Neuro - Auditory System Flashcards
How many sections is the ear generally divided into?
→ outer
→ middle
→ inner
Where is the auditory organ located?
embedded in the petrous portion of the temporal bone (hardest bone in body)
What are the functions of the outer ear?
→ captures sound and focuses it in the tympanic membrane
→ amplify some frequencies by resonance in the canal
→ protect ear from external threats (done by hair and wax)
What are the functions of the middle ear?
amplification by:
→ focusing vibrations from large surface area (tympanic membrane) to smaller surface area (oval window). The change in surface area means the pressure is increased.
→ Using leverage from the incus-stapes joint to increase the force on the oval window
What is the hearing part of the inner ear?
cochlea
What is the function of the inner ear?
→ transduce vibration into nervous impulses
→ produces a frequency (or pitch) and intensity (or loudness) analysis of sound
What are the 3 compartments of the cochlea when it’s uncoiled?
→ scala vestibuli
→ scala media
→ scala tympani
What are the scala vestibuli and scala tympani?
bone structures, contain perilymph (high in sodium)
What is the scala media?
→ membraneous structure that contains endolymph (high in potassium)
→ contains the hearing organ / Organ of Corti
→ sandwiched between scala vestibuli and tympani
What membrane contains the organ of Corti?
basilar membrane
How is the basilar membrane arranged?
→ arranged tonotopically (thicker in one end, thinner the other) like a xylophone
→ base = narrow + tight
→ apex = wide + loose
→ sensitive to different frequencies at different points along length
What are the two types of hair cells in the Organ of Corti? How are they arranged?
→ inner hair cells, arranged in 1 column
→ outer hair cells, arranged in 3 columns
What is the purpose of the tectorial membrane in the Organ of Corti?
→ located above the hair cells
→ always in constant contact w OHCs, which assist in contact with IHCs
→ allows for hair deflection
→ this depolarises the cells
What are the functions of IHCs?
→ carries 95% of afferent info of the auditory nerve
→ transduces sound into nerve impulses
What are the functions of OHCs?
→ carries 95% of efferent info of the auditory nerve
→ modulates sensitivity of the response
What are the hairs of the hair cells called?
stereocilia
What does deflection of the stereocilia do?
→ mechanically opens the potassium open channels
→ ionic interchange = depolarisation of cell
→ neurotransmitter is liberated
How does amplitude of sound affect deflection?
higher amplitudes (louder sounds) causes greater deflection of stereocilia, more hair cells involved, etc.
What does hyper-polarisation of hair cells cause?
closes potassium channels
What is the auditory pathway from cochlea to cerebral hemispheres?
→ spiral ganglions form each cochlea project via auditory vestibular nerve (VIII) to ipsilateral cochlear nuclei (monoaural neurones)
→ auditory info crosses at the superior olive level in brainstem
→ after this point, all connections are bilateral
→ goes through inferior colliculus and medial geniculate body in thalamus
→ ends in auditory cortex
What happens if a sound is too soft to cause deflection?
OHCs contract to bring the tympanic membrane closer to the stereocilia of the IHCs to cause deflection so sound can be heard
What happens if a sound is too loud?
→ OHCs can elongate
→ pushes tympanic membrane away from stereocilia of IHCs
→ less deflection = less loud sound transduced
How is hearing arranged in the auditory cortex?
→ tonotopically organised (like a xylophone)
→ low freq to high freq anterior-posteriorly in the brain
What is frequency / pitch? What is it measured in?
→ cycles per second, perceived tone
→ measured in Hertz
What is amplitude / loudness? What is it measured in?
→ sound pressure
→ subjective attribute correlated with physical strength
→ decibels
What is the human range of hearing for frequency?
20Hz - 20,000 Hz
What is the human range of hearing for amplitude?
0 dB - 120 dB
Why is the decibel scale useful?
→ log scale, so range of sensitivity is high
→ allows compression of scale onto a graph
→ reflects on idea that many physiological processes are on-linear
How does hearing acuity change with age?
decreases with age, especially high frequencies
How are medium + low frequencies affected through life?
progression of hearing loss
What are the aims of a hearing assessment?
