Auditory Systems Flashcards
What is the ear divided into?
Three sections:
Outer
Middle
Inner
Where is the part of the ear responsible for balance located?
Inner
Why are the sections of the ear significant?
Each part has a specific function during the hearing process.
Where is the ear located?
The organ is embedded in the petrous portion of the temporal bone, the hardest bone in the body.
What forms the outer ear?
Pinna
External auditory canal
What are the functions of the outer ear?
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
What is involved in protecting the ear from threat?
Wax - toxic pH
Hairs
What comprises the middle ear?
Ossicles Oval window Tensor tympani muscle Tympanic membrane Stapedius muscle Auditory tube Round window Cochlea
What is the function of the middle ear?
Amplification
What are the names of the three ossicles?
Malleus
Incus
Stapes
How does the middle ear carry out amplification?
- 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 function of the inner ear?
Hearing - transduce vibration into nervous impulses
What is the hearing part of the inner ear?
Cochlea
What is the function of the cochlea?
transduce vibration into nervous impulses
also produces a frequency (or pitch) and intensity (or loudness) analysis of the sound
What are the three compartments of the cochlea?
Scala vestiboli
Scala Tympani
Scala Media
What are the features of the Scala vestibule and Scala tympani?
Bone structures
contain perilymph (high in sodium)
What are the features of the Scala media?
Membranous structure
contains endolymph (high in potassium)
Here is where the hearing organ or Organ of Corti is located
Where is the Organ of Corti found?
Basilar membrane
How is the basilar membrane organised?
tonotopically
using the same principle as a xylophone
How does the structure of the basilar membrane relate to its function?
sensitive to different frequencies at difference point along its length
high = base (narrow and tight)
low = apex (wide and loose)
What does the organ of corti contain?
thousands of hair cells: inner hair cells (IHC) and outer hair cells (OHC)
How are the hair cells in the organ of corti arranged?
IHC are arranged on one column and OHC on three
Where is the tectorial membrane located?
located above the hair cells
roof of the organ
What is the function of the the tectorial membrane?
Will allow the hair deflection, which in turn will depolarise the cell
Only OHC hair are in constant contact with the tectorial membrane
They bring the membrane closer or away to/from the IHC
What are the main features of IHC?
Carry 95% of the afferent information of the auditory nerve
Their function is the transduction of the sound into nerve impulses.
What are the main features of the OHC?
Carry 95% of efferents of the auditory nerve
Their function is modulation of the sensitivity of the response
Act as an active amplifier. Can bring membrane further away/closer to tectorial membrane deciding whether IHCs transmit sounds
What are the hairs of the hair cells called?
Stereocilia
What does the deflection of the stereocilia do?
Mechanically opens potassium channels causing depolarisation and releasing NTs
NT goes to afferent nerve and the impulse is transmitted to the CNS
How are louder sounds dealt with?
Bigger vibration of tympanic membrane
More vibration of the ossicles
More pressure of the endolymph
Bigger movement of the basilar membrane
Wider area of the cochlea is stimulated
Tectorial membrane will come into contact with the IHC
Greater deflection of stereocilia and K+ channel opening
What happens if a sound is too loud?
OHC elongate to push away the tectorial membrane from the hairs of the IHC not allowing the cell to transduce all the sound
Why are the OHC so important?
If the OHC do not work you would only be able to hear a narrow range of loudness
e.g. you can hear whispers
What happens if a sound is too quiet?
OHC contract to pull the tectorial membrane towards the IHC
Simply describe the auditory pathway
Cochlea
Cochlear nucleus
Superior olive
Crosses and becomes bilateral
Inferior colliculus
Medial geniculate body
Auditory cortex in temporal lobe
Outline the auditory pathway
Spiral ganglions from each cochlea project via auditory vestibular nerve (VIII) to the ipsilateral cochlear nuclei (monoaural neurons)
Auditory information crosses at the superior olive level
After this point all connections are bilateral
What is the function of the auditory cortex?
