Auditory and vestibular system Flashcards
what are the 3 sections of the ear
The ear is divided in 3 sections: outer , middle and inner
where is the ear found
The ear is embedded in the petrous portion of the temporal bone
What parts make up teh outer ear and what is the function of the outer ear
Capture sound and focus it in the tympanic membrane
Amplify some frequencies by resonance in the canal
Protect ear from external threats
Has hairs to protect inner ear
Formed by pina and external auditory canal
what makes up the inner ear and what is the function
ossicles , oval and round window , tympanic membrane and muscles
Function is amplification:
Focussing vibrations from large SA (tympanic membrane) to smaller SA (oval window) → change in SA increases pressure
Leverage from incus-stapes joint to increase force on oval window
what makes up the inner ear and what is its function
Cochlea is the hearing part of the ear; transduces vibrations into nervous impulses (while doing so, also produces a frequency and intensity analysis of the sound - pitch and volume)
what is the 3 compartments
scala vestibuli
scala tympany
scala media
what is the fluid in the scala vestibuli and scala tympani
perilymph ( high in Na)
what is the fluid in the scala media
endolymph high in K
what other structure is found in the scala media
Organ of corti in the media basilar membrane
how is the scala media basilar membrane arranged and why
Basilar membrane arranged tonotopically ( aka thicker at one end + tinner at the other) meaning it is sensitive to different frequencies at different points along length
what are the two types of hair cells found in the Organ of corti and what is thier function
Inner hair cells –> afferent cells carry out transduction
Outer hair cells –> can contract / elongate to act as an active amplifier by moving the tectorial membrane towards or away from IHC
what is the tectorial membrane
located above the hair cells and will allow the hair deflection which in turn will depolarise the cell
OHC hair are in constant contact with the tectorial membrane, and these assist the contact with the IHC
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Describe the process of transaction
1) Sound come in causing the tectorial membrane to move ( depends on OHC) causing vibration of hairs on the basilar membrane
2) Deflection of the stereocilia towards the longest cilium will open potassium ion channels within the inner hair cell, as the tectorial membrane is brought into contact.
3) Ionic interchange of K+ within the endolymph and inner hair cell leads to cell depolarisation.
4) Depolarisation induces the opening of voltage-gated calcium channels which release excitatory neurotransmitters (Glutamate) onto the afferent nerve.
5) Higher amplitudes ( louder) sounds will cause greater deflection of the stereocilia and K+ channel
6) If a sound it too loud the cell will hyperpolarise and close K+ channels
how does the OHC effect transduction
act as an active amplifier . OHC can contract to move the tectorial membrane away / towards the IHC.
1) For softer sounds :
OCH contract to move tectorial membrane towards IHC then more sound is transduced so more APs and visa versa
This normally happens if a sound is too soft to increase APs
2) for louder sounds;
If a sound is too loud , OHC elongate and push tectorial membrane away from IHC to prevent APs and reducing noise
Describe the auditory pathway
Spiral ganglions from each cochlea project via the auditory vestibular nerve (VIII) to the ipsilateral cochlear nuclei (monoaural neurons).
Auditory information crosses the superior olive level, beyond this point all connections are bilateral. ( aka travels up both the same and opposite side)
Ultimately information is relayed to the auditory cortices located within the temporal lobe.
Wernicke’s area: comprehension and understanding of language.
What are the areas of the brain that the auditory sensory system is connected to
Primary
Primary: exclusively carries messages from the cochlea
Short pathway: 3 to 4 relay
Each relay nucleus does a specific work of decoding and integration
First relay in cochlear nuclei of brainstem: receive type 1 spiral ganglion axons (auditory nerve)
Important decoding of brain signal here: duration, intensity, frequency
Second major relay in superior olivary complex: majority of auditory fibres synapse here having crossed the midline
Third relay in mesencephalon: localisation of sound along with final neuron
Final relay in median geniculate body of thalamus: preparation of motor (vocal) response
Fast: large myelinated fibers
Final neuron links thalamus to primary auditory cortex
PAC in temporal area within lateral sulcus
Message already largely decoded by previous neurons but at PAC it is recognised, memorised and integrated into voluntary response
Reticular sensory
non-primary/reticular sensory: carries all types of sensory messages
From the cochlear nuclei, small fibers connect with the reticular formation: this is where the auditory message joins all other sensory messages
Next relay is in the non-specific thalamus nuclei before the pathway ends in the polysensory (associative) cortex
Main function of pathways: select the type of sensory message to be treated first
Pathways connected to wake and motivation centers as well as to vegetative and hormonal systems
Central auditory cortex
What is freq? , Amplitude?
Frequency/pitch (Hz): Cycles per second, perceived tone
Amplitude/loudness (dB): Sound pressure, subjective attribute correlated with physical strength.
what sounds are heard by humans and what happens to these sounds with age
Frequency: 20-20,000Hz (8000Hz is the discrimination maximum)
Loudness: 0 dB to 120 dB sound pressure level (SPL). ( can hear >120 but will cause damage to ear)
Hearing acuity decreases with age, particularly higher frequencies. Medium and low frequencies could be affected with the progression of a hearing loss.
