Auditory And Vestibular System Flashcards
What type of frequency is captured by vestibular organs?
Low frequency motion (movements)
What type of frequency is captured by hearing organs?
High frequency motion (sound)
What comprises sound?
Frequency/pitch (Hz): Wave cycles per second - perceived tone
Amplitude/loudness (dB): Sound pressure - subjective attribute correlated with physical strength
What is the human range of hearing?
Frequency: 20-20,000Hz
Loudness. 0 dB to 120 dB sound pressure level (SPL)
What are the functions of the outer ear?
Capture sound and focus it to the tympanic membrane
Modest amplification (10dB) of upper range of speech frequencies due to resonance in canal
Protects the ear
What is the main function of the middle ear?
Mechanical amplification (20-30dB)
Useful for detecting low levels of sound
What are the functions of the tensor tympani and stapedius muscle?
Work to reduce sound (e.g. when things are too loud)
Tensor tympani contracts and stiffens tympanic membrane - reduces vibrations
Stapedius muscle contracts and stops stapes from vibrating as much
What is the hearing part of the inner ear?
Cochlea
What is the function of the cochlea?
Transduces vibration (through fluid) into nervous impulses
Able to capture frequency and intensity of the sound
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Label the image
What are the three compartment of the cochlea?
Scala vestibuli - bone
Scala tympani - bone
Scala media - membranous
What do the scala vestibuli and tympani contain and what is the substance high in?
Perilymph
High in sodium
What does the scala media contain and what is the substance high in?
Endolymph
High in potassium
Where is the hearing organ (Organ of Corti) located?
In the scala media of the cochlea
Does the Organ of Corti lie in the vestibular or basilar membrane?
Basilar
Describe the arrangement of the basilar membrane
Tonotopically arranged
What are the two types of hair cells of the organ of Corti?
Inner hair cells (arranged in one line)
Outer hair cells (arranged in lines of three)
What is the function of the tectorial membrane?
Located above hair cells
Allows deflection of hairs which leads to depolarisation of the cells
What is the function of the inner hair cells?
Carry 95% of the afferent information of the auditory nerve
Involved in transduction of sound into nerve impulses
What is the function of the outer hair cells?
Carry 95% of the efferent information of the auditory nerve
Involved in modulation of the sensitivity of the response
What is the name for the ‘hairs’ of the hair cell?
Stereocilia
Stereocilia deflect towards the what…?
Kinocilium (longest cilium)
What does deflection of the stereocilia cause?
Opens K+ channels
This depolarises the cell causing release of neurotransmitter to the afferent nerve which then depolarises
How are higher amplitudes of sound detected during transduction?
Higher amplitudes cause greater deflection of the stereocilia and K+ channel opening
Describe the auditory pathways
Spiral ganglions via the vestibulo-cochlear nerve travel to ipsilateral cochlear nuclei (monoaural neurons) in the pons
Auditory information crosses over at the superior olive level -> after this point all connections are bilateral
Where is the problem located with conductive hearing loss?
Outer or middle ear
Where is the problem located with sensorineural hearing loss?
Sensory organ (cochlear) or the nerve (auditory nerve)
What are the most and least common types of hearing loss?
Sensorineural - accounts for 90% of all hearing loss
Central hearing loss - very rare; originates in brain and brainstem
What is the difference between sudden and progressive hearing loss?
Sudden - minutes to days
Progressive - months to years
What are some outer ear causes of conductive hearing loss?
Wax (cerumen impaction)
Foreign body
What are some middle ear causes of conductive hearing loss?
Otitis - bubbles can be seen through ear drum suggesting presence of liquid inside the middle ear
Otosclerosis
What are the inner ear causes of sensorineural hearing loss?
Noise
Presbycusis (old age)
Ototoxicity - e.g. chemotherapy or antibiotics (specifically gentamicin and other aminoglycosides, commonly given for UTIs in hospital)
What are nerve causes of sensorineural hearing loss?
Acoustic neuroma/vestibular schwannoma - unilateral
What does this image show?
Acoustic neuroma
What are some clinical beside assessments for hearing?
Whisper in ipsilateral ear whilst rubbing fingers in contralateral ear
Tuning fork - two tests assess the presence of gross hearing loss - Weber and Rinne test
Does hearing loss usually affect the whole frequency range?
No, tends to be frequency specific, e.g. high frequencies
What is an audiogram?
