The ear and hearing Flashcards
What are the two systems of the ear?
- Vestibular system
- Auditory system
What is the function of vestibular system in information?
Inform the brain of the position of head and body and how they are moving
Function of vestibular system
Balance
Function of auditory system info
Detect sounds
Locate them in space
Identify sound nuances
Function of auditory system
Hearing
What does pitch (tone) depend on?
Frequency
What does intensity (loudness) depend on?
Amplitude
What does timbre (quality) depend on?
Overtones
What is sound amplitude expressed in>
db
What is sound amplitude expressed in>
db
What is the auditory threshold?
0db
Auditory system pathway
Sound waves move tympanic membrane ➡
Tympanic membrane moves ossicles ➡
Ossicles move oval window membrane ➡
Movement in oval window moves fluid in cochlea ➡
Fluid in cochlea induces response in sensory neurons
Auditory system stimulus pathway
Auditory Receptors (Cochlea) ➡ Brainstem ➡ Thalamus (Medial geniculate nucleous-MGN ➡ Primary Auditory Cortex
Pressure in oval window vs tympanic membrane
Greater pressure (force/surface) in the oval window than in the tympanic membrane,
What happens after a loud sound? (attenuation reflex)
The brain will diminish hearing sensitivity by tensing the muscles → impairment of amplification system
Attenuation reflex
- Protection (delayed)
- Adaptation to loud sounds
- Not hearing our own speech
Where are malleus and stapes linked and how?
Link to skull by muscles
What communicates at apex?
Scala vestibuli and scala tympani
Where does perilymph flow from?
From vestibuli to tympany when stapes taps the oval window
Where does endolymph flow?
Inside the scala media
Basilar membrane at apex is ..
Wider and more flexible
When perilymph is displaced…
Displaces endolymph (vestibular membrane very flexible) and generating waves in the basilar membrane.
Where are high frequencies detected?
Near oval window narrow and stiff
Where are low frequencies detected?
Near apex wide and flexible
Frequency discrimination in
the cochlea diagram
Inner ear: Transduction pathway
A. Upward displacement of basilar membrane creates a shearing force that leads to lateral displacement of the stereocilia
B. Mechanical displacement of the stereocilia in a lateral direction cause depolarization of the hair cell.
C. Downward displacement of the basilar membrane creates shearing force that results in lateral displacement of the stereocilia in the opposite direction hyperpolarization of the hair cell.
What does perilymph have a similar composition to?
Cerebral spinal fluid
What does perilymph have a similar composition to?
Cerebral spinal fluid
Endolymph in scala media ion concentration
High K+ and low Na+
Ion channel in inner ear transduction
Mechanically gated ion channel
How is the inner ear mechanically gated ion channel connected?
In different stereocilia by an elastic filament (tip-link)
Cilia bending in one direction in inner ear
Channel opens: K+ influx and cell depolarization
Cilia straight in inner ear
Channel semi-closed: K+ influx at resting potential
Cilia bending in opposite direction in inner ear
Channel closed: cell hyperpolarization
What does channel opening lead to and why?
K+ influx because of the exceptionally high [K+] in the endolymph!
What do most of fibres in auditory nerve innervate?
Inner hair cells, and the contribution of outer cells is somehow small
What are fibres in auditory nerve
Axons of Spiral ganglion cells
Amplification system by outer hair cells
- Sound depolarizes the outer hair cell
- That change in membrane potential induces the cell to shorten or lengthen (electromotility)
- The change on length amplifies the movement of the basilar membrane
NB!! Mechanism of hearing
- Sound waves induce oscillation in the tympanic membrane
- Pressure waves are transmitted through the ossicles to the oval window
- Pressure on the oval window induces movement of the basilar membrane
- Displacement of the basilar membrane depolarizes hair cells (sensory cells)
- Depolarization of outer hair cells change their morphology, amplifying the signal
- Depolarization of inner hair cells induce neurotransmitter release
- Neurotransmitter evokes AP in the auditory (cochlear) nerve
What are hair cells innervated by?
An efferent fibre
Acetylcholine function in hearing
Can induce hair cell hyperpolarization, reducing the sensitivity of the cochlea locally
When is a cochlear implant useful?
When cause of deafness is sensory-neural (damage/death of hair cells but mostly functioning auditory nerve)
NB! Describe cochlear implant
Headpiece with microphone (converts sound into electrical signals), digital processor, a radio transmitter (conveys a digital code to an internal receiver). The receiver will send electrical impulses through electrodes that enter the cochlea and stimulate the auditory nerve → from base to apex
- Uses natural tonotopy to differentiate frequencies
Cochlear implant diagram
