louder in normal
What is the Rinne test?
- place base of tuning fork against the patients mastoid process
- lift and place 1 cm from external auditory meatus
- air louder than bone: normal
- bone louder than air: conductive hearing loss
- air louder than bone: sensorineural hearing loss
What does the Rinne test do?
compares bone and air conduction
Other than tuning forks, what tests can be done to assess hearing loss?
- audiometry
- central processing assessment
- tympanometry
- otoacoustic emission
- electrocochelography
- evoked potentials
What does a Pure tone audiometry measure?
hearing acuity for variations in sound intensity and frequency
What is an audiometry?
a device used to produce the sound of varying intensity and frequency
What is an audiogram?
- hearing thresholds are plotted to define if there is hearing loss or not
- hearing loss: progressive increase in the hearing thresholds required to hear the sound at the specific frequency
What is a Central Processing Assessment?
assessment of hearing abilities other than detection: sound localisation, filtered speech and speech in noise
(assesses sound discrimination and comprehension)
What happens in a Tympanometry?
tests the condition of the middle ear and the mobility of the ear drum and ossicles by generating variations of air pressure in the ear canal
What would cause a non-compliant tympanic membrane?
- middle ear effusion
- perforation of the tympanic membrane
- pharyngomatic tube dysfunction
- occluded ear canal
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 happens in auditory evoked potentials electrochleography?
- 0.2-4ms
- electrical activity from the cochlea and the VIII nerve
- evoked by clicks or tone burst
What is done in an auditory brainstem response test?
- 1.5-10ms
- electrical activity from the VIII nerve and brainstem nuclei (evoked by clicks)
- alterations in latency of waves can indicate the location of the deficit
When is an auditory brainstem response test done?
in babies and children
What is tested in a late response?
80-500+ms
- activity from the primary auditoty and association cortex
- evoked by tone burst and oddball paradigm