Chart where hearing thresholds are plotted to define if there is hearing loss or not
Note: normal hearing threshold is located between 0-20dB
Describe the defining thresholds for hearing loss in dB
What type of hearing loss does each graph show?
What are Otoacoustic Emission tests typically used in?
Newborn screening and hearing loss monitoring
Does not require a response - unlike audiometry
How do Otoacoustic Emissions (OAEs) work?
Normal cochlea produces low-intensity sounds called OAEs
Sounds produced specifically by outer hair cells as they expand and contract
Sounds can be detected through a machine
What are the treatments for hearing loss?
Treat underlying cause - e.g. infection
Hearing aids
Cochlear implants
Brainstem implants
What are the inputs of the vestibular system and what detects them?
Movement and gravity
Detected by mechanical sensors
What are the outputs of the vestibular system?
Ocular reflex
Postural control
Help maintain posture and gaze
What components make up the vestibular organ?
Utricle and saccule (otoliths) are located in the vestibule and are joined to the cochlea (saccule is joined to the cochlea)
Three semicircular canals on each ear - anterior, posterior, lateral
Semicircular canals have an ampulla on one side and are connected to the utricle
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Where do the hair cells sit in the otolith organs?
In the maculae
How are the maculae placed in the saccule and utricle organs?
Saccule - vertically
Utricle - horizontally
What do the maculae contain?
Hairs cells
A gelatinous matrix
Otoliths on top - carbonate crystals that help the deflection of the hair cells
What causes otolith movement?
Linear acceleration and tilt
Utricle - horizontal movement
Saccule - vertical movement
Where are the hair cells of the semicircular canals located?
In the crista of the ampulla
What surrounds the hair cells of the semicircular canals and what is the function of that structure?
Cupula - helps with hair cell movement
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The semicircular canals are high in what fluid?
Endolymph - high in potassium
What kinds of movement do the semicircular canals detect?
Orientation of canals in the head defines three planes
Anterior and posterior canals form a 90 degree angle
Lateral canals are horizontal to the other canals
Work in pairs
Can detect movement at different angles - angular acceleration
How are cells depolarised in the vestibular organ?
Hair cells have kinocilium and stereocilia
Movement of the endolymph causes deflection of cilia leading to depolarisation
Describe hair cell potentials
Hair cells have a resting potential which has a basal discharge to the nerve
Movement towards kinocilium causes depolarisation- increase in nerve discharge
Movement away from kinocilium causes hyperpolarisation - reduction in nerve discharge
Where do the primary afferent of the vestibular nerve end?
Vestibular nuclei of pons
What other areas does the vestibular nerve communicate with?
Vestibulospinal tract
Abducens nucleus
Medial longitudinal fasciculus (connects 3rd and 6th nerve)
Oculomotor nucleus
What are the key functions of the vestibular system?
Detect and inform about head movements
Keep images fixed in retina during head movements
Balance
What is the vestibular-ocular reflex (VOR)?
Keeps images fixed in retina
Connection between vestibular nuclei and oculomotor nuclei
Eye movement in opposite direction to head movement, but same velocity and amplitude
What is the purpose of the vestibulo-spinal reflex do?
Helps to maintain posture, e.g. when falling
How do you categorise vestibular disorders?
Timing and laterality:
Acute and unilateral
- Main complaints are imbalance, dizziness, vertigo and nausea
Slow and unilateral or bilateral loss
- Main complaints are imbalance and nausea - no vertigo
Give examples of peripheral vestibular disorders
Affect vestibular organ and/or VIII nerve:
Vestibular neuritis
Benign Paroxysmal Positional Vertigo (BPPV)
Meniere’s disease
Give examples of central vestibular disorders
Stroke
Multiple sclerosis
Tumours
What is the clinical approach to diagnosis of suspected vestibular disorders?
What is it called when the eyes change in the laterality of how they move?
Skew
Name two acute balance disorders
Vestibular neuritis
Stroke
HINTS exam useful to differentiate
Name an intermittent balance disorder
Benign Paroxysmal Positional Vertigo
Dix-Hallpike Test - useful in diagnosis
Name a recurrent balance disorder
Migraine ( Meniere’s disease)
Name two progressive balance disorders
Schwannoma vestibular (VIIIth nerve)
Degenerative conditions (MS)
Describe the steps of the HINTS exam
Head Impulse test
- Horizontal rotational VOR
Nystagmus
- Vestibular organ vs cerebellar/brainstem nystagmus
Test of Skew deviation
- Vertical misalignment - usually absent in peripheral pathology
