Auditory system Flashcards
what is the anatomy of the auditory system?
- Divided into 3 sections
- Outer, middle and inner ear
- Each part has specific function
- Organ embedded in petrous portion of temporal bone (hardest bone in body)
what is the function of the outer ear?
capture sound and focus it on the tympanic membrane
amplify some frequencies by resonance in the canal
protect the ear from external threats
what is the function of the middle ear?
- Main function is amplification by:
- Focusing vibration from large surface area (tympanic membrane) to smaller surface area (oval window)
- Change surface area means pressure in increased
- Using leverage from incusstapes joint to increase force on oval window
- Focusing vibration from large surface area (tympanic membrane) to smaller surface area (oval window)
what is the function of the inner ear?
- Hearing part inner ear is cochlea
- Function = transduce vibration into nervous impulses
- While doing so produces a frequency (pitch) and intensity (loudness) analysis of sound
what does the cochlea contain?
3 compartments
- Scala vestibuli
- Scala tympani
- Scala media
what are the scale vestibuli and tympani?
bone structures
contain perilymph (high in sodium)
what is the structure of the scala media?
membranous structure
contains endolymph (high potassium)
hearing organ of Corti location
what is the structure where the organ of corti lies?
basilar membrane
how is the basilar membrane arranged?
tonotopically
sensitive to different frequencies at different points along length
what is the hearing organ?
organ of corti
what does the organ of corti contain?
- Contains hair cells: inner (IHC) and outer (OHC)
- IHC arranged on one column and OCH on three
- tectorial membrane above hair cells
- allow hair deflection, which will depolarise the cell
- only OHC in constant contact with membrane
- assists contacts with the IHC
what are the function of inner hair cells?
carry 95% afferent information of auditory nerve
function= transduction of sound into nerve impulses
what are the function of outer hair cells?
carry 95% efferent information of auditory nerve
function= modulation of sensitivity of response
what is the difference between inner and outer hair cells?
IHC arranged in one column, OHC on 3
OHC in constant contact with tectorial membrane
what are sterocilia?
hairs of hair cells
responsible for transduction
how does deflection of stereocilia work?
open K+ channels= deflection of sterocilia towards longest cilium
- ionic interchange depolarises the cell= NT liberated
what are the effects of depolarisation in hair cells?
K+ channel opening
what happens to deflection of stereocilia with higher amplitude of sound?
higher amplitude of sound= greater deflection of stereocilia & K+ channel opening
what happens to stereocilia on hyperpolarisation?
closes K+ channels so noises arent as loud
what is the pathway from the cochlear to cerebral hemisphere?
- spiral ganglions from each cochlear project via auditory vestibular nerve (VIII) to ipsilateral cochlear nuclei
- auditory information crosses at superior olive level
- after this point all connections are bilateral
how is hearing organised centrally?
tonotopically organised
what is frequence / pitch?
(Hz)= cycles per second, perceived tone
what is amplitude/loudness?
(db)
sound pressure, subjective attribute correlated with physical strength
what is the human range of hearing?
- freq = 20-20,000 Hz
- loudness= 0dB- 120 dB sound pressure level (SPL)
what is hearing acuity affected by?
decreases with age, particularly higher frequencies
what are the types of hearing assessments?
tuning forks
audiometry
central processing assessment
tympanometry
otoacoustic emission
auditory evoked potentials
what do tuning forks do?
- establish probable presence or absence of hearing loss with significant conductive component
- provide early and general information when audiometry is not available or possible
- webber and rinne test
- vibrate fork and see if hearing loss asymmetric or symmetric
- not very specific tests
what is audiometry?
- science of measuring hearing acuity for variations in sound intensity and frequency
- produce an audiogram where hearing thresholds plotted
what is central processing assessment?
- assessment hearing abilities via verbal and non verbal tests
- e.g sound localisation, speech in noise, filtered speech
what is tympanometry?
- test middle ear and mobility of eardrum and conduction bones by creating vibrations of air pressure in ear canal
what is otoacoustic emission?
- low intestity sounds produced by normal cochlea
- produced specifically by outer hair cells as expand and contract
- part of newborn hearing screening and hearing loss monitoring
- require no cooperation of subject
what are the types of auditory evoked potentials?
- electrocochleography
- electrical activity from cochlea and 8th nerve- evoked by clocks or tone burst
- auditory brainstem response (ABR)
- electrical activity from 8th nerve and brainstem nuclei and tracts- evoked by clicks
- late responses
- electrical activity from primary auditory and association cortex- evoked by tone burst and oddball paradigm
what are auditory brain stem responses and how are they used?
- electrical responses from auditory pathway
- ABR more commonly used in clinics
- Does not require attention from patients
- Alterations in latency of waves can point the location of deficit
when might cortical potential be affected?
neurological condition or processing problems
what are the types of hearing loss?
conductive hearing loss
sensorineural hearing loss
mixed hearing loss
what is conductive hearing loss due to?
problem located in outer or middle ear
what is sensorineural hearing loss due to?
problem located in the inner ear or auditory nerve
what is mixed hearing loss due to?
conduction and transduction affected
problem affects more than one part of ear
what are the degrees of hearing loss?
depends on where hearing thresholds are located
can be mild, moderate, severe or profound
what are the causes of conductive hearing loss?
what are the causes of sensorineural hearing loss?
what are the treatments for hearing loss?
- Underlying cause
- Hearing aids/ cochlear implants/ brainstem implants
what do hearing aids do?
- Amplify sound, does not replicate any structure
what do cochlear implants do?
- Replaces function of hair cells by receiving sound, analysing and transforming into electrical signals
- Send electrical impulse directly to auditory nerve
- Required functional auditory nerve to function
what do brainstem implants do for hearing loss?
Electrical signal sent to electrode in brainstem when auditory nerve affected- risky! Only bilateral nerve damage
