Auditory & Vestibular System Flashcards
Define frequency / pitch (Hz).
Cycles per second, perceived tone.
What is the human range for frequncy / pitch?
20 - 20,000Hz
Define amplitude / loudness (dB).
Sound pressure, subjective attribute correlated with physical strength
What is the human range for amplitude / loudness?
0 - 120 dB
What structures make up the outer ear (3)?
- Pinna
- External auditory meatus
- Tympanic membrane
What are the functions of the outer ear (3)?
- 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
What structures make up the middle ear (6)?
- Ossicles:
- Malleus (hammer)
- Incus (anvil)
- Stapes (stirrup)
- Oval window
- Tensor tympani muscle
- Stapedius muscle
The muscles in the middle ear act as a protection against loud noises by limiting the movement of the ossicles.
What is the function of the middle ear?
- Mechanical sound amplification
Can provide an additional 20-30dB
What is the function of the inner ear?
- Transduce vibration into nervous impulses
Captures the frequency (pitch) and intensity (loudness) of the sound
What is the hearing part of the inner ear called?
Cochlea
What are the 3 compartments that make up the cochlea?
- Scala vestibuli
- Scala tympani
- Scala media
What is the scala vestibuli?
Bone structure
Contain perilymph (high in Na+)
What is the scala tympani?
Bone structure
Contain perilymph (high in Na+)
What is the scala media?
Membranous structure
Contains endolymph (high in K+)
Here is where the hearing organ or Organ of Corti is located
How is the basilar membrane arranged?
Tonotopically
Using the same principle as a xylophone
How is the base of the basilar membrane structured? What frequency does it detect?
- Narrow and tight
- High frequency
How is the apex of the basilar membrane structured? What frequency does it detect?
- Wide and loose
- Low frequency
Where does the organ of Corti lie?
In scala media, above the basilar membrane
What 2 types of hair cells does the Organ of Corti contain?
- Inner hair cells (IHC)
- Outer hair cells (OHC)
What is the relation between the tectorial membrane and the hair cells?
- The tectorial membrane is above the hair cells and allows hair deflection, which in turn will depolarise the cell
Like a ceiling to push the hair cells against.
What is the function of inner hear cells (IHC)?
- Transduction of the sound into nerve impulses
IHC carry 95% of the afferent information of the auditory nerve
What is the function of outer hear cells (OHC)?
- Modulation of the sensitivity of the response
OHC carry 95% of efferents of the auditory nerve
Describe the process of hearing (Sound to Cochlea).
- The auricle collects sound waves and channels them into the ear canal (external auditory meatus), where the sound is amplified.
- The sound waves then travel towards the eardrum.
- Sound waves cause the eardrum to vibrate.
- The vibrations from the eardrum move the ossicles into motion. The ossicles further amplify the sound.
- The tiny stapes bone attaches to the oval window that connects the middle ear to the inner ear.
- The sound waves enter the inner ear and then into the cochlea.
- The auricle (pinna) is the visible portion of the outer ear
- The ossicles are actually tiny bones — the smallest in the human body. The three bones are named after their shapes: the malleus (hammer), incus (anvil) and stapes (stirrup).
- The cochlea is filled with a fluid that moves in response to the vibrations from the oval window.
Describe the process of transduction (vibrations to electrical impulses).
- The deflection of the stereocilia towards the longest cilium (kinocilium) will open K+ channels
- This depolarises the IH cell releasing the neurotransmitter (glutamate) to the afferent nerve which then depolarises
Higher amplitudes (louder) of sound will cause greater deflection of stereocilia and K+ channel opening
Outline the auditory pathway from the auditory nerve to the auditory cortex.
- Spiral ganglions via the vestibulo-cochlear nerve (CNVIII) 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
- Impulse travels to the Inferior colliculus
- Then to the Medial geniculate body
- Finally to the auditory cortex
What are the anatomical causes of hearing loss divided into?
- Conductive
- Sensorineural
- Central
What are the causes of conductive hearing loss (4)?
- Outer ear
- Wax
- Foreign Body
- Middle ear
- Otitis
- Otosclerosis
What are the causes of sensorineural hearing loss (4)?
- Inner ear
- Noise
- Age-related hearing loss (Presbycusis)
-
Ototoxicity
- Chemotherapy / Some antibiotics (i.e. gentamicin)
- Nerve
- Acoustic neuroma (vestibular schwannoma) (unilateral)
What is central hearing loss?
- Very rare hearing loss that originates in the brain and brainstem
What bedside tests can be used to clinically assess hearing?
