Auditory system Flashcards
What is the function of a vestibular organ?
Capture low frequency motion (movement)
What is the function of a hearing organ?
Capture high frequency motion (sound)
What is the primitive mechanism of vestibular organs?
Statocyst - crystallised calcium ball moves freely across cells with hairs during movement which is transduced to an electrical signal
What are the evolved vestibular and hearing organs present in humans?
vestib: Utriculus and Sacculus,
audio: Cochlea and Canals
What relative frequencies do vestibular and hearing organs detect?
Vestibular - low frequency (movement), Hearing - high frequency (sound)
What is frequency?
Cycles per second
pitch (Hz)
What is amplitude?
Sound pressure, associated with physical strength
volume (dB)
Human range of hearing
20 - 20k Hz
0 - 120 dB
What are the structures in the outer ear?
Pinna,
External Acoustic Meatus,
Tympanic Membrane (eardrum)
What are the three functions of the outer ear?
Capture sound and focus to tympanic membrane Amplification of sound (by 10dB)
Protection of ear from external threats
What is the bony structure in the middle ear and what is its functions?
Ossicles - vibrate to create mechanical amplification of sound by 30dB
What are the muscular structures in the middle ear and what are their functions?
Tensor tympani muscle, Stapedius muscle
- protection by reducing sound that reaches the middle ear during loud noises
What is the main structure in the inner ear and what is its function?
Cochlea, transduce vibration signals into nervous impulses, capturing frequency and amplitude
What are the three compartments in the cochlea, and what do they contain?
Scala vestibuli
Scala tympani
scala media
What are the bony structures in the cochlea
Scala vestibuli (vestibular side)
scala tympani (basilar side)
- contain perilymph (high in Na+)
What is the middle membranous layer of the cochlea
Scala media - contains endolymph (K+),
contains the Organ of Corti (hearing organ, has hairs)
What are the three structures in the Organ of Corti?
Basilar membrane, Hair Cells, Tectorial Membrane
How is the basilar membrane arranged?
Tonotopically (like a xylophone) -
narrow base is for high frequencies, wider apex is for lower frequencies
What type of hair cells does the Organ of Corti contain and how are they arranged?
Inner hair cells (in one line) and Outer hair cells (form three lines)
What is the function of the IHC?
Inner hair cells - carry 95% of afferent information, transduction of sound into nerve impulses
What is the function of the OHC?
Outer hair cells - carry 95% of efferent information, modulation of sensitivity of response
What is the role of the tectorial membrane?
Above the hair cells, allowing hair deflection which in turn will depolarise the cell
What are the hair cells also known as and what is the largest one called?
Stereocilia // Kinocilia
How does transduction occur across the IHC?
Deflection of stereocilia towards kinocilia opens K+ ion channels,
cell depolarisation
Ca2+ channels open, stimulates excitatory glutamate neurotransmitter release.
Deflection of stereocilia away from kinocilia causes hyperpolarisation and inhibits nerve discharge
Effect of amplitude on stereocilia
High amplitudes (volume) will cause greater deflection of stereocilia and k+ channel opening leading to greater neural signal
Effect of frequency on stereocilia
Has no effect on movement of stereocilia
Main points of transduction
Depolarisation - opens k+ channels
Hyperpolarisation - closes k+ channels
How does auditory information travel from the cochlea to the cerebral hemisphere?
Stereocilia -> vestibulocochlear nerve’s spiral ganglions ipsilaterally to the cochlear nuclei (monoaural) in the brainstem.
bilateral superior olives
inferior colliculus.
medial geniculate body in the thalamus,
cerebral hemisphere
What are the three types of anatomical hearing loss?
Conductive (outer or middle ear),
Sensorineural (cochlear or auditory nerve - 90% of cases),
Central Hearing Loss (rare, brainstem)
What is conductive hearing loss
Affects outer or middle ear
What is sensorineural hearing loss
affects the cochlear or auditory nerve
What is central hearing loss
affects the brain and brainstem
What are the two types of hearing loss with regards to timing?
Sudden hearing loss (minutes and days),
Progressive hearing loss (months to years)
What are four causes of conductive hearing loss?
Outer ear - cerumen impaction (wax),
foreign body //
Middle ear - otitis (inflammation causes bubbles of liquid to be present in middle ear) ,
Otosclerosis (ossicles fuse together with age and do not vibrate properly) abnormal bone growth
What are four causes of sensorineural hearing loss?
Inner ear - noise, presbycusis (old age), ototoxicity (chemotherapy, antibiotics - gentamicin)
nerve - vestibular schwannoma (acoustic neuroma)
tumour pushes on the 8th nerve causing unilateral hearing loss
What are two quick bedside tests for clinical assessment of hearing loss?
Whispering numbers in ear and rubbing fingers in other ear - can they hear,
Tuning fork test
What are the tuning fork tests and what do they show?
Weber test - fork on top of head, tests both ears hearing
Rinne test - on mastoid process behind ear, compares left and right hearing
How does audiometry work?
Measure the patients hearing threshold - the lowest amplitude required to hear different frequencies
What lowest levels of audiometry is normal?
Lowest - 10-20dB
How can you observe conductive hearing loss on an audiometry graph?
Two different graphs for bone and air
Bone conduction is normal (showing there is no problems with auditory nerves) but air conduction is impaired (showing there is a problem with the outer and middle ear)
How can you observe sensorineural hearing loss on an audiometry graph?
Both bone and air hearing is impacted
How can you do newborn hearing screening?
Otoacoustic emissions are low-intensity sounds produced by outer hair cells as they expand and contract.
What are the two main treatments for hearing loss?
Treatment of the underlying conditions (wax buildup or tumour) or hearing aids
What are two rare treatments for hearing loss?
Cochlear implants and brainstem implants
