1
Q
What is sound ?
A
a displacement of air that creates regions of compressed air (peaks) and rarefied air (troughs)
2
Q
pitch
A
sound frequency
3
Q
volume
A
sound intensity
4
Q
speed of sound at room temperature
A
343m/s
5
Q
velocity
A
frequency x wavelength
6
Q
frequency
A
number of peaks per second ( in Hertz)
7
Q
wavelength
A
distance between successive peaks (in meters)
8
Q
low frequency =
A
longer wavelength
(e.g. 1Hz = 343m )
9
Q
higher frequency =
A
shorter wavelength
(e.g. 343Hz = 1m)
10
Q
volume
A
measured in decibels
11
Q
what is sound detected by
A
the ear
12
Q
structure of the ear: outer ear
A
pinna, auditory canal
13
Q
structure of the ear: middle ear
A
tympanic membrane, ossicles
- maleus
- stapes
- incus
14
Q
structure of the ear: inner ear
A
- oval window
- cochlea
- auditory-vestibular nerve
15
Q
structure and functions of the ear
A
- sound travels better in air than in fluid
- inner ear (cochlea) is filled with fluid
- the ear bones (ossicles) in the middle ear amplify sound intensity at the oval window to compensate for this air -to-fluid dilution of sound
16
Q
function of middle ear: lever effect
A
- when air displaces the tympanic membrane it pushes on the handle of the malleus
- malleus acts as lever with a fulcrum point that is nearer to the handle
- as it vibrates the handle is displaced and the head moves
- malleus passes this movement to incus, incus to stapes
- due to lever effect pressure at oval window increased x20
17
Q
muscles in the middle ear
A
- tensor tympani muscle
- stapedius muscle
18
Q
tensor tympani muscle connection
A
malleus
19
Q
stapedius muscle connection
A
stapes
20
Q
function of muscles in middle ear
A
when the tensor tympani and stapedius muscle contract they stiffen the ossicles and dampen the sound intensity that enters the ear via the oval window
21
Q
attenuation reflex
A
- contraction of muscles in middle ear reduce sound intensity that is transmitted to inner ear
22
Q
descending effector reflex circuits
A
dampen loud low frequency sound input to protect hair cells from mechanical damage
(from >60dB to 20dB)
23
Q
what happens when tensor tympan contracts
A
contracts to pull on malleus and tighten the tympanic membrane
24
Q
what happens when stapedius muscle contracts
A
contracts to dampen stapes movement to reduce displacement of oval window
25
structure of inner ear
- cochlea consists of three fluid filled chambers separated by membranes
- scala media, scala tympani, scala vastibuli
26
scala media
contains endolymph
- high potassium low so sodium due to ion transportation by stria vascularis cells
27
endolymph in scala media
fluid enters hair cells when they open
28
scala vestibuli / scala tympani
contains perilymph
- low potassium high sodium
29
organ of corti
sits on basilar membrane between scala media and scala tympani
30
stria vascularis
- in scala media
- group of cells that pump potassium into scala media and take up sodium
31
structure of cochlea
- basilar membrane increases in width from base to apex
- scala media is sealed off at apex
32
function of cochlea
- stapes taps on oval window
- when sound enters via oval window is causes perilymph to generate waves
- this causes deflection of basilar membrane
33
high vs. low frequency sounds in fluid
low sounds travel better, further down the basilar membrane towards the apex / tip of cochlea
34
the basilar membrane + tonotopic organisation
- basilar membrane is narrower and stiffer at the base and wider and floppier at the apex
- high frequency sounds displace the stiffer BM base and are dampened
- low freq. sounds travel further and displace BM near apex
- each freq. causes maximal displacement at a particular region along the membrane
35
in the organ corti
- contains hair cells that have stereocilia at their tip
- these are arranged into outer hair cells and inner hair cells
36
hair cells in the organ of corti
- outer hair cells have their sterocilia buried in the reticular lamina
- inner hair cells have free stereocilia that sit below the tectorial membrane
37
hair cell innervation
- cells carry signal to sensory neurons whose cell bodies are in the spiral ganglion
- spiral ganglion neurons form auditory nerve that passes information to brain
38
mechanical displacement of hair cells
- when basilar membrane is deflected up the stereocilia bend outward
- deflection of stereocilia in one direction increases receptor potential (depolarisation)
- in the opposite direction decreases receptor potential (hyperpolarisation)
39
hair cells in resting state
- still movement of potassium, but signal can be increased or decreased depending on which way the hair cells are bent
40
mechanically gated channels in hair cells
- tips of stereocilia have mechanically gated TRPA1 ion channels
- when stereocilia bend channels open
- endolymph has high postassium so potassium floods into hair cells
- depolarizes cell and opens voltage-gated Ca2+ channels
- calcium influx = neurotransmitter release = activates spiral ganglion neuron
41
what are the channels in stereocilia
TRPA1 ion channels
42
Spiral ganglion cell structure
- outer cells have poor innervation by spiral ganglion cells as not as many connected compared to inner hair cells which can activate multiple SGCs
- higher intensity sound activates more SGCs
43
coding of sound intensity in SGNs in inner hair cells
- High spontaneous rate (low threshold)
- medium spontaneous rate (mid threshold)
- low spontaneous rate (high threshold
44
High spontaneous rate SGNs
fire from ~0dB to ~20dB
45
medium spontaneous rate
fire from ~20dB to ~40dB
46
low spontaneous rate
fire from ~40dB to ~80dB
47
place theory of hearing
- pitch coded by hair cell /SGN position (tonotopy)
- volume coded by SGN firing rate and number of activated SGNs
48
coding of sound intensity for high frequencies >4,000 Hz or 4kHz
- increasing intensity (volume) leads to increased firing rate - up to threshold
- further increase in intensity activity activates higher threshold SGNs
49
the cochlear amplifier
- outer hair cells are attached to the basilar membrane and reticular limina at both ends
- have motor proteins that allows the to elongate or compress
- when cells are activated by potassium influx it activates motor proteins that compress the cell.
- this amplifies basilar membrane movement (x100) and leads to increased bending of inner hair cell stereocilia
50
motor protein in outer hair cells
prestin
51
medial olivocochlear reflex inhibition is for what
high intensity sounds at high frequencies
52
what is medial olivocochlear reflex inhibition
- loud high frequency noise activates reflex descending (effector) circuits to dampen sound input to protect hair cells from mechanical damage
53
medial olivocochlear reflex inhibition mechanism
- from inner hair cells
- when they detect sounds they send it to posterior ventral cochlear nucleus
- neurons send this to medial olivocochlear system
- these then signal to outer hair cells to release acetylcholine
- this hyperpolarises them to relax prestin to elongate them/ stop them from compressing and getting damaged
- pushing tectorial membrane away from basilar membrane so that loud sound cannot bend inner hair cells as much
54
medial olivocochlear reflex inhibition mechanism what does it do
dampens outer hair cell amplifier to reduce gain causing less displacement of inner hair cell stereocilia
55
conductive hearing loss
- defect in outer or middle ear
- excessive wax or bones in middle of ear become separated to cannot tap effectively
56
how to test for conductive hearing loss
Rinne's test
- each ear tested separately
57
sensorineural hearing loss
- defect in inner ear
- cochlea or auditory nerve damage
58
how to test for sensorineural hearing loss
- Weber's test (Rinne's test first)
- comparison between ears (lateralisation test)
- can detect if one ear is more impaired than the other
59
audiogram
shows the hearing level for each ear at various frequencies
(for hearing loss)
60
BIOL21341/ Sensory Systems
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