- Whisper in ipsilateral ear whilst rubbing fingers in contralateral ear
- Weber test
- Rinne test
Bedside tests are quick but not accurate
What test can be used to assess the hearing of an individual (awake and alert)?
Audiometry
How is audiometry conducted?
The audiogram is where the hearing thresholds are plotted to define if there is a hearing loss or not
What test can be used to assess the hearing of a newborn?
Otoacoustic Emissions (OAEs)
On what principle does the Otoacoustic Emissions (OAEs) test depend on?
- The normal cochlea produces low-intensity sounds called OAEs
- These sounds are produced specifically by the outer hair cells as they expand and contract
What treatments are available to treat hearing loss (4)?
- Treat underlying cause
- Hearing aids
- Cochlear implants
- Brainstem implants
What is the function of the vestibular system?
Divide into 3 simple steps.
- Mechanical sensors (canals and otoliths) detect movement & gravity
- The CNS integrates this information and generates responses
- The outputs are perception and reflexes that maintain postural control & gaze (ocular reflex)
Name structures 1-6.
- Anterior semicircular canal
- Lateral semicircular canal
- Posterior semicircular canal
- Utricle
- Saccule
- Cochlea
Where are the utricule and saccule located?
In the vestibule, joined by a conduit
What structure attaches to the saccule?
Cochlea
What are the 3 semicircular canals?
- Anterior semicircular canal
- Posterior semicircular canal
- Lateral semicircular canal
Describe the structure of the semicircular canals.
The semicircular canals have an ampulla on one side, and they are connected to the utricle
How are the semicircular canals arranged?
Anterior and posterior canals form a 90° angle. Lateral canals are horizontal to the other canals.
Therefore they work in pairs.
What are the 2 otolith organs?
- Utricle
- Saccule
Where are the maculae placed horizontally?
Utricle
Where are the maculae placed vertically?
Saccule
What do the maculae contain?
- Hair cells
- Gelatinous matrix
- Otoliths
The otholiths are carbonate crystals that help the deflection of the hairs.
How are the hair cells arranged?
-
Located in the ampullary crista in the ampulla
- The rest of the canal only has a liquid high in potassium called endolymph
- Surrounded by the cupula which helps the hair cell movement
How do vestibular hair cells work?
- 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 an inhibition in nerve discharge.
What is the dizziness and what is vertigo?
Dizziness: Light-headed
Vertigo: Sensation of spinning
Where do the primary afferents from the vestibular organs end?
In the vestibular nuclei in the brainstem (pons)
What are the 2 vestibular reflexes?
-
Vestibulo-ocular Reflex (VOR)
- To keep images fixed in the retina during head movements
-
Vestibulo Spinal Reflex (VSR)
- Balance
How does the vestibulo-ocular reflex (VOR) work?
- Eye movement in opposite direction to head movement, but same magnitude of velocity
- Achieved by a connection between the vestibular nuclei and oculomotor nuclei
How are vestibular disorders categorised?
- Timing
- Laterality
What are the main complaints in an acute & unilateral vestibular disorder (4)?
- Imbalance
- Dizziness
- Vértigo
- Nausea
What are the main complaints in a slow & unilateral or any bilateral loss vestibular disorder (2)?
- Imbalance
- Nausea
No vértigo
What are the causes of peripheral vestibular disorders (vestibular organ and / or VIII nerve) (3)?
- Vestibular neuritis
- Benign Paroxysmal Positional Vertigo (BPPV)
- Meniere’s disease
What are the causes of central vestibular disorders (brainstem / cerebellum) (3)?
- Stroke
- Multiple Sclerosis
- Tumours
What are the red flags when examining a patient with suspected vestibular disorder (5)?
- Headache
- Gait problems
- Hyper-acute onset
- Hearing loss
- Prolonged symptoms (>4 days)
What is the differential diagnosis of an acute balance disorder presentation (2)?
- Vestibular Neuritis
- Stroke
How can one distinguish between vestibular neuritis & stroke in an acute balance disorder presentation?
HINTS exam
What is involved in a HINTS exam?
-
HeadImpulseTest
- Stroke: Normal horizontal rotational VOR
-
Nystagmus
- Stroke: Gaze-direction nystagmus
-
TestofSkewDeviation
- Stroke: verticalmisalignment
What is the differential diagnosis of an intermittent balance disorder presentation (1)?
- Benign Paroxysmal Positional Vertigo (BPPV)
What is the differential diagnosis of a recurrent balance disorder presentation (1)?
- Vestibular migraine (Meniere’s Disease)
What is the differential diagnosis of a progressive balance disorder presentation (2)?
- Schwannoma vestibular (VIIIth nerve)
- Degenerative conditions (MS)