How is sound measured
The decibel scale is a logarithmic scale since the range of sensitivity is very large.
A logarithmic scale enables us to graphically compress the scale. Outer hair cells support sound amplification beyond 60dB
what tests are used for hearing assessment
tuning for pure tone audiometry verbal and non verbal tests Tympanometry Otoacoustic emissions Auditory evoked potentials cortical potentials
what are the 2 tests for tuning forks and what happens in a tuning fork test
Rinne test directly stimulates vibration of hair cells ie only inner ear
Weber test tests normal hearing ie outer / middle inner
Used to establish probably presence or absence of hearing loss with a significant conductive component
Used to provide early and general information, when audiometry is not available or possible.
( NOT really used anymore main function is to see if hearing loss is symmetric or asymmetric)
what happens in a PTA
Science of measuring hearing acuity for variations in sound intensity and frequency
Audiometer = device used to produce sound of varying intensity and frequency
Machine plates different sounds and freq and person tells you if they can hear it or not
Test produces an audiogram = hearing threshold are plotted to define if there is hearing loss of not
Normal = 0 - 20dB
what happens in a verbal and non verbal test
can a person who hears sounds understand the sound ie :
Sound localisation
Filtered speech
Speech in noise
What is tympanometry and what is it used for
Used to test condition of the middle ear and mobility of the eardrum (tympanic membrane) and the conduction bones ( ossicles) by creating variations of air pressure in the ear canal
( see google docs for results)
What is an OAE and where is it used for
Normal cochlea produces low-intensity sounds called OAEs
These are produced by OHCs as the expand and contract
Test often part of newborn hearing screening and hearing loss monitoring
what are auditory evoked potentials and what responses are measured
Measure electrical signals prod from a sounds ,
Can measure 3 things
Electrocochleography
0.2-4.0ms; electrical activity from the cochlea and eighth nerve. Evoked by clicks or tone burst
Auditory Brainstem Response (ABR)
1.5-10.0ms; electrical activity from eighth nerve and brainstem nuclei and tracts. Evoked by clicks
Late responses (N1-P2, P300, MMN, etc.) 80-500+ ms; electrical activity from the primary auditory and association cortex. Evoked by tone burst and oddball paradigm
see wave forms from google docs
what are cortical potentials used for
Could be affected on neurological conditions or processing problems
what are cortical potentials used for
Could be affected on neurological conditions or processing problems
what are the types of hearing loss and describe them
Conductive hearing loss - problem in outer or middle ear
Sensorineural hearing loss - problem is location in inner ear or auditory nerve
Mixed hearing loss - conduction and transduction of sound are affected; problem affects more than one part of the ear
What are the degrees of hearing lose
mild, moderate, severe or profound
Causes of conductive hearing loss
Outer ear: wax, foreign body
Inner ear: otitis, otosclerosis
Causes of sensorineural hearing loss
Inner ear: presbycusis, ototixicity ( drugs/ toxins causing damage)
Presbycusis: OCH loss with age
Nerve: CN VIII tumour
What are treatments of hearing loss
1) Treat underlying cause ie wax
2) hearing aids: depending on the type, degree and characteristics of the patient there are many options
hearing aids amplify the sound; does not replace any structure
3) cochlear implant: replaces the function of hair cells by receiving sound, analysing it, transforming it into electrical signals and sending an electrical impulse directly to the auditory nerve
4) brainstem implant: when the auditory nerves are the affected structures, the electrical signals can be sent to a set of electrodes placed directly into the brainstem. If it is very risky, advised for people with bilateral important auditory nerve damage
what is the vestibular system and where is it found
Vestibular organ is in the posterior part of inner ear (contains hair cells for hearing and balance)
what are the inputs and outputs of the vestibular system
vestibular system inputs (3): visual, proprioceptive, vestibular information( organ information )
outputs: reflexes to maintain stable posture and gaze
CNS integrates this information and generates the responses
what is the function of teh vestibular system
To detect and inform about head movements, postural control, keep images fixed in the retina during head movements. ( aka why you can move your head and keep your eyes on the same spot)
what are symptoms of damage to vestibular system
Symptoms to damage of vestibular system: nausea , problems with ocular reflex , problems with gait
what are the 2 vestibular reflexes
Vestibulo-oculo reflex ( VOR)
vestibulo spinal reflex ( VSR)
what is the VOR
reflex ensures that the images remained fixed within the retina. This involves a connection between vestibular nuclei and oculomotor nuclei.
Eye movement in opposite direction to head movement (with same velocity and amplitude).
what is the VSR
this concerns motor neurones to limb muscles (lateral corticospinal tract). The contraction of muscles is dependent on head movement. Motor neurones to neck and back muscles (medial tract).
Postural control, there is compensatory body movement according to the head position.
