Auditory system Flashcards
The human auditory system can distinguish between these 2 properties of sound
Frequency and amplitude
Describe the rate of response of the auditory system (fast/slow)
Fast
What is the general purpose of the auditory system for survival?
Orient the head and the body towards novel stimuli as an “early warning system”
What is sound?
A wave of mechanical energy produced by moving air molecules
What is upper limit of the range of human auditory detection in volume level? (decibels)
Just over 120-140 decibels
Describe wave frequency
The number of cycles in a second
Musical tone is made up of…
One fundamental frequency or pitch, but it is not perfect
An octave increase corresponds to this increase in frequency
2-fold increase
Describe the characteristic frequency of background noise
No characteristic frequency
What frequencies of sound can human ears detect?
20-20,000 Hz
Sound above the frequency of human detection is called…
Ultrasound
Sound below the frequency of human detection is called…
Infrasound
As we age, how does sound perception tend to change?
Lose sensitivity to higher frequencies
What are 3 examples of animals which can hear sounds higher than humans?
Dogs, Dolphins, mice
If an animal has a lower range of sounds for communications, what would this be useful for?
Communicating over long distances
Auditory neurons are derived from this sensory placode
Otic placode
Which germ layer do hair cells originate from?
Ectoderm
What structures make up the outer ear?
The pinna, auditory canal, tympanic membrane
Which structures make up the middle ear?
Ossicles and oval window
Which of the ossicles is connected to the tympanic membrane?
Malleus
What is the order of the ossicles starting at the tympanic membrane?
Malleus > incus > stapes
The middle ear is filled with…
Air
The inner ear (cochlea) is filled with…
Fluid
What is the name of the tube which connects the middle ear to the nasal cavities?
Eustachian tube
Describe how middle ear infection can cause conductive hearing loss
Accumulation of scar tissue from infection can immobilize the bones
Is conductive hearing loss permanent?
No, it can often be repaired
Describe Rinne’s test
Activate a tuning fork, hold next to ear and then nearby ear with tuning fork on a bony surface (skull)
If you can only hear when the fork is directly against the skull, the person has conductive hearing loss
Hearing aids would be beneficial to someone who…
Still has functional hair cells, just fewer of them
Cochlear implants would be beneficial to someone who…
Has a hair cell defect
What structures are in the inner ear?
Cochlea
What are the 2 key roles of the inner ear?
- Deconstruct complex sounds into constituent sounds
- Confines the action of each component tone to a discrete segment of the cochlea (tonotopic map)
The apex of the cochlea is called…
Helicotrema
The membrane in the middle of the cochlea is called the…
Basilar membrane
What are the names of the fluid in:
1. The scala vestibuli
2. The scala tympani and media
- Perilymph
- Endolymph
Where is the round window?
Below the oval window
Where are hair cells located in the inner ear?
On the basilar membrane
The basilar membrane is (wider/narrower) at the apex than at the base
Wider
High frequency sounds are detected at the (apex/base) of the cochlea
Base
Low frequency sounds are detected at the (apex/base) of the cochlea
Apex
Describe the stiffness of the cochlea at the base and the apex
Stiff at base, floppier at the apex
Describe what is meant by tonotopic map
The inner ear arranges vibrational frequencies on the basilar membrane
Describe the role of the tectorial membrane
Overlies hair cells and moves with vibration to allow hair cells to be deflected and open
Are outer or inner hair cells more important for the detection of sound?
Inner
What is the organ of corti?
Structure along the basilar membrane which is made of hair cells and supporting cells
Describe the formation of outer hair cells
Form little V’s in rows
Are there more outer hair cells or inner hair cells in an ear?
Outer hair cells more abundant
Do hair cells have axons?
No, synapse to ganglion cells
When hair cells are lost, can they be replaced?
No
What neurotransmitter do hair cells release in response to activation?
Glutamate
What is the proposed role of outer hair cells?
Might contribute to amplification of sound or protection (moving tectorial membrane so that hair cells are not activated at high volumes)
Describe the input/output of outer hair cells
Few input and a lot of output
Inner hair cells connect to (a single/multiple) spiral ganglion cells
Multiple
Describe stereocilia
The little staircase projections on the tops of hair cells which contact the tectorial membrane
Describe kinocilia
Large stereocilia which disappear in adult hair cells in humans
What structure connects the stereocilia of the hair cells to each other?
Tip links
Describe the arrangement of stereocilia on the apical surface of a hair cell
Staircase arrangement: arranged from shortest to tallest
When stereocilia are bent in one direction, what happens?
If bent enough to reach threshold, the cell will depolarize
In hair cells, describe how mechanical energy is transformed into electrical energy
Vibrations of the fluid move the basilar membrane, pushing the stereocilia of the hair cells over to one side and causing channels to open, depolarizing the cell
What initiates bending of the stereocilia?
Upward motion of the basilar membrane
Describe how hearing aids can be customized
If a person is struggling with hearing specific frequencies, the hearing aids can amplify only those frequencies
Describe a cochlear implant
An electrode with frequency bands is inserted into the cochlea surgically, it detects certain frequencies and stimulates the basilar membrane in the area which corresponds with that frequency
Where does a cochlear implant get inserted within the cochlea?
