the ear and sound detection Flashcards
what is sound?
the displacement of air that creates regions of compressed air (peaks) and rarefied air (troughs)
what is the speed (velocity) of sound at room temperature?
343 m/s
calculate velocity
frequency x wavelength
what is frequency
number of peaks per second (in Hz) (number of compressed areas per second)
what is a wavelength
distance between successive peaks (in m)
relationship between frequency and wavelength
lower frequency= longer wavelengths
higher frequency = shorter wavelengths
intensity
higher displacement of air but the same frequency
outer ear
pinna, auditory canal and tympanic membrane
middle ear
malleus, incus, stapes and oval window
inner ear
cochlear
sound pathway from the pinnae to the oval window
1) sound waves travel through air down the auditory canal, displacing the tympanic membrane
2) tympanic membrane pushes on the handle of the malleus
3) the malleus acts as a lever with a fulcrum point that is nearer to the handle, as the handle is displaces to the right, the head is displaced to the left
4) malleus passes the movement to the incus, the incus to the stapes, and the stapes to the oval window
what does the lever effect have on the oval window?
pressure at the oval window is increased 20x
how many muscles are there in the inner ear? what are they attached to?
two muscles, one attached to the malleus and one attached to the stapes bone
what happens when muscles in connected to the malleus and stapes contract?
they stiffen the ossicles and dampen the intensity that enters the inner ear via the oval window
intensity and frequency the attenuation reflex is needed at
High intensity (>60dB) Low frequency (<2,000Hz)
what is the attenuation reflex?
feedback mechanism to dampen loud sounds and protect hair cells in the inner ear
pathway when the inner hair cells are being put under too much mechanical pressure
inner hair cells
project to the posterior ventral cochlear nucleus (PVCN)
projects to the superior olivary complex
1)
projects to the facial motor nucleus (cranial nerve 7)
contracts the stapedius muscle
2)
projects to the NLL
projects to the motor 5 nucleus (cranial nerve 5)
contracts the muscle attached to the malleus
pathway of internally generated sounds
motor V nucleus
cranial nerve 5
contracts muscle attached to the malleus
what three chambers are found in the cochlear
scala vestibuli
scala media
scala tympani
which chambers is perilymph found?
scala vestibuli
scala tympani
which chambers is endolymph found?
scala media
endolymph composition
1mM Na+ low
150mM K+ high
why does the endolymph have high K+ and low Na+?
due to ion transport by stria vascularis cells
why is the composition of endolymph in the scala media important?
high level of potassium outside of hair cells allows it to move down the concentration gradient into the cell
perilymph composition
low K+ and high Na+
like extracellular fluid
where is the organ of corti located
on the basilar membrane, between the scala media and scala tympani
where is the basilar membrane?
between scala tympani and scala media
where is reissner’s membrane?
between scala vestibuli and scala media
where is the stria vascularis
in the scala media
what happens when the stapes taps on the oval window?
it causes displacement of the perilymph
what chambers are continuous with each other?
scala vestibuli and scala tympani
what does vibration of the basilar membrane determine?
how the hair cells in the organ of corti are going to be bent, determining if they open
where will high frequency sound vibrate the basilar membrane?
closer to the base, and will quickly be dampened
where will low frequency sounds vibrate the basilar membrane?
allow the basilar membrane to vibrate to the apex
structural features of the basilar membrane
thicker and stiffer at the base
thinner and floppier at the apex
where are hair cells found?
the organ of corti
frequency and the basilar membrane
each frequency causes a maximal displacement at a particular region along the membrane
outer hair cells structure
have their stereocilia buried in the reticular lamina
inner hair cells structure
have free stereocilia that sit below the tectorial membrane
mechanical displacement of hair cells
- When basilar membrane is deflected up the stereocilia bend outward
- Deflection of the stereocilia in one direction increases receptor potential (depolarisation)
- Deflection of the stereocilia in the opposite direction decreases receptor potential (hyperpolarisation)
depolarisation of inner hair cell
- The tips of the stereocilia have mechanically gated TRPA1 ion channels
- When the stereocilia bend these open
- Endolymph has high K+ so potassium floods into the hair cell
- This depolarises the cell and opens voltage gated Ca2+ channels
- Calcium influx
- Neurotransmitter release
- Activate spiral ganglion neuron
movement of potassium at resting inner hair cell
leaky movement into hair cells
channel inner hair cells
TRPA1
inner hair cells to spiral ganglion
one inner hair cell to many spiral ganglion cells
well innervated
outer hair cells to spiral ganglion cells
many outer hair cells to one spiral ganglion cell
poorly innervated
what intensity activates more spiral ganglion cells
higher intensity
spiral ganglion cells with high spontaneous activity require…
lower frequency to up the signal to threshold for them to respond
spiral ganglion cells that have low spontaneous activity require
a higher frequency to reach threshold for them to respond
high spontaneous rate = x threshold
low
low spontaneous rate= x threshold
high
high spontaneous rate SGNs fire from
0dB up to ~20dB saturation
medium spontaneous rate fire from
~20dB to ~40dB saturation
low spontaneous rate fire from
~40dB to ~80dB
what frequencies are coded by low, mid and high spontaneous rate spiral ganglion cells? what organisation is this?
4000Hz and above (high frequency)
tonotopic organisation
what happens as you increase frequency
basilar membrane vibration will increase to reach the particular area
how do spiral ganglion cells respond below 4000Hz
they phase lock their firing to frequency instead of the intensity
what is volume coded by >4000Hz
SGN firing rate and number of activated SGNs
what cells act as a cochlear amplifier?
outer hair cells
what protein allows outer hair cells to elongate or compress?
the motor protein prestin
what happens when an outer hair cell is activated?
K+ influx activates motor proteins that compress the cell
this amplifies the basilar membrane movement and leads to an increased bending of the inner hair cell stereocilia
how much do outer hair cells amplify the basilar membrane movement
x100
when is the medial olivocochlear reflex inhibition used? why
High intensity, high frequency (>2,000Hz)
Loud high frequency noise activates reflex descending (effector) circuits to dampen sound input to protect the hair cells from mechanical damage
medial olivocochlear reflex mechanism/ pathway
inner hair cells
project to posterior ventral cochlear nucleus (PVCN)
project to ventral nucleus of the trapezoid body’s (VNTB) medial olivocochlear system (MOCS)
cranial nerve 8 projects to the outer hair cells
acetylcholine hyperpolarises OCHs to relax prestin, elongating the cells and pushing the basilar membrane down
result of elongation of outer hair cells
dampens the outer hair cell amplifier causing less displacement of the inner hair cell stereocilia
conductive hearing loss
- Defect in outer or middle ear
- Checked using Rinne’s test
- Each ear is tested separately, check if there is conductive impairment in each ear
sensorineural hearing loss
- Defect in inner ear (cochlear or auditory nerve)
- Checked using Weber’s test
- Comparison between ears (lateralisation test)- can detect if one ear is more impaired than the other
Rinne test
place the base of struck tuning fork on the mastoid bone
have patient indicate when the sound is no longer heard
now move the fork beside the ear an ask if it is audible
conclusion of rinne test
if patient can hear the fork at their ear they have normal hearing
if the patient cannot hear the fork at their ear they have conductive hearing loss
weber test
place base of struck tuning fork on the bridge of forehead, nose or teeth
results of weber test
normal: no lateralisation
unilateral conductive loss: lateralisation to affected side
unilateral sensorineural loss: lateralisation to normal or better hearing side
audiogram shows
the hearing level for each ear at various levels
