2.10 - Auditory and vestibular systems Flashcards
What does the vestibular organ (canals) do?
Captures low frequency motion (movements)
What does the hearing organ (cochlea) do?
Captures high frequency motion (sound)
What is frequency/pitch (Hz)?
Cycles per second, perceived tone
What is amplitude/loudness (dB)?
Sound pressure, subjective attribute correlated with physical strength
What is the human range of hearing in terms of frequency and loudness?
- frequency: 20-20000 Hz
- loudness: 0-120 dB sound pressure level (SPL)
What three sections are the ear divided into?
- outer ear
- middle ear
- inner ear
What is found in the outer ear?
- auricle (pinna)
- external auditory meatus
What is found in the middle ear?
- ossicles: malleus, incus, stapes
- tympanic membrane (eardrum) and cavity
- stapedius muscle
What is found in the inner ear?
- cochlea
- semicircular canals
- vestibular nerve
- cochlear nerve
What are the functions of the outer ear?
- to capture sound and to focus it to the tympanic membrane
- modest amplification (10dB) of upper range of speech frequencies by resonance in the canal
- to protect the ear from external threats (by hairs and wax)
What is the main function of the middle ear?
Mechanical amplification (can provide an additional 20-30 dB)
What is the hearing part of the inner ear?
Cochlea
What is the function of the inner ear (cochlea)?
- transduce vibration into nervous impulses
- it does so in a way that captures the frequency (pitch) and intensity (loudness) of the sound
What three components is the cochlea divided into and describe them?
- scala vestibuli, scala media and scala tympani
- scala vestibuli and scala tympani - bone structures, contain perilymph (high in sodium)
- scala media - membranous structure, contains endolymph (high in potassium)
What is located in the scala media of the cochlea?
The hearing organ (Organ of Corti), which lies in the basilar membrane
How is the basilar membrane arranged?
Tonotopically, using the same principle as a xylophone (base is narrow and tight, detecting high frequencies; apex is wide and loose, detecting low frequencies)
What two kinds of hair cells does the organ of Corti contain?
- inner hair cells (IHC)
- outer hair cells (OHC)
What would happen to high frequency vs low frequency sounds in the basilar membrane?
- high frequency sounds = thicker base moving more than apex = cells in base will be participating more in transduction of sound
- low frequency sounds = thinner apex moving more than base = cells in apex will be participating more in transduction of sound
What is the tectorial membrane?
Located above the hair cells (organ of Corti) and allows hair deflection, which in turn will depolarise the cell
What is the function of inner hair cells (organ of Corti)?
- carry 95% of the afferent information of the auditory nerve
- their function is the transduction of the sound into nerve impulses
What is the function of the outer hair cells (organ of Corti)?
- carry 95% of the efferents of the auditory nerve
- function is modulation of the sensitivity of the response
What are the stereocilia?
Hairs of the hair cells
How does transduction of sound work?
- inner hair cells
- the deflection of the stereocilia towards the longest cilium (kinocilium) will open K+ channels
- this depolarises the cell, releasing the neurotransmitter (glutamate released as Ca2+ enters) to the afferent nerve which then depolarises
What will higher amplitudes of sound do to the transduction process?
Higher amplitudes (louder) will cause greater deflection of stereocilia towards kinocilium in inner hair cell, and K+ channel opening
What is the difference between depolarisation and hyper-polarisation during transduction?
- depolarisation opens K+ channels
- hyper-polarisation closes K+ channels
What is the overall pathway for sound through the ear (until the cochlea)?
signal (sound) –> outer ear (moderate amplification) –> external auditory meatus –> tympanic window –> reverberates against 3 ossicles (middle ear) –> oval window (from stapes) amplifies further –> cochlea (liquid) –> scala vestibular –> scala media –> scala tympani (sheer force causes hair cells to move) –> K+ in, Ca2+ in, GLUT out –> neuronal signal
What three mediums do sound travel through during the process?
Air –> bone (middle ear) –> liquid (inner ear)
What is the auditory nerve pathway like after the cochlea?
- spiral ganglions from each cochlea project via the vestibulocochlear nerve (CN VIII) to the ipsilateral cochlear nuclei (monoaural neurons) in the brainstem (pons)
- auditory information crosses at the superior olive level (brainstem)
- after this point, all connections are bilateral (combination of L + R hearing)
- superior olive (BS) –> inferior colliculus (BS) –> medial geniculate body (thalamus) –> auditory cortex
A lesion where in the auditory nerve pathway causes unilateral hearing loss?
Lesion between cochlear nucleus and superior olive (as superior olive is where information becomes bilateral)
What are two ways to classify hearing loss by?
- anatomical - conductive hearing loss, sensorineural hearing loss, central hearing loss
- timing - sudden / progressive
What is conductive hearing loss?
Problem is located in outer or middle ear (cells working fine, problem with sound getting into ear)
What is sensorineural hearing loss?
Problem is located in the inner ear - sensory organ (cochlear) or auditory nerve –> 90% of all hearing loss
What is central hearing loss?