Along the scala tympani
What is Usher syndrome?
Loss of visual and auditory input - mutation in a gene common to both systems
Describe the mutation in Usher syndrome
Mutation in the tip links of hair cells and cilia of rod photoreceptors - results in supporting cells of the retinal pigmented epithelium not being able to support photoreceptors
In a person with complete loss of hair cells, what 3 treatment approaches are available?
Cochlear implants
Stem cell replacement therapy
Gene therapy
What are three key requirements in cell replacement therapy?
- Cells need to have the correctly specified fate
- Cells need to be incorporated at the correct location
- Cells need to integrate into the correct circuit
Describe endogenous stem cells
Adult supporting cells which are induced to become stem cells
What are 2 exogenous sources of stem cells?
Embryonic stem cells
Induced pluripotent stem cells
What is an important structural protein in the stereocilia of hair cells?
Actin
Other than tip links, what other links connect adjacent stereocilia?
Lateral links (in the middle of stereocilia) and ankle links (at the base of stereocilia)
What are the 2 ends which connect to either end of a tip link called?
The upper tip link density and the lower tip link density
Describe how tip links can be removed experimentally
If hair cells are treated with BAPTA (A calcium chelator) this destroys the tip links
What happens to hair cell activity if tip links are removed with BAPTA?
Hair cells do not respond: tip links are required for hearing
Can tip links be regenerated?
Yes, tip links can regenerate. Hair cells cannot
Where are the mechanotransduction channels located on stereocilia?
At the lower tip link density
How was it determined that mechanotransduction channels of stereocilia were located at the LTLD and not the ULTD?
In calcium imaging experiments, there was no cation influx at the tips of the tallest cilia, suggesting that channels must be at LTDL
If they were at UTLD, we would see activity at the top
Describe the mechanical action exerted by tip links upon vibration of the basilar membrane
Tip link “tents” the shorter tip link, pulling on this and initiating a signaling cascade in the LTLD
What is the proposed mechanosensory channel in the LTLD of hair cells?
TMC1/2 complex: complex of 4 proteins
TMC1/2 (main)
PCDH15-CD2
TMHS
TMIE
Describe what is known about the mechanosensory channels in the stereocilia of the hair cells
TMC1/2 complex, associated with the LTLD and coupled to the mechanotransducer (MET) channel
Do hair cells produce graded potentials or action potentials?
Graded potentials
To depolarize a cell, which ions flow into the cell and why?
Ca and K+ go into the cell
This is because K+ is higher in the endolymph than it is in the cell
Endolymph has (high/low) K+
High
Perilymph has (high/low) K+
Low
Describe which parts of the hair cells are exposed to endolymph and perilymph
Stereocilia: endolymph
Soma: perilymph
Mechanotransduction channels of the stereocilia are closed when cilia are (direction)
Upright
When stereocilia are pushed the “opposite direction” (force leading from tall side of staircase), the hair cell is…
Hyperpolarized
Describe the intracellular pathway starting at deflection of stereocilia towards depolarization to recovery
Deflection > K+ and Ca++ influx > Opening of V-gated Ca++ channels > further Ca++ influx > Vesicle release > when K+ reaches soma (in perilymph, low K+) get K+ efflux > recovery
Describe calcium-dependent fast adaptation
Ca++ influx helps depolarize the cell, but when inside the cell Ca++ acts on channels to close them to prevent further influx - this could involved dissociation from a gating spring, but this is just a theory
Describe “slow” adaptation of hair cells
Calcium-dependent movement (via a myosin motor) of the upper tip link density down the stereocilium, which decreases tension on the tip link
Describe how outer hair cells may be involved in cochlear amplification
Receive input and uses motor proteins such as prestin to make the soma of the cells grow or shrink, which could change the pressure of the stereocilia on the tectorial membrane - can amplify or quiet sounds to protect hair cells
The auditory nerve is made of a collection of these neurons
Spiral ganglion cells
What brain structures does afferent sound information pass through to get to the auditory cortex?
Cochlea > cochlear nucleus > superior olive > inferior colliculus > Medial geniculate nucleus (thalamus) > auditory cortex
At which brain structure does sound information from both ears get combined?
Superior olive
True or false: the superior olive maintains a tonotopic map
True
At which brain centre is the tonotopic map of sounds lost?
None: maintained all the way to the auditory cortex
Sounds associated with the apex of the cochlea are more (frontal/posterior) on the auditory cortex
Frontal
What are the 3 vestibular organs of the inner ear called?
Utricle
Saccule
Ampullae (semicircular canals)
What is the purpose of the vestibular system?
Information used to maintain body posture, eye position, and balance
What are the sensory cells of the vestibular system?
Hair cells
The semicircular canals are sensitive to this kind of movement
Head rotation (side to side and nodding movements)
The otolith organs are sensitive to this kind of movement
Gravity and head tilts
What is structurally different about hair cells of the vestibular system compared to the auditory system?
They maintain their kinocilia
Describe how bending of stereocilia is initiated in the vestibular system
There is a gelatinous layer which covers hair cell stereocilia and when the head is moved the stereocilia are deflected
What is otoconia in the otolith organ?
Bits of calcium carbonate which weigh down the gelatinous layer on top of the hair cells
