L23 Hearing And The Ear Flashcards
Define sound
Longitudinal pressure waves travelling through air or other medium
What is frequency?
Pitch of sound
- measured in Hertz = cycles per second
What is amplitude?
Intensity - loudness
- measured in decibels
- log scale to better match perception
What is the equation for frequency?
Freq = 1/t
Units = hertz/s-1
t = period/s
What is the middle ear?
It is air filled space
- needs equal pressure either side of the tympanic membrane otherwise hearing is impaired
Why do we need a middle ear?
- sound waves travelling from air to water are mostly reflected
- inner ear is fluid filled
- without middle ear there’s a poor transfer of energy from air to cochlear fluid
What can the middle ear overcome and how?
Overcomes air-fluid mismatch
- ossicles act as levers
- area of tympanic membrane > footplate of the stapes
How many hair cells do human ears have?
- one row of inner hair cells
- three rows of outer hair cells
What are at the tips of hair cells?
Small processes called stereocilia
- bend in response to pressure changes
- stereocilia of the tallest row of outer hair cells are embedded in the tectorial membrane
In which direction ones the travelling wave move?
Moves from the base to the apex
Base - stiff - high frequencies
Apex - floppy - low frequencies
What happens when sound enters the cochlea?
- the basilar membrane vibrates
- travelling waves travel up the basilar membrane
- position of maximal displacement depends on frequency of sound
- low freq produce maximal displacement at the apex of cochlea
- high freq produce maximal displacement at the base
- displacement of basilar membrane excites hair cells
What does the excitation of inner hair cells lead to?
Leads to transmitter release and excitation of auditory nerve fibres
What happens when stereocilia bend towards to longest one?
Entry of K+ leads to depolarisation, Ca2+ entry and transmitter release
-> transmitters activate nerve fibres
What happens when stereocilia bend towards the shortest one?
No K+ entry leads to:
Hyperpolarisation
No transmitter
Nerves stop firing
How are inner and outer hair cells activated?
- movement of basilar membrane + interaction with tectorial membrane
- bending towards tallest stereocilium
- bending towards shortest stereocilium
How are only inner hair cells activated?
- release of neurotransmitters
- activate cochlea nerve fibres associated with the hair cell
How does the cochlea filter by frequency?
- Each neuron responds best to one frequency
- Each location along the cochlea amplifies one frequency
How can outer hair cells be damaged?
Damaged by ototoxic drugs
- some antibiotics
How do outer hair cells act as cellular motors?
- change length as stereocilia bend back and forth
- shorten and lengthen in response to sound stimulation
- driven by motor protein in OHC membrane - Preston
- inject energy and amplify basilar membrane movement
- the reason hearing is so sensitive - detect BM movements in nm range
What increases the displacement of basilar membranes?
The elongation and shortening of outer hair cells
- enhances hearing sensitivity and frequency selectivity
At what sound pressure level is the threshold for hearing damage?
90dB
What is the main function of the pinna?
Localisation of sound
How are inner hair cells involved in the excitation of auditory nerves?
They release neurotransmitters as stereocilia bend towards the tallest
Which structure of the cochlea changes shape due to the Preston protein and increases sound sensitivity?
Outer hair cells
- outer hair cells increase in length due to stereocilia bending
- this injects energy into the basilar membrane so it can vibrate more
- increases sensitivity to sound
The stapedius muscle connects to which cranial nerve?
Stapes (CN VII)
What is the role of the stapedius muscle?
It dampens the amplitude of the sound wave
The tensor tympani muscle connects to which cranial nerve?
Malleus (CN V)