Recognising sound
Being conscious of sound
Comprehension of sound (language complexes)
How is hearing organised?
tonotopically
distribution of frequencies represented in different nerve fibre innervated the apex/base
continues up brainstem and into the cortex
frequencies need to be discriminated to allow processing of speech
What is frequency?
Cycles per second, perceived tone (Hz
What is amplitude?
Sound pressure, subjective attribute correlated with physical strength (dB)
What is the human range of hearing?
Frequency: 20–20,000Hz
Loudness: 0 dB to 120 dB sound pressure level (SPL)
Why is the dB scale used?
The decibel scale (a log scale) is useful because the range of sensitivity is very large
Why is a logarithmic scale used for dB?
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)
What happens to hearing with age?
Hearing acuity decreases with age, particularly higher frequencies.
Medium and low frequencies could be affected with the progression of a hearing loss.
How do you conduct a hearing assessment?
- Tunning fork
- Audiometry
- Central processing assessment
- Tympanometry
- Otoacustic Emission
- Electrocochleography
- Evoked potentials
What is the tuning fork used for?
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.
Tell you if the hearing loss is symmetric/asymmetric
What are the aims of a hearing assessment?
Is there a hearing loss? If yes…
- Of what degree?
- Of what type?
What is PTA?
Pure Tone Audiometry
What is Pure Tone Audiometry?
Typical test
Science of measuring hearing acuity for variations in sound intensity and frequency
audiometer is the device used to produce sound of varying intensity and frequency
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
What is central processing assessment?
Assessment of hearing abilities other than detection.
Verbal and non verbal tests
What are examples of tests used in a central processing assessment?
Sound localization
Filtered speech
Speech in noise
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.
What are Otoacoustic Emissions?
The normal cochlea produces low-intensity sounds called OAEs
These sounds are produced specifically by the outer hair cells as they expand and contract
If these sounds cannot be heard it is indicative of hearing loss
This test is often part of the newborn hearing screening and hearing loss monitoring (do not need cooperation of patient)
What are auditory evoked potentials?
Potentials generated by auditory stimuli
Record electrical changes to tell you whether structures are functioning
What is electrocochleopgraphy?
0.2-4.0 ms, electrical activity from the cochlea and eighth nerve. Evoked by clicks or tone burst.
latency relates to how far away the activity being detected is
What is auditory brainstem response?
1.5-10.0 ms, electrical activity from the eighth nerve and brainstem nuclei and tracts. Evoked by clicks.
What is late responses?
80-500+ ms, electrical activity from the primary auditory and association cortex. Evoked by tone burst and oddball paradigm.
What is ABR?
Auditory brainstem response
What is 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.
Used in babies and children
What are cortical potentials?
Cortical potentials could be affected on neurological conditions or processing problems.
What are the 3 types of hearing loss?
Conductive
Sensorineural
Mixed
What is conductive hearing loss?
Problem is located in outer or middle ear.
What is sensorineural hearing loss?
Problem is located in the inner ear or the auditory nerve.
Info is not transduce and taken to cortex
What is mixed hearing loss?
Conduction and transduction of sound are affected. Problem affects more than one part of the ear.
How can hearing loss be classified?
Mild
Moderate
Severe
Profound
What are the causes of conductive hearing loss?
Outer ear: Wax, foreign body
Middle ear: Otitis, Otoscerosis
What are causes of sensorineural hearing loss?
Inner ear: Prebycusis, Ototoxicity
Nerve: VIII nerve tumor
What are the treatments?
Treat underlying cause e.g. remove wax
Hearing aids
Cochlear implants
Brainstem implants
What are the main features of 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 the main features of 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 needs a functional auditory nerve to function.
What are the main features of brainstem implants?
When the auditory nerves are the affected structures, the electrical signals can be send to a set of electrodes placed directly into the brainstem.
What is the issue with brainstem implants?
It is very risky, then it is advised for people with bilateral important auditory nerve damage.
Not great results as the brainstem is not as frequency sensitive as the cochlea