Very rare and originates in the brain and brainstem - affects both conduction and transduction of sound and affects more than one part of the ear
How can hearing loss be classified in terms of timing?
- sudden hearing loss - minutes to days
- progressive hearing loss - months to years
What are the causes of conductive hearing loss in the outer ear?
- wax
- foreign body
What are the causes of conductive hearing loss in the middle ear?
- otitis (inflammation, liquid in middle ear)
- otosclerosis (extra bone produced causing hearing loss)
What are the causes of sensorineural hearing loss in the inner ear?
- noise
- presbycusis - normal hearing loss due to loss of hairs as you get old
- ototoxicity - exposure to meds/drugs can be toxic
What are the causes of sensorineural hearing loss in the auditory nerve?
Acoustic neuroma (vestibular schwannoma) (unilateral) - pushes CN VIII
How can you quickly test someone’s hearing as part of a clinical assessment?
Whisper in ipsilateral ear whilst rubbing fingers in contralateral ear
What is the tuning fork used for?
- two tests assess the presence of gross hearing loss
- used to compare hearing on both sides and see if issue is in outer/inner ear
- Weber test - sensorineural hearing loss - tap ringing fork on head, sound should be in middle
- Rinne test - conductive hearing loss - sound through mastoid should not be as loud as sound in ear (reduced conduction through air e.g. due to wax would impact this)
What is an audiogram?
- where the hearing thresholds are plotted to define if there is a hearing loss or not
- normal hearing threshold is located between 0-20 dB
What are the degrees of hearing loss based on where the hearing thresholds are located?
- normal hearing: 0-20dB
- mild hearing loss: 20-40dB
- moderate hearing loss: 40-70dB
- severe hearing loss: 70-90dB
- profound hearing loss: 90-130dB
What does the audiograph for conductive hearing loss look like?
- air conduction impaired - line shifted lower on graph (lower hearing level)
- bone conduction normal
What does the audiograph for sensorineural hearing loss look like?
Both air and bone conduction lines are together and are lower down
What are otoacoustic emissions (OAEs)?
- the normal cochlea produces low-intensity sounds called OAEs
- these sounds are produced specifically by outer hair cells as they expand and contract
- this test often part of the newborn hearing screening and hearing loss monitoring
What are the treatments for hearing loss?
- underlying cause e.g. earwax, infection
- hearing aids - amplify sounds and different frequencies
- cochlear implants - replaces function of hair cells in transduction
- brainstem implants - risky and rare
What are the inputs of the vestibular system?
Movement and gravity via mechanical sensors (canals and otoliths)
What are the outputs of the vestibular system?
Perception and reflexes that maintain posture and gaze - ocular reflex and postural control
What is the role of the CNS in the vestibular system?
Integrates information from inputs and generates the output responses
Where is the vestibular organ?
- posterior area of inner ear
- inner ear contains hair cells for hearing and balance
What parts is the vestibular organ split into?
- vestibule (containing utricle and sacule - otolith)
- semicircular canals
What is in the vestibule?
- utricle and saccule (otolith) are located in the vestibule
- joined by a conduit
- saccule is also joined to the cochlea, utricle also joined to semicircular canals
What are the semicircular canals?
- there are three semicircular canals on each ear - anterior, posterior, lateral (6 total)
- semicircular canals have an ampulla on one side, and are connected to the utricle
What are otolith organs?
And describe their structure in terms of hair cells.
- utricle and saccule are the otolith organs
- their hair cells are located on the maculae - gelatinous matrix with otoliths on top
- the maculae are placed horizontally in the utricle and vertically in the saccule
Why are they called otolith organs?
They have carbonate crystals on top of the hair cells that help with the movement (deflection) of the hairs
What kind of movement do the otolith organs deal with?
Linear acceleration and tilt
What happens to the otolith organs when upright?
The otoliths have a resting potential so the head and neck muscles are contracted/relaxed to keep head in that position
What happens to the otolith organs when accelerating/tilting head?
Otoliths read the movement and allow for compensatory movements e.g. if head is tilting back, we can snap it forward
What kind of movement is the utricle and saccule responsible for sensing?
- utricle - horizontal movement (horizontal maculae, vertical hairs)
- saccule - vertical movement (vertical maculae, horizontal hairs)
Where are the hair cells in the semicircular canals located?
- hair cells in the canals are located in the ampulla
- ampulla has crista, where the hair cells are located
- hair cells surrounded by cupula which helps with the hair cell movement
- rest of the canal only has liquid high in potassium (endolymph)
What are the hair cells in the semicircular canals surrounded by?
The cupula - a gelatinous substance which the endolymph moves to help with hair cell movement
What kind of movement do the semicircular canals deal with?
- angular acceleration
- cupula moves and displaces hair cells
- one side of the vestibular system reports depolarisation and the other reports hyperpolarisation
- information –> brainstem –> integration = which direction head is moving and the acceleration
What are the planes of the semicircular canals?
