Hearing part 2 Flashcards
What are the three components of the outer ear?
Pinna
Auditory canal
eardrum/tympanic membrane
What is the function of the pinna?
helps focus sound towards ear canal
Helps determine direction of sound because it is highly asymmetrical
How long is the auditory canal?
3 cm long
What is the function of the auditory canal?
protects eardrum
Enhances frequencies of 1000 Hz to 5000 Hz due to resonance
What is the function of the eardrum?
transmits sound as vibration to middle eat by flexing back and forth
What is another name for the eardrum?
tympanic membrane
What are the components of the middle ear?
ossicles - malleus, incus, stapes
What happens in the middle ear when sound pressure causes eardrum to vibrate?
Eardrum causes ossicles to vibrate:
Malleus connected to incus — connected to stapes
stapes transmits vibrations to oval window of inner ear
What is the purpose of the middle ear?
amplifies sounds by a factor of 10 to 50 from air pressure in outer ear to liquid pressure in inner ear
How does the muddle ear amplify sounds?
Concentrates pressure from large eardrum onto small stapes footplate
Uses mechanical leverage to generate large force on stapes footplate from small force on eardrum
What are the two muscles of the inner ear?
Tensor tympani and stapedius
Where are the two muscles in the inner ear connected to?
stapedius muscle is connected to stapes
tensor tympani muscle is connected to malleus
What is the function of the auditory reflex?
Protects inner ear and reduces interference from internal sounds (speech, chewing etc)
How does the auditory reflex work?
Stapedius contracts in response to high sound levels (amplitude) and own speech - holds bones still so less force gets passed to inner ear
Reduces transmission of low frequency/high amplitude sounds
Protects inner ear and reduces interference from internal sounds (speech, chewing, etc)
What are the two components of the inner ear?
semicircular ducts and cochlea
What are the semicircular ducts involved in?
balance and equilibrium
What are the three semicircular ducts called?
anterior, lateral, posterior
What is the cochlea?
coiled snail-like structure that contains sensory receptors for sound
What is the size of the cochlea?
2 mm wide but 35 mm long uncoiled
What are the three main compartments of the cochlea?
Scala vestibuli (vestibular duct)
Scala tympani (tympanic duct)
Cochlear partition (cochlear duct)
What does the cochlear partition contain?
sensory receptors
Is the cochlea filled with liquid?
yes
Where are the three parts located in the cochlea?
Scala vestibuli is located on the top
Scala tympani is located on the bottom
Cochlear partition is in the middle
What organ does the cochlear partition contain that runs along the length?
Organ or corti
What does vibration of oval window by stapes do?
sends pressure waves down scala vestibuli to apex and back up scala tympani towards round window
What are the three parts of the cochlear partition?
Tectorial membrane (upper)
Organ of corti
Basilar membrane
What are the two parts of the organ or corti?
Inner hair cells
Outer hair cells
Describe the inner and outer hair cells?
Inner is one row of cells an the cilia are free
Outer is 3 rows of hair cells and the cilia are embedded in tectorial membrane - move with membrane
What does the round window do?
moves back and forth and absorbs sounds so they don’t go back
How many inner and outer hair cells do we have in the ear?
Not many
3500 inner hair ells and 12000 outer hair cells - easily damaged
What do pressure waves in the cochlea cause?
Basiliar membrane to move up and down
Tectorial membrane to move back and forth
Cilia of hair cells to bend
Where are hair cells embedded? What do they have coming in and out of them?
in tissue
afferent and efferent nerve endings
What are the cilia of hair cells connected by?
Tip-links
Where does transduction occur?
in hair cells
What does bending of cilia to the right and left do?
Bending of cilia to the right causes K+ ion channels to open
Bending of cilia to the left causes K+ ion channels to close
How does transduction occur in the hair cells?
Increased membrane potentials due to K+ entering cause neurotransmitter release on auditory nerve fibers
How are K+ channels opened?
tip links pull on it to open the mechanically gated channels
What part of the ear is most succeptible to damage from from loud sound?
tip-links because they can break from too much movement
How does phase locking work?
Movement of cilia follows changes in pressure
Thus, neurotransmitter release follows changes in pressure
Thus, action potentials in auditory nerve are synchronized with sound waves
How does phase locking work at low frequencies?
firing can occur with every wave –> fires only at peak
How does phase locking work at high frequencies?
firing is intermittent because a single hair cells cannot hit all peaks but when you combine the firing of many hair cells you get phase locking
What is the main message of Bekesy’s place theory of hearing?
location of maximum firing indicates frequency
According to Bekesy’s theory how is the base of the basiliar membrane (by stapes)?
three to four times narrower than at apex
100 times stiffer than at apex
How does Bekesy’s place theory of hearing work?
sound causes travelling waves along basilar membrane whose position of maximum displacement depends on frequency
Location of peak displacement has highest firing rate by auditory nerve cells - indicates frequency
How does the cochlear amplifier work for neural response (Bekesy missed this)?
Basilar membrane is not a passive object
Outer hair cells change shape to amplify and focus motion of basilar membrane - actively interact with sound
What happens when hair cells elongate? What about when they contract?
amplify the motion of the hair cells - increasing firing
stiffen against pressure waves
How do hair cells elongate and contract?
They have efferent and afferent connections - incoming signals tell it to stiffen or relax
How is frequency represented along the basilar membrane?
sound causes waves to travel along basilar membrane
Location of waves peak depends on frequency
Where are low (25 Hz) and high frequency represented along the basilar membrane (1600 Hz) ?
Low is closer to apex and high is closer to base
How does the basilar membrane respond to pure tone frequencies increasing?
as frequency goes up the location of maximum displacement moves towards base
What does basilar membrane activation look like for sound composed of two frequencies (1000 and 2500 Hz)?
gets broken up based on frequency so there are two areas of maximum displacement along membrane
What do complex sounds look like on basilar membrane?
many different locations of max displacement
What does a tonotopic map show?
location of maximum response as a function of frequency, not location
highest frequencies at base
Lowest at apex
What do frequency tuning curves of auditory nerve cells show?
each auditory nerve cell responds to certain frequencies, based on position along cochlea
Does each auditory nerve cell only respond to one frequency?
No a range of frequencies
Why is the cochlear amplifier very important for frequency tuning? What would happen if the outer hair cells were destroyed?
if they were destroyed there would be a very broad tuning
having the cochlear amplifier allows for more specific tuning
What is the function of the ear?
Transduces sound waves into a tonotopic neural signal
