auditory systems Flashcards
what is the anatomy of the ear?
The ear is divided in three sections: outer, middle and inner ear.
Each part has a specific function during the hearing process.
The organ is embedded in the petrous portion of the temporal bone, the hardest bone in the body.
this protects it. it is a very delicate structure
what is the outer ear and its functions?
Pinna and external auditory canal/meatus
functions:
To capture sound an to focus it the tympanic membrane
To amplify some frequencies by resonance in the canal.
To protect the ear from external threats. - this is done with hairs and wax. the wax capture microbes and trap or kill them (pH). the hairs expel the wax
what is the middle ear and its functions?
tympanic membrane (ear drum), ossicles (malleus, incus, stapes), muscles (tensor tympani, stapedius), oval window
The main function of the middle ear is 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 inner ear and its functions?
cochlea (hearing), semicircular canals
hearing functions:
Its function is to transduce vibration into nervous impulses
While doing so, it also produces a frequency (or pitch) and intensity (or loudness) analysis of the sound
what is the cochlea?
The cochlea contains 3 compartments:
- Scala vestibuli and 2. scala tympani:
Bone structures, contain perilymph (high in sodium)
(where the hair cells are) - Scala media: Membranous structure, contains endolymph (high in potassium). Here is where the hearing organ or Organ of Corti is located.
(where the stereocilia of the hair cells are)
what is the basilar membrane of the cochlea?
The structure where the organ of Corti lies in is the basilar membrane
Basilar membrane is arranged tonotopically, using the same principle as a xylophone
base: narrow and tight, high frequency, high pitched sounds
apex: wide and loose, low frequency, low pitched sounds
This means it is sensitive to different frequencies at difference point along its length
what is the organ of corti/hearing organ?
The organ of Corti contains thousands of hair cells: inner hair cells (IHC) and outer hair cells (OHC)
(cillia)
IHC are arranged on one column and OHC on three
The tectorial membrane is located above the hair cells and will allow the hair deflection, which in turn will depolarise the cell. Only OHC hair are in constant contact with the tectorial membrane, and these assist the contact with the IHC
what are the hair cells of the organ of corti?
IHC carry 95% of the afferent information of the auditory nerve. Their function is the transduction of the sound into nerve impulses
OHC carry 95% of efferents of the auditory nerve. Their function is modulation of the sensitivity of the response. (they act as an active amplifier, they can contract, to make the tectorial membrane closer or farther from the IHC, changing the afferents) (closer means more transduction)
(there are 3 columns of outer, and one column of inner)
how does trandsuction work?
The hairs of the hair cells are called stereocilia.
The deflection of the stereocilia towards the longest cilium will open K+ channels
The ionic interchange depolarises the cell, which leads to calcium influx, and the neurotransmitter (glutamate, or in its family) is liberated.
Higher amplitudes (louder) of sound will cause greater deflection of stereocilia and K+ channel opening which leads to calcium influx into the cells, and release o
endolymph is high in potassium, so acts as a store
for quiet sounds, the basement membrane will vibrate less, so the tectorial membrane will be in contact with fewer hair cells, so sound is transduced less
what happens with the tectorial membrane during depolarisation and re-polarisation?
Depolarisation:
as the basilar membrane moves up (upwards phase), this pushes the hair cells up so they come into contact with the tectorial membrane, which deflects the hairs in the direction of the longest hair
causes potassium influx and depolarisation
re-polarisation:
basilar membrane moves down (downward phase).
hairs still in contact with tectorial membrane are pushed the wrong way, and potassium channels close
how do the outer hair cells help in transduction?
active amplifying
if the amplitude of a sound doesnt move the basement membrane sufficiently upwards for the IHCs to be displaced by the tectorial membrane
the OHCs can contract to bring it closer
if the sound is too loud, the OHCs can elongate to push the tectorial membrane away from the cilia of the IHCs
OHCs are the ones that get damaged more commonly so these abilities are lost
what are the auditory pathways?
Spiral ganglions from each cochlea project via vestibulocochlear nerve (VIII) to the ipsilateral cochlear nuclei (monoaural neurons)
Auditory information crosses at the superior olive level (in brainstem)
After this point all connections are bilateral
(both ears impulses travel on both sides)
CNVIII -> cochlear nucleus -> superior olive (more decussate, some stay) -> inferior colliculus -> medial genticulate body -> auditory cortex in temporal lobe
(first 4 are brainstem, 5 is thalamus, 6 is cerebral cortex)
how is hearing organised?
Hearing is tonotopically organised
by frequency, only basal cells will transmit high frequencies, only apical will transmit low frequencies (this is very necessary for language)
in primary and secondary auditory cortices in temporal lobe
how does sound work?
longitudinal wave
Frequency/pitch (Hz): Cycles per second, perceived tone.
Amplitude/loudness (dB): Sound pressure, subjective attribute correlated with physical strength
Human range of hearing:
Frequency: 20–20,000Hz
Loudness: 0 dB to 120 dB (can hear wider but its uncomfortable)sound pressure level (SPL)
(although we do start to lose the top end progressively) (150-4000Hz is the main range used in speech)
what is sound amplitude?
The decibel scale (a log scale) is useful because the range of sensitivity is very large.
