Audio-vestibular system Flashcards
What did a Statocyst evolve into?
Utriculus –> Sacculus –> Cochlea and Canals
What is a vestibular organ?
Captures low frequency motion (movements)
What is a hearing organ?
Captures high frequency motion (sounds)
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 range of human hearing?
Frequency: 20–20,000Hz
Loudness: 0 dB to 120 dB sound pressure level (SPL)
What are the outer ear functions?
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.
Pinna–> External auditory meatus –> Tympanic membrane
What is the function of the middle ear?
Tympani membrane –> Oval window
Mechanical amplification (can provide an additional 20-30dB)
What happens at the tympanic membrane?
Vibrations in the air are changed to mechanical movement and now moves at the exact frequency of sound
What happens when the sound is too loud in the middle ear?
Tensor tympani muscle and stapedius muscle both contract
What is the function of the inner ear?
The hearing part of the inner ear is the cochlea
Its function is to transduce vibration into nervous impulses
It does so in a way that captures the frequency (or pitch) and intensity (or loudness) of the sound
What is the cochlea composed for?
The cochlea contains 3 compartments:
-Scala vestibuli and scala tympani: Bone structures, contain perilymph (high in sodium)
-Scala media: Membranous structure, contains endolymph (high in potassium). Here is where the hearing organ or Organ of Corti is located.
What is the basilar membrane?
The structure where the organ of Corti lies in is the basilar membrane
Basilar membrane is arranged tonotopically, using the same principle as a xylophone
What type of hair cells does the organ of Corti contain?
Inner hair cells (1 line)
Outer hair cells (3 lines)
Where is the tectorial membrane?
What does it do?
The tectorial membrane is above the hair cells and allows hair deflection, which in turn will depolarise the cell.
What do the inner hair cells do?
Carry 95% of the afferent information of the auditory nerve. Their function is the transduction of the sound into nerve impulses
Carry the information via the auditory nerve once you get the signal. First stage of transduction from a motion to a neuronal signal here
To spiral ganglion cells and then to the auditory nerve
What do the outer hair cells do?
Carry 95% of efferents of the auditory nerve. Their function is modulation of the sensitivity of the response.
How does transduction occur?
The hairs of the hair cells are called stereocilia.
The deflection of the stereocilia towards the longest cilium (kinocilium) will open K+ channels and the voltage gated Ca2+ channels open, with Ca 2+ entering
This depolarises the cell releasing the neurotransmitter to the afferent nerve which then depolarises.
How does higher amplitude affect deflection?
Higher amplitudes (louder) of sound will cause greater deflection of stereocilia and K+ channel opening
What happens in depolarisation
What happens in hyperpolarisation?
Opens K+ channels (upward phase)
Closes K+ channels (downward phase)
Explain the auditory pathways*
Auditory nerve combines with the vestibular-cochlear never (8th cranial nerve) to the ipsilateral cochlear nuclei in the brainstem (pons)
Sends a message when it gets to cochlear nuclei to the other side and also to superior olive
Auditory information crosses at the superior olive level
After this point all connection are bilateral
Superior olive connects to inferior colliculus in brain stem
This then connects with medial geniculate in thalamus
Then to auditory cortex in temporal lobe
Why does brain damage affect a small part and why is it rare to lose hearing
Because it is bilaterally innervated very early stage in pons
Why does brain damage only affect a small part
Bilaterally innervated at very early stage in pons
Where in the thalamus is related to hearing
Medial geniculate body
What are anatomical types 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
What are the timing types of hearing loss
Sudden hearing loss minutes to days
Progressive hearing loss months to years
What are the causes of conductive hearing loss
Outer ear:
Wax
Foreign body
Middle ear:
Otitis - inflammation due to infection - Bubbles can be seen through the ear drum, suggesting there is liquid inside the middle ear.
Otosclerosis - joints fusing
What are the causes of sensorineural hearing loss
Inner ear:
Noise
Presbycusis (old age)
Ototoxicity - chemotherapy / antibiotics e.g. gentamycin
Nerve:
acoustic neuroma (vestibular schwannoma) (unilateral)
What are examples of clinical test
Bedside tests are quick but not accurate-Whisper in ipsilateral ear whilst rubbing fingers in contralateral ear
Tuning Fork – 2 tests assess the presence of gross hearing loss.