- the orientation of the canals in the head defines three planes
- anterior and posterior canals form a 90 degree angle
- lateral canals are horizontal to the other canals
- therefore they work in pairs
What are the pairs that the semicircular canals work in?
- both lateral canals
- anterior one side and posterior other side and vice versa
What do vestibular hair cells have coming off them?
- stereocilia
- a kinocilium (biggest cilium)
What do the cilia coming off vestibular hair cells do?
- allow cells to depolarise with movement of the endolymph, generated by head movement
- movement of stereocilia towards kinocilium = depolarisation
- movement of stereocilia away from kinocilium = hyperpolarisation
What are hair cell potentials in the vestibular organ?
- hair cells have a resting potential which has a basal discharge to the nerve (posture upright when not moving)
- hairs moving towards the kinocilium generates depolarisation and an increase in nerve discharge (excitation)
- hairs moving away from kinocilium generates hyperpolarisation and a reduction in nerve discharge (inhibition e.g. if one ear is being depolarised, the other one is hyperpolarised)
What is the nerve path from the vestibular nerve to the brain?
- primary afferents end in vestibular nuclei in the brainstem (pons)
- vestibular nuclei are main generators of vestibular reflexes
What are the main three functions of the vestibular system?
- to detect and inform about head movements
- to keep images fixed in the retina during head movements
- balance
What are two vestibular reflexes?
- vestibulo-ocular reflex (VOR)
- vestibulo-spinal reflex (VSR)
What does the vestibulo-ocular reflex do?
- keeps images fixed in retina
- connection between vestibular nuclei and oculomotor nuclei
- eye movement in opposite direction to head movement with the same velocity and amplitude
- e.g. staring at dot and moving head but dot stays fixed in eyesight
What is the pathway for the vestibulo-ocular reflex?
Vestibular organ detects movement –> vestibular nerve –> vestibular nucleus –> abducens nucleus (contralateral) –> via medial longitudinal fasciculus –> oculomotor nucleus (back to other side) –> eye movement (think in terms of direction e.g. move head right means both eyes need to move left, so respective LR and MR contraction)
How do we classify vestibular disorders?
- timing and laterality
- location
- timing (evolution)
What are the main complaints in acute and unilateral vestibular disorder?
- imbalance
- dizziness
- vertigo
- nausea
What are the main complaints in slow AND unilateral, or any bilateral loss?
- imbalance
- nausea
- NO VERTIGO
What are some peripheral vestibular disorders (vestibular organ and/or CN VIII)?
- vestibular neuritis (acute)
- benign paroxysmal positional vertigo - BPPV (intermittent)
- Meniere’s disease (recurrent)
What are some central vestibular disorders (CNS)?
- stroke (acute)
- multiple sclerosis (progressive)
- tumours (progressive)
What are acute balance disorders?
- vestibular neuritis
- stroke
What are intermittent balance disorders?
Benign paroxysmal positional vertigo (BPPV)
What are recurrent balance disorders?
- migraine
- (Meniere’s disease)
What are progressive balance disorders?
- schwannoma vestibular (VIIIth nerve)
- degenerative conditions (MS)
What are the main diagnoses?
- BPPV
- vestibular neuritis
- vestibular migraine
- stroke (cerebellar)
What is tested in the core exam?
- eyes
- ears
- legs
What are some red flag symptoms of vestibular disease? (5)
- headache
- gait problems
- hyper-acute onset
- hearing loss
- prolonged symptoms (>4 days)
How do you test your VOR?
- fast movements of head hard to compensate for vs slow
- hold head, move fast suddenly and see whether eyes stay fixed
- if you move left, left horizontal canal responsible
- e.g. if issue in left canal, if you look right VOR normal but if you look left the eye will not stay fixed initially
What is the HINTS exam used for?
- clinical exam in acute dizziness
- vestibular neuritis (peripheral) or stroke (CNS)?
What does the HINTS exam include?
- Head Impulse test - horizontal rotational VOR
- Nystagmus - vestibular organ vs cerebellar/BS nystagmus
- Test of Skew deviation - vertical misalignment, usually absent in peripheral pathology (vestibular neuritis)
What is benign paroxysmal positional vertigo (BPPV)?
- peripheral disorder where otoliths from the utricle detach from maculae and float around the semicircular canals
- floating otoliths induce a bigger endolymph flow when the head moves (crystals carry more liquid)
- movement of head processed by the brain as bigger and faster than in reality
What is the nature of BPPV attacks?
- intermittent vertigo attacks every time the head moves quickly, especially when lying down or standing up
- attacks are short as they end when the endolymph settles
- when head is still, usually no vertigo symptoms (but some feel unstable/lightheaded when walking)
How is BPPV treated?
Disorder continues until crystals are taken back to the utricle with repositioning manoeuvres e.g. Epley or Semont
Does BPPV cause hearing problems?
No - only affects semicircular canals where crystals are floating around (but hearing loss can occur if they have a different condition coexisting e.g. presbycusis)
