Hearing Flashcards
What is sound?
It is a compressive wave which travels at 343m/sec in air
It has 2 properties:
Frequency (Hertz(Hz))
Volume (Decibels(dB))
What is the human auditory range?
How does it change with age?
20Hz to 20000Hz
With age, normal loss of the upper range of frequency
Describe the cochlea.
It is a one dimensional (linear) sense-organ
It consists of 3 layers:
Scala vestibule
The basilar membrane
Scala tympani
How does the organ of Corti sense sound?
It has hair cells that detect movements and convert it to signals
These cells are covered by a tectorial membrane
What is the travelling wave theory?
von Bekesy showed that the basilar membrane resonates and so mechanically amplifies sound with progressively lower frequencies along the length of the cochlea
So the lower the frequency the further along the cochlea it is sensed
So the place at which a sound is sensed is determinate on its frequency.
This is tonotopy
How are the hair cells within a cochlea arranged?
Inner hair cells - 1 row
Sense sound
Outer cells - 3 rows
Serve as amplifiers
They are mechanically tuned, by their position along the cochlear
And
Electrically tuned by expression of particular ion channels
How do the stereocilia on hair cells work to interpret sound.
The stereocilia are of differing heights and have potassium channels on them connected to each other by protein chains
If the stereocilia are displaced:
If pushed towards the smaller ‘hair’ then the protein chains become shorter and close the K+ channels
If pushed towards the larger ‘hair’ then the protein chains become longer and open the channels
Because the environment of the endolymph (ECF around hair cells) has a high conc. of K+ and the ICF is low in K+ so the influx of K+ depolarises the cell and causes neurotransmission to the spiral ganglion neurite.
Tell me about the spiral ganglion neurons.
45,000 SGN (afferent axons 8th nerve
SGN types:
- Type 1 (95%) inner hair cell (IHC)
- Type 2 (5%) outer hair cell (OHC)
What do the outer hair cells do?
They are supplied by efferent feedback from the olivocochlear bundle (OCB) within the olivocochlear system
They serve as cochlear amplifiers
They receive a signal and create a force by changing in length to help amplify the sound for the IHC
How is sound encoded to account for volume?
At a low level of sound not many action potentials fired, at higher levels more fired and at even higher levels, surrounding IHC fire action potentials as well
Where does the signal travel from the cochlea?
Via CN VIII, to the cochlea nucleus
Which travels to the medial nucleus of the trapezoid body (MNTB) on the opposite side
Which then has links to the lateral and medial superior olives (LSO/MSO)
What is the auditory pathway?
Cochlea Spiral ganglion nucleus Cochlear nucleus Superior olivary complex Inferior colliculus Medial geniculate nucleus Auditory cortex
What can cause hearing impairments?
Loud noise Congenital defects (inherited) Infections (e.g. rubella, glue ear) Ototoxic comp. (aminoglycosides) Trauma (damage to temporal bone) Age
How can you assess haring?
Visual inspection (otoscope)
Audiograms: plot sensitivity against frequency
- Provides an overall measure of hearing sensitivity
Otoacoustic emissions (OAE) This is the sound generated by OHC Tests for amplifier function
Auditory brainstem response (ABR)
Also used to general hearing function in babies
What are the different sites of hearing loss?
Conductive hearing loss
- Blockage, ruptured eardrum
- Fluid accumulation (otitis media)
- Otosclerosis (progressive oscicle immobilisation)
Sensory loss
- Hair cell destruction (physical, noise related)
- Hair cell death (ototoxic; e.g. kanamycin)
Neural hearing loss
- Spiral ganglion damage (e.g. acoustic neuroma)
- Age-related hearing loss (ARHL, possible links with
dementia)
- Tinnitus - Phantom sound (associated with hearing loss)
- Auditory neuropathy (with hyperbilirubinaemia,
neonatal jaundice)
- Monaural deafness - destroys ability to localise sound