This allows us to compress the scale on a graph, and reflect the fact that many physiological processes are non-linear (i.e. they can respond to both very low and very high values)
(conversational speech is about 60 dB, loo flushing is 70, stadium crowd is about 130, shotgun is 170)
what happens to hearing acuity with age?
Hearing acuity decreases with age
particularly higher frequencies.
Medium and low frequencies could be affected with the progression of a hearing loss.
what is a hearing assessment, and what happens in it?
Aims of the assessment?
To answer these questions:
- Is there a hearing loss? If yes…
- Of what degree?
- Of what type?
Procedures:
- Tunning fork
- Audiometry
- Central processing assessment
- Tympanometry
- Otoacustic Emission
- Electrocochleography
- Evoked potentials
what is the tuning fork method of hearing testing?
not really used now days
Used to establish the probable presence or absence of a hearing loss with a significant conductive component.
They are used to provide early and general information, when audiometry is not available or possible.
weber test, rinne test
weber: symmetric or asymmetric
weber: tuning fork on skull bone. vibes go directly to hairs of cochlea, stimulates endolymph internally
what is the pure tone audiometry method of hearing testing?
Science of measuring hearing acuity for variations in sound intensity and frequency
An audiometer is the device used to produce sound of varying intensity and frequency
patient raises hand when they hear sounds of different frequency and amplitudes
what is an audiogram?
The audiogram is where the hearing thresholds are plotted to define if there is a hearing loss or not. A normal hearing threshold is located between 0 – 20dB
in normal hearing, the line will be fairly level
if hearing loss, the line will be downhill (high freq go first
what is a central processing assessment?
Assessment of hearing abilities other than detection.
Verbal and non verbal tests
Examples:
Sound localization
Filtered speech
Speech in noise
if abnormal, there is a problem with sound processing
what is tympanometry?
Examination used to test the condition of the middle ear and mobility of the eardrum (tympanic membrane) and the conduction bones by creating variations of air pressure in the ear canal.
anything below normal means there is some sort of resistance
Most common results: A – normal C – negative middle ear pressure B Middle ear effusion Perforation of tympanic membrane Eustachian (pharyngotympanic) tube dysfunction Occluded ear canal
what are otoacoustic emissions (OAEs)?
The normal cochlea produces low-intensity sounds called OAEs
These sounds are produced specifically by the outer hair cells as they expand and contract
This test is often part of the newborn hearing screening and hearing loss monitoring.
test will be negative if their hair cells arent working, so theyll have a hearing deficit
what are auditory evoked potentials?
Electrocochleography
– 0.2-4.0 ms, electrical activity from the cochlea and eighth nerve. Evoked by clicks or tone burst.
Auditory Brainstem Response (ABR)
– 1.5-10.0 ms, electrical activity from the eighth nerve and brainstem nuclei and tracts. Evoked by clicks.
Late Responses (N1-P2, P300, MMN, and more) – 80-500+ ms, electrical activity from the primary auditory and association cortex. Evoked by tone burst and oddball paradigm.
what is the auditory brainstem response?
Electrical responses from the auditory pathway
ABR is more commonly used in clinic.
Does not require attention from the patient.
Alterations in the latency of waves can point to the location of the deficit.
Objective measurement commonly used in babies and children.
what are cortical potentials?
not commonly used in clinics
Cortical potentials could be affected on neurological conditions or processing problems.
what are types of hearing loss?
Conductive hearing loss: Problem is located in outer or middle ear. (sound isnt getting to the inner ear)
Sensorineural hearing loss: Problem is located in the inner ear or the auditory nerve. (hair cells or nerve isnt working)
Mixed hearing loss: Conduction and transduction of sound are affected. Problem affects more than one part of the ear.
what does audiometry look like in different types of hearing loss?
conductive:
will be two distinct lines depending on where the sound is coming from (inner ear hair cells, vs through air)
sensorineural:
lines are on top of eachother, so problem is not in conductance of sound.
the info isnt being transduced probably
mixed:
looks more random, two different lines
how are degrees of hearing loss graded?
Depending on where the hearing thresholds are located, hearing loss can be classified as mild, moderate, severe or profound.
on audiometry
done by decibels/volume
what are causes of conductive hearing loss?
conductive: cerumen impaction (tympanic membrane dysruption)
outer ear -
wax
foreign body
middle ear -
otitis
otosclerosis
what are causes of sensorineural hearing loss?
inner ear -
presbycusis (Progressive hearing loss due to
age)
ototoxisity (eg. drugs)
nerve-
VIII nerve tumour
how is hearing loss treated?
underlying cause (eg. wax removal)
hearing aids
cochlear implants
brainstem implants
what are 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.
what are cochlear implants?
The cochlear implant replaces the function of the hair cells by receiving sound, analysing it, transform it into electrical signals and sending an electric impulse directly to the auditory nerve
it inserts into the cochlea
It needs a functional auditory nerve to function.
helps coz lots of hearing loss is caused by hair cell dysfunction
what is a brainstem implant?
When the auditory nerves are the affected structures, the electrical signals can be send to a set of electrodes placed directly into the brainstem.
It is very risky, then it is advised for people with bilateral important auditory nerve damage.
and doesnt necessarily give great results
