AUDITORY SYSTEM Flashcards
In which bone is the organ of the ear embedded in?
Petrous portion of the temporal bone - hardest bone in body
What are the functions of the outer ear?
Capture sound and focus it on the tympanic membrane
Amplify some frequencies by resonance in canal
Protect ear from external threats
How does the outer ear protect against external elements?
Wax to cover external elements and hairs to move it out.
Wax also slightly acidic to kill micro-organisms
What are the functions of the middle ear?
Amplification by:
- Focusing vibrations from large SA (tympanic mem.) to
small SA (oval window) increasing pressure
- Use leverage from incus-stapes joint to increase force
on the oval window
Name the 3 ossicles in their order
Malleus, Incus, Stapes
What is the entrance to the inner ear?
Oval window
What is the hearing part of the inner ear called and its specific function?
Cochlea
Transduces vibration into nervous impulses whilst producing a frequency and intensity analysis of the sound
Describe (and draw) the structure of the cochlea
Contains 3 compartments:
- Scala vestibuli (bone structure containing perilymph)
- Scala tympani (bone structure containing perilymph)
- Scale media (membranous structure floating in
perilymph containing endolymph)
What is perilymph high in?
Sodium
What is endolymph high in?
Potassium
Where is the Organ of Corti located?
Scala media, specifically basilar membrane
Describe the structure of the basilar membrane and what this allows it to be
Base is narrow and tight
Apex is wide and loose
Hair cells all along basilar mem.
Similar to a xylophone (tonotopically) this allows the basilar membrane to be sensitive to diff frequencies at diff points along it.
High freq = higher amplitude of waves at start/base
Low freq = higher amplitude of waves at end/apex
What is the Organ of Corti AKA?
Hearing organ
Name the hair cells which are present in the organ of Corti
Inner hair cells
Outer hair cells
Describe the structure of the organ of Corti
Hair cells arranged on top of basilar membrane with afferent nerve fibres:
IHCs - 1 row
OHCs - 3 rows
Tectorial membrane above hair cells
What is the relationship between the tectorial membrane and hair cells?
Allows for the deflection of hairs which depolarises the hair cell.
OHCs in constant contact with tectorial membrane and and bring it closer into contact with IHC
What is the function of IHCs?
Carry 95% of afferent information of the auditory nerve. Transduces sound into nervous impulses
What is the function of OHCs?
Carry 95% of efferent information of the auditory nerve. Modulates the sensitivity of the response
How do OHCs modulate sensitivity?
OHCs have contractile proteins in their membrane and can thus shorten to bring the tectorial membrane closer/away from the IHC. If IHCs make contact with the tectorial mem. sound is transduced.
How does transduction occur?
Deflection of stereocilia on IHCs towards longest cilium opens K+ channels causing depolarisation and VGCaC to open. Calcium into cell causes release of NTs e.g. glutamate to afferent nerve.
What are the hairs of the hair cells called?
Stereocilia
How are louder and softer sounds differentiated in the ear?
Loud sound:
- Increased tympanic mem. vibration
- Increased vibration speed of ossicle chain
- Increased movement of endolymph
- Increased movement of basilar mem.
- Increased area + cells of cochlea stimulated
- Increased deflection of stereocilia and opening of K+
channels
- Increased NT release
Vice versa (decreased movement of basilar mem. and the tectorial mem. touches less IHCs)
Explain how and why we can hear extremely soft sounds that don’t produce enough mechanical movement for the tectorial membrane to touch the IHCs
The OHCs wil contract to allow the IHC to touch the tectorial membrane (depolarisation of OHC)
What occurs in the ear when a sound is too loud?
OHCs elongate pushing the tectorial membrane away from the IHCs preventing transduction of sound (hyper-polarisation of OHC)
Describe the auditory pathway
Spiral ganglions from each cochlea project via auditory vestibular nerve (VIII) to the ipsilateral cochlear nuclei.
Some then cross to the other side’s superior olive and some stay same side. Then pathway continues bilaterally through thalamus to auditory cortex
What frequencies do base nerve fibres transmit?