→ is there hearing loss?
→ what is the degree of hearing loss, if yes?
→ what is the type of hearing loss, if yes?
What are the procedures used to test hearing during a hearing assessment?
- Tuning fork
- Audiometry
- Central processing assessment
- Tympanometry
- Otoacustic Emission
- Electrocochleography
- Evoked potentials
How and why is the tuning fork used?
→ Weber test or Rinne test
→ Used to establish the probable presence or absence of a hearing loss with a significant conductive component
→ provide early and general information, when audiometry is not available or possible
What is PTA?
→ pure tone audiometry
→ science of measuring hearing acuity for variations in sound intensity and frequency
What is a audiogram?
→ where hearing thresholds are plotted to define if there is a hearing loss or not
→ normal hearing threshold is located between 0-120dB
What is an audiometer?
device used to produce sound of varying intensity and frequency
What is a CPA?
→ central processing assessment
→ assessment of hearing abilities other than detection
→ verbal + non-verbal tests
→ examples: sound localisations, filtered speech, speech in noise
What is the Weber test?
vibrate tuning fork at the top of the head to assess whether hearing loss (if any) is symmetrical
What is the Rinne test?
compares direct stimulation of outer + middle ear with direct stimulation of the inner ear
When is the tuning fork method used?
in GPs, usually when there’s no alternative method available at the moment
What is tympanometry?
examination used to test the condition of the middle ear + mobility of the eardrum (tympanic membrane) + the conduction bones by creating variations of air pressure in the ear canal
What are OAEs? Why are they tested?
→ normal cochlea produces low-intensity sounds called OAEs
→ sounds are produced specifically by OHCs as they expand and contract
→ often part of the newborn hearing screening and hearing loss monitoring.
What are auditory evoked potentials? What are 3 most common types that clinics look for?
action potentials evoked by auditory stimuli:
→ electrocochleography
→ ABR (auditory brainstem response)
→ late response
What is electrocochleagraphy?
→ 0.2-4.0 ms
→ electrical activity from the cochlea and eighth nerve
→ evoked by clicks or tone burst
What is ABR (auditory brainstem response)?
→ 1.5-10.0 ms
→ electrical activity from the eighth nerve and brainstem nuclei and tracts (auditory pathway)
→ ABR is more commonly used in clinic
→ evoked by clicks
→ does not require attention from patient
What are late responses?
→ 80-500+ ms
→ electrical activity from the primary auditory and association cortex
→ evoked by tone burst and oddball paradigm
What do alterations in latency of waves show?
can point to the location of the deficit
What are cortical potentials useful?
useful in investigations of neurological conditions or processing problems
What are the 3 types of hearing loss?
→ conductive hearing loss
→ sensorineural hearing loss
→ mixed hearing loss
What is conductive hearing loss?
problem is in outer + middle ear
What is sensorineural hearing loss?
problem is in inner ear or auditory nerve
What is mixed hearing loss?
→ conduction + transduction of sound are affected
→ more than one part of the ear is affected
What are the different degrees of hearing loss?
→ normal → mild → moderate → severe → profound
What are some of the causes of conductive hearing loss affecting the outer ear?
→ wax
→ foreign body
What are some of the causes of conductive hearing loss affecting the middle ear?
→ otitis
→ otosclerosis
What are some of the causes of sensorineural hearing loss affecting the inner ear?
→ presbycusis
→ ototoxicity
What are some of the causes of sensorineural hearing loss affecting the nerve?
8th cranial nerve tumour
What are the different treatments for hearing loss?
→ underlying cause
→ hearing aids
→ cochlear implants
→ brainstem implants
What is the main purpose of hearing aids?
amplifies sounds but doesn’t replace any specific structure
What is the main purpose of cochlear implants?
replaces functions of hair cells by receiving sound, analysing it + transforming it into electrical signals + sending an electric impulse directly to the auditory nerve
What does a cochlear implant need in order to function?
needs a functional auditory nerve
What is the main purpose of a brainstem implant?
→ when the auditory nerves are the affected structures, the electrical signals can be sent to a set of electrodes directly into the brainstem
→ very risky, advised for people w bilateral important auditory nerve damage
