Flashcards in Auditory I-III Deck (45)
How does sound work?
Sound radiates from vibrating sources (a tuning fork or the vocal