Lecture 17 Flashcards
How is sound generated?
Particles vibrate back and forth within a restricted area
The sound wave is travelling along the whole length
What is found with the compressed air?
many particles
What is found in rarefied air?
few particles
What is the unit of frequency?
Hz
- 1 peak per second: 1 Hz
- 1000 peaks per second 1000 Hz
What is amplitude?
Air pressure difference between peaks and troughs, expressed as decibels (dB)
What is the human hearing range?
20 - 20 000Hz
What forms the outer ear?
Pinna, auditory canal, tympanic membrane, oval window
What is the pinna?
Latin for “wing)
More sensitive to sounds from in front than behind
Convolutions of pinna play a role in sound localization in vertical direction (not horizontal)
Fixed in humans but mobile in other humans
What is the auditory canal?
Extends about 2.5cm into skull
Moves forward in through bone of the skull until it hits tympanic membrane
What is the tympanic membrane?
Vibrates backwards and forwards and causes movement in the ossicles in the middle ear
What is the oval window?
Moves backwards and forwards causing fluid movement in the cochlea
What are the ossicles of the middle ear?
Malleus (hammer)
Incus (anvil)
Stapes (stirrup)
- Malleus to incus has a rigid connection
- Incus to stapes has a flexible connection
How does the middle ear transfer sound?
Pressure spreads out on tympanic membrane to minimize sound
Pressure is concentrated in middle ear to amplify sound
How does the air to fluid transition transfer sound?
More resistance at oval window than at tympanic membrane
Must overcome resistance for sound to reach cochlea
Round window helps pressure to be relieved somewhere else
What are the inward and outward movements of the tympanic membrane called?
Inward - peak
Outward - trough
- Indent in round window retains same volume of fluid
How does the stimulus gain at the middle ear?
Ossicles amplify sounds to exert 20 times more pressure on the oval window than there is on the tympanic membrane
The oval window would barely move it if was moved directly by sound due to the air-fluid interface, as fluid has a greater inertia (impediance)
What is impedance matching?
Air and water have different inpedance?
- the tendency of each medium to oppose movement brough about by a pressure wave
How does fluid movement occur towards the base of the cochlea?
Scala vestibuli is filled with perilymph and connects at its large end to the oval window
- perilymph has high Na and low K
Scala tympani is filled with perilymph and connects at its large end to the sound window
Scala media is filled with endolymph
- High K and low Na
- Scala media is the only place where endolymph is found
- High K is crucial hearing
Stria vascularis regulates high K amounts in endolymph
Organ of corti is the organ of auditory sensory
How do hair bundles line up?
At the top of hair cells line up along cochlea
How many rows of hair cells are there in the outer hair cells and inner hair cells?
outer: 3 rows
inner: 1 row
How are inner hair cells lined up?
Primary sensory receptors
Send most of the information of sound to the brain
Are there many hair cells?
Very few in number compared to retinal cells
- very precious!
- causes hearing loss if you use inner hair cells
What two membranes cause movement of hair cells?
Tectoral membrane and basilar membrane
What are the afferent fibers that send signals to the hair cells?
Primary afferent neurons by the vestibular cochlear nerve to the auditory hair cell
How do different frequency sounds travel in the basilar membrane?
Cause maximal displacement of the basilar membrane in different regions
tonotopic map
Higher frequency at base, lower frequency at apex
How does basilar membrane displacement affect hair cells?
At peak: - Stapes moves forward - Basilar membrane moves upwards - Hair cells depolarize At trough: - Stapes moves inwards - Basilar membrane moves downwards - Hair cells hyperpolarize Outer hair cells have tallest hair bundles that are attached to the tectorial membrane
What are hair bundles surrounded by?
Endolymph within the scala media
What are hair bundles?
Mechanoelectrical transducer channel
What are inner hair cells?
Stereocilia = tall hair bundles
Are stereocilia a type of cilia?
no: different structure
What is the structure of stereocilia?
Taller at the back, shorter at the front
Sound causes hair cells to be pushed one way or another
Tip links connect stereocilia
What happens when stereocilia move outwards?
Move toward longer stereocilia
Causes channels to open to allow cations in to depolarize membrane of outer membrane
What happens when stereocilia move inwards?
Move toward shorter stereocilia
Causes channels to shut
What does the extra electrochemical gradient provide?
Cations move via hair cells
Channels within hair cells bundle open
First depolarization: due to K
Second depolarization: due to Ca
What are the synaptic vesicles filled with?
glutamate
- activates receptors on afferent nerve
What does glutamate generate?
EPSPs
Stimulate action potentials to fire
To auditory brain cell
What do outer hair cells act as?
Cochlea implant
- electromotile
- basilar membrane moves upward
- hair bundles move towards taller stereocilia
- outer hair cell depolarizes
- outer hair cell contracts
- outer hair cell becomes shorter
- amplifies upwards movement of basilar membrane
What happens when hair cells are stimulated?
Change in size
Outer hair cell depolarization DOES NOT release synaptic vesicles
Inner hair cells are able to respond to sounds greater
Outer hair cells respond to sounds at lower thresholds