High
What frequencies do apex nerve fibres transmit?
Low
How is hearing organsied?
Tonotopically with different frequencies representing different areas
What is the human hearing range of frequency?
20 - 20,000 Hz
What is the amplitude range of human hearing?
0 - 120 dB (sound pressure level)
Which frequencies are most affected when age increases?
High frequencies
List hearing assessment procedures
Tuning fork Audiometry Central processing assessment Tympanometry Otoacustic emission Electrochochleography Evoked potentials
What are the aims of a hearing assessment?
Of what degree?
Of what type? (location and structures affected)
What are the 2 tests you can do with a tuning fork?
What are their differences?
Weber test (inner ear test): - fork put on top of head, vibrations into bone directly to endolymph
Rinne test (outer/middle hear test): - fork next to ear and behind
When are tuning forks used as an assessment?
To provide early and general info when audiometry is not possible
What is an audiometer?
Device used to produce sounds of various intensity and frequency to give an idea of the freq. range and minimum hearing for the patient
What is the normal hearing threshold on an audiogram?
0-20dB
How do you read an audiogram?
The closer the points are to the top, the softer the sounds a person can hear
What is a central processing assessment?
Assessment of hearing abilities other than detection e.g. sound localisation, filtered speech and speech in noise.
Verbal and non-verbal tests
If the patient can’t hear speech in noise what is the problem?
Processing of sound info not doing its job
What is tympanometry?
Examination used to test the condition of the middle ear: mobility of the tympanic membrane and conduction of ossicles by creating variations of air pressure in ear canal
What are OAEs and how is it used as an examination?
Otoacoustic emmisions which are low intensity sounds produced by the OHCs as the expand and contracts.
Test to screen and monitor hearing loss in newborns
What are auditory evoked potentials?
Examination which records nervous impulses from the ears to tell us if structures are working properly
List 3 different auditory evoked potentials
Electrocochleography
Auditory brainstem response (ABR)
Late responses
Explain electrochochleography
0.2-4.0 ms
electrical activity from cochlea and 8th nerve
evoked by clicks/tone burst
Explain auditory brainstem response (ABR) and patients it is good for
1.5-10 ms
Electrical activity from 8th nerve and brainstem nuclei and tracts
Evoked by clicks
Alterations in latency of waves can point to location of deficit
Doesn’t require attention from patient so good for babies and children
Explain late responses
80-500+ ms
electrical activity from the primary auditory and association cortex
Evoked by tone burst and oddball paradigm
What are the 3 types of hearing loss and describe where the problem lies for each type
Conductive hearing loss: outer and middle ear
Sensorineural hearing loss: inner ear/auditory nerve
Mixed hearing loss: Conduction and transduction of sound affected with more than one part of ear affected
What are the 5 degrees of hearing loss?
Normal hearing Mild hearing loss Moderate hearing loss Severe hearing loss Profound hearing loss
Name some causes of conductive hearing loss
Outer ear: wax, foreign body
Middle ear: otitis, otosclerosis
What is otitis?
Infection leading to inflammation of middle ear and presence of fluid buildup behind tympanic membrane
Name some causes of sensorineural hearing loss
Inner ear: presbycusis (loss of OHCs with age), ototoxicity (exposure to ototoxics)
8th nerve: nerve tumour…
What are the treatments for hearing loss?
Treat underlying cause
Hearing aids
Cochlear implants
Brianstem implants
How do hearing aids work?
Amplifies the sound, doesn’t replace any structure
Won’t hep patients who lack hair cells, they need cochlear implant
Can be custom made e.g. only amplifies high freq. if patient only has hearing loss in high freq.
How do cochlear implants work?
Replaces the function of the hair cells by receiving sound analysing it and transforming it to an electrical impulse to send directly to the auditory nerve.
Needs functional auditory nerve
How do brainstem implants work? And when are they used
Used when auditory nerve isn’t functioning properly
Instead the electrical signals can be sent to a set of electrodes placed into the brainstem.
Very risky operation and only advised for patients with bilateral auditory nerve damage