-Weber test - top
-Rinne test - on mastoid, behind ear
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
What is conductive hearing loss
Can hear through headphones as going directly through bone to inner ear, organ of corti, and inner ear works so neurone problem
What is a a special test in audiometry that can be done
Otoacoustic Emissions (OAEs)
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.
What is the treatment for hearing problems
Underlying cause e.g. treat infection
Hearing aids
Cochlear implants
Brainstem implants
What goes into the vestibular system
What comes out
Movement and gravity
-Via 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
What is the vestibular organ made of
The utricule and saccule are located in the vestibule and are joined by a conduit. The saccule is also joined to the cochlea
There are three semicircular canals on each ear, anterior, posterior and lateral
The semicircular canals have an ampulla on one side, and they are connected to the utricle.
What are otolith organs
Utricle and saccule are the otolith organs. Their cells are located on the maculae, placed horizontally in the utricle and vertically in the saccule
What are maculae
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 is otolith movement
What is horizontal movement
What is vertical movement
Linear acceleration and tilt
Utricule
Saccule
Describe the semi-circular 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
The ampulla has the crista, where the hair cells are located. The cells are surrounded by the cupula which helps the hair cell movement
Describe the semi-circular 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
The ampulla has the crista, where the hair cells are located. The cells are surrounded by the cupula which helps the hair cell movement
Describe the orientation of the canals
The orientation of the canals in the head defines three planes.
Anterior and posterior canals form a 90° angle. Lateral canals are horizontal to the other canals.
Therefore they work in pairs
Describe the hair cells and how they work
Vestibular hair cells have a kinocilium (the biggest cilium) and stereocilia
Cilia allows the cells to depolarise the cell with movement of the endolymph generated by head movement
Describe hair cell potentials
Hair cells have a resting potential which has a basal discharge to the nerve
Hairs moving towards the kinocilium generates depolarization and an increase in nerve discharge.
Hairs moving away from the kinocilium generates hyperpolarization and a reduction in nerve discharge.
What are the vestibular system functions
To detect and inform about head movements
Balance
To keep images fixed in the retina during head movements
What are the vestibular reflexes
Vestibulo-ocular Reflex
(VOR)
Vestibulo Spinal Reflex
(VSR)
Describe the vestibulo-ocular relfex
Keeps images fixed in the retina
Connection between vestibular nuclei and oculomotor nuclei
Eye movement in opposite direction to head movement, but same velocity and amplitude
How to categorise vestibular disorders
Timing and Laterality
In acute AND unilateral:
Main complaints - imbalance, dizziness, vértigo and nausea
In slow AND unilateral or any bilateral loss:
Main complaints – imbalance and nausea – NO vertigo
Where are peripheral vestibular disorders and give some examples
Vestibular organ and/or VIII nerve
Vestibular neuritis
Benign Paroxysmal Positional Vertigo (BPPV)
Meniere’s disease
Where are central vestibular disorders and give some examples
CNS (brainstem/cerebellum)
Stroke
Multiple Sclerosis
Tumours
Explain the clinical approach for a physician
The main diagnoses
BPPV (crystals moving)
Vestibular Neuritis
Vestibular Migraine
Stroke (cerebellar)
The core exam
EYES
EARS
LEGS
Red Flags
Headache
Gait problems
Hyper-acute onset
Hearing loss
Prolonged symptoms (>4 days)
What are the balance disorders in terms of timing
Acute:
Vestibular Neuritis
Stroke
HINTS exam
Intermittent:
Benign Paroxysmal Positional Vertigo (BPPV)
Dix-Hallpike test
Recurrent:
Migraine
(Meniere’s Disease)
Progressive:
Schwannoma vestibular (VIIIth nerve)
Degenerative conditions (MS)
Explain the HINTS exam
HeadImpulseTest
Horizontal rotational VOR
**Vestibular Neuritis or Stroke?
Nystagmus
Vestibular organ Vs Cerebellar/brainstem nystagmus
TestofSkewDeviation
Verticalmisalignment - usually absent in peripheral pathology
Vestibular nerve goes to ganglion and joins with the auditory nerve and goes together with auditory nerve as vestibulo-cochlear nerve into brain through pons
Vestibular nerve goes through vestibuli nuclei, then cerebellum and then nuclei of eye movement - 3rd, 4th and 6th nerves that control eyes
