physiology of hearing Flashcards
what kind of wave is sound
longitudinal
what is the speed of sound in air (room temp)
340 m/s
what is the course of sound through the ear (6)
- the sound wave is funnelled into the ear by the pinna
- travels along the external auditory meatus to the tympanic membrane
- sound energy is passed through to the tympanic membrane causing it to vibrate
- this causes the vibration of ossicles in the middle ear
- the force of the sound is increased and the sound energy is transmitted to the inner ear via the oval window
- sound transduction occurs in the cochlear of the inner ear
what is the name of the 3 ossicles in the middle ear
Malleus, incus and stapes
what are the middle ear ossicles responsible for
Increasing the force of and transmitting sound energy - the force is increased x20 fold
what is the cocktail party effect
Modulation of amplification by OHCs so that specific sounds (i.e. speech) can be focused on in a noisy environment
what muscles are in the middle ear and what is their function
tensor tympani and stapedius -> They act to dampen loud and low frequency sounds by contracting and restricting movement of the ossicles thus protecting the middle ear from damage
what are the 3 main functions of the cochlear
- to split complex sounds (e.g. speech) into simpler ones
- to amplify/dampen certain sounds
- sensory transduction of sounds
what does the cochlear contain(4)
- basilar membrane
- endolymph
- perilymph
- organ of corti
describe the basilar membrane
The basilar membrane acts as a tonotopic map - certain frequencies resonate at different points on the membrane -> The base of the BM is stiff and narrow, while the apex is wider and more flexible allowing for low frequency sounds to resonate in the apex while high pitched sounds resonate at the base
how is sensory transduction achieved by the inner hair cells
- the movement of the BM causes the stereocilia of the IHCs to move against the scala tympani, resulting in the opening of the tip links (containing TRPA1 K+ channels) which allow potassium ions to enter from the surrounding endolymph - the endolymph is K+ rich and so this allows for K+ to flow down its gradient into the cell -> The bottom of the IHC is surrounded by perilymph rather than endolymph which is Na+ rich -> This allows K+ to be actively transported out and Na+ to enter thus maintaining the ion gradient that allows K+ to flow in from the stereocilia
- When force is applied in the direction towards the tallest stereocilium, the tip links open and potassium ions can enter
- This causes the cell to depolarise and release neurotransmitters (glutamate) which bring the type 1 auditory nerve fibres (to which they synapse) closer to generating an action potential.
how is sound understood by the brain (IHC -> brain)
- BM movements are picked up by the Inner Hair Cells (IHCs), which are tuned to be most sensitive to a particular frequency
- The action potentials are propagated up auditory nerve fibres to the inferior colliculus, which, like the BM, has a tonotopic map
- From here, signals (along with other information from rate and temporal coding) are transmitted to high brain areas and complex sounds can be understood
why is sound amplification and modulation important
in order to filter out certain sounds and amplifying others, such as speech
describe how outer hair cells achieve sound amplification (5)
- The OHCs are electromotile and by changing length to induce concordant movements with the BM, they can cause an exaggeration of its motion (i.e. increase the amplitude)
- In mammals, the protein prestin is interwoven in the plasma membrane of the OHCs which is then attached to the tectorial membrane
- When a voltage change is experienced, the structure of prestin is compressed, causing the OHC to change length
- The voltage change occurs due to the movement of stereocilia, causing tip links to open and an influx of potassium ions into the cell, generating a current
- This amplifies the movement of the BM and thus amplifies the signals to the IHCs
describe how the outer hair cells achieve sound modulation
The OHCs are also innervated by efferent fibres which bring signals back down from the brain and allow the OHCs to modify their amplifying effect, akin to positive feedback -> This allows focus on one particular sound while filtering out others - this is known as the “cocktail party effect”
what wave can sound be converted into
sine wave
what is the human hearing range
20-20,000 Hz -> most sensitive around 1000-4000 Hz as this is the range for speech
3 functions of the outer ear in hearing
- gathers sound and transforms sound pressure at the tympanic membrane
- amplifies sound (only by 10-15 dB)
- aids in sound localisation
components of the middles ear (4)
- ossicles
- tympanic membrane
- middle ear muscles
- eustachian tube
what structure is the eustachian tube connected to
the pharynx
what is impedance mismatch and how is it overcome
transference of energy from the air of the middle ear to the inner ear fluid -> overcome by the lever action of the ossicles which increase the force of the sound energy
extra:
F = P x A
P2 = P1 x A1/A2 where A1 is the area of the eardrum and A2 is the area of the oval window
what bone is the cochlear situated in
the temporal bone
what are the compartments of the cochlaer known as
scalae
what is the purpose of the round window
decompress acoustic energy that enters the cochlea via stapes movement against the oval window i.e. allows for vibration to occur in the endolymph
which cochlaer compartments contain perilymph (2)
- scala vestibuli
- scala tympani
what membrane sits above the hair cells
tectorial membrane
what is the labelled line principle
reserves the specificity of a receptor class in encoding a sensory modality to the designated brain area i.e. the neurons attached to specific IHCs which respond to specific sound frequencies run all the way up to the brain meaning that the nerves act as a tonotopic map as well and can form a tonotopic structure in the brain
what kind of nerve fibres synapse with ICHs/OHCs
ICH - myelinated type 1
OHC - unmyelinated type 2 + efferent fibres (can stop the BM from moving as much and thus modulate certain sounds)
what is the attenuation reflex of the middle ear
sabilisation of the ossicles by the contractions of tensor tympani and stapedius in order to protect against loud or low frequency sounds
where is the receiver and transmitter in a cochlaer implant
receiver on outside of head, transmitter inside -> sound transmitted into the cochlear
apart from loud/ low freq exogenous sounds, when does the attenuation reflex happen
when you yourself are speaking -> masks the sound of your own voice so as to allow for other sounds to be heard aswell
where do inner hair cells synapse to
spiral ganglion of corti
what is prestin
a transmembrane protein that mechanically contracts and elongates leading to electromotility of outer hair cells (OHC)
what drugs affect OHCs and how (3)
reduce the amplification effect:
1. furosemide
2. salicyclic acid
damage the OHCs:
3. some abx e.g. kanamycin or gentamicin
what is Presbycusis
age related bilateral hearing loss -> higher frequencies lost first
what happens in presbycusis (physiologicaly - 7)
- loss of HCs
- loss of spiral ganglion nerves
- atrophy of stria vascularis (part of the lateral wall of the cochlear duct)
- stiffening of BM
- ROS leading to Mt. dysfunction => increased cell death
- calcium homeostasis problems ->CCBs may cause decreased hearing threasholds due to loss of Ca2+ Channels on HCs)
- in women - loss of oestrogen means loss of neuroprotective effect
what are otaucstic emissions
spontaneous sounds made by the ear -> filtered out by central auditory nerves
what builds up in OHCs due to loud noise
ROS -> leads to damage
what is the auditory pathway from nerve -> brain
spiral ganglion –(CN VIII)–> ventral cochlear nucleus -> superior olive –(lat. leminiscus)–> inferior colliculus -> medial geniciate nucleus -> auditory cortex (temporal lobe)
