Auditory System Flashcards
Outer ear
To capture sound and to focus it the tympanic membrane.
To amplify some frequencies by resonance in the canal.
To protect the ear from external threats.
Middle ear
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
Inner ear
The hearing part of the inner ear is the cochlea
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
Cochlea
The cochlea contains 3 compartments:
Scala vestibuli and scala tympani: Bone structures, contain perilymph (high in sodium)
Scala media: Membranous structure, contains endolymph (high in potassium). Here is where the hearing organ or Organ of Corti is located.
Basilar membrane
The structure where the organ of Corti lies in
Basilar membrane is arranged tonotopically, using the same principle as a xylophone
This means it is sensitive to different frequencies at difference point along its length
The organ of Corti
contains thousands of hair cells: inner hair cells (IHC) and outer hair cells (OHC)
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.
Hair cells
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.
Transduction
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 and the neurotransmitter is liberated.
Higher amplitudes (louder) of sound will cause greater deflection of stereocilia and K+ channel opening
Auditory pathways
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
Travels from brainstem to thalamus to medial geniculate body
Cerebral hemisphere
Frequency/pitch
Cycles per second, perceived tone.
Amplitude/loudness
Sound pressure, subjective attribute correlated with physical strength.
Hearing assessment
- Tunning fork
- Audiometry
- Central processing assessment
- Tympanometry
- Otoacustic Emission
- Electrocochleography
- Evoked potentials
Tunning fork
Used to establish the probable presence or absence of a hearing loss with a significant conductive component.
Weber - on middle of skull
Rinne - in front of ears
Conductive problem - in bad ear, BC>AC
Sensorineural problem - in good ear, AC>BC (false positive)
Pure Tone Audiometry (PTA)
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
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
Central processing assessment
Assessment of hearing abilities other than detection.
Verbal and non verbal tests
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.
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.
Auditory evoked potentials
Electrocochleography
Auditory Brainstem Response (ABR)
Late Responses (N1-P2, P300, MMN, and more)
Auditory Brainstem Response (ABR)
Electrical responses from the auditory pathway
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.
Cortical potentials
Cortical potentials could be affected on neurological conditions or processing problems.
Types of hearing loss
Conductive hearing loss: Problem is located in outer or middle ear.
Sensorineural hearing loss: Problem is located in the inner ear or the auditory nerve.
Mixed hearing loss: Conduction and transduction of sound are affected. Problem affects more than one part of the ear.
Degree of hearing loss
20-40dB mild
40-70dB moderate
70-90dB severe
90-120dB profound
Causes of hearing loss - conductive
Outer Ear - wax, foreign body
Middle ear - otitis, oterosclerosis
Causes of hearing loss - sensorineural
Inner ear - presbycusis, ototoxicity
Nerve - VIII nerve tumour
Treatment
Hearing aids
Cochlear implants
Brainstem implants
Hearing aids
Hearing aids amplify the sound, does not replace any structure.
Cochlear implant
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.
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.