( aka helps you avoid falls and stops you from tripping and eating shit)
what are the otolith organs
utricle and saccule
what are the functions of the otolith organs
These organs detect linear acceleration ( ie when walking forward) and tilt : due to otolith movement
Utricle detects horizontal movement and saccule vertical movement
Where are the hair cells found in the utricle and the saccule
maculae
what can be found on the hair cells in the
Otoliths
carbonate crystals on top of hair cells that help deflection of hairs determining de/hyperpolarisation )
what is the structure of the otoliths
utricle and saccule are located in the vestibule and are joined by a conduit, saccule is also joined to the cochlea
maculae contain hair cells, gelatinous matrix and otoliths (carbonate crystals on top of hair cells that help deflection of hairs determining de/hyperpolarisation )
what are the 3 semi-circular canals
anterior . posterior , lateral
each has an ampulla on one side and are connected to the utricle)
The right anterior and left posterior semicircular canals are along the same orientation.
how do the semicircular canals work
in pairs
which pairs can semicircular canals form
The semicircular canals work accordingly in pairs according to planes: .
Both laterals
Anterior from one side with posterior of the opposite side
What shouldn’t be present on hair cells in the semi-circular canals
otoliths
what is the ampulla and what does it contain
a structure at the end of semi-circular canals
ampulla includes the crista, where the hair cells are located
Surrounded by the cupula, which facilitates hair cell movement.
The endolymph contains a high concentration of potassium ions within the semicircular can
what is the fluid in the canals
The endolymph contains a high concentration of potassium ions within the semicircular canals.
what is the function of the semicircular canals
These organs detect angular acceleration
Velocity is output through the VIII th ( vestibulocochlear nerve (
what is the labryinth
Labyrinth (posterior interior ear) concerned with the vestibular system
The bony labyrinth resides within the petrous part of the temporal bone.
what are the 2 planes formed
anterior and posterior semi-circular canal form a 90-degree angle, the anterior canal is 45 degrees to the front
planes determine which structure will be stimulated with a specific head movement
what is the kinocilium and sterocilia
Kinocilium = biggest cilium
stereocilia = cillia that move allowing cell to depolarise
what causes hair cell depolarisation
movement of the endolymph within the crista generated by head movement
where are hair cells found
In utricles + saccule with otoliths
all 3 canals with no otoliths
what is the hair cell resting potential
Hair cells have a resting potential which has the basal discharge of the nerve
This is due to muscle contraction against gravity
Gives an indication that the head is not moving
what causes excitation of teh hair cells
movement of the head moving the stereocilia + otholiths
causes the stereocilia to move towards the kinocilium, generating a depolarisation and an increase in nerve discharge. ( see hyperpolarisation above)
Stereocilia away from kinocilium generating hyperpolarisation and a reduction in nerve discharge.
what is the relationship of hair cells between opposite ears
antagonistic relationship between the left & right ear (excitation and inhibition simultaneously occur at that specific velocity).
What is the function of the hair cells
Detects linear acceleration and tilt (otolith movement).
what is the vestibular nuclei
where primary afferents from the vestibular system end
where is teh vestibular nuclei found
cerebellum
What are the projections of the vestibular nuclei
Spinal cord ( reflexes)
nuclei of the extraocular muscles ( eye muscle movement)
Cerebellum ( coordination of movement)
centres of cardiovascular and respiratory control
which cortex does the vestibular system integrate into
Many cortical areas participate
Main processing centre thought to be in the parietal lobe - in parieto-insular vestibular cortex ( PIVC)
Describe the pathway of the vestibular system
see google docs
what assessments of teh vestibular system are done
Anamnesis (History): Disorders are specific with patient history in order to determine the specific cause of the pathology.
Posture & gait: Can be determined by Posturography.
Cerebellar function: Using imaging (CT scan & MRI)
Eye movements: This can be examined during consultation.
what tests are done to test the vestibular system
Caloric test (Stimulates the inner ear with different temperature to identify how symmetric the responses area – it is a test of the lateral semi-circular canals)
Video head impulse test (vHIT): Measures the Vestibulo-ocular reflex.
Vestibular evoked myogenic potential (VEMP)
Rotational test
Imagining ie CT , MRI
Symptoms and impact assessment
What is the main symptom of balance disorders
dizziness or vertigo
what can dizziness also be caused by
Dizziness can also be due to heart disorders, presyncopal episodes, orthostatic hypotension, anaemia, hypoglycaemia, psychological, gait disorders
How can balance disorders be categorised
Categorised based on location of affected structure and evolution of signs and symptoms
What are the 2 types of balance disorders categorised by location
Peripheral vestibular disorders (labyrinth and/or viii nerve): ie vestibular neuritis, BBPV, Meniere’s disease, unilateral and bilateral vestibular hypotension
Easier to manage
Central (CNS) disorders : stroke, MS, tumours
what are the 4 groups of balanced disorders categorised by evolution
Acute: vestibular neuritis, stroke
Intermittent: BPPV
Recurrent: Meniere’s, migraine
Progressive: Schwannoma vestibular (VIIth nerve) , Degenerative conditions(MS)Acute: vestibular neuritis, stroke
Intermittent: BPPV
Recurrent: Meniere’s, migraine
Progressive: Schwannoma vestibular (VIIth nerve) , Degenerative conditions(MS)
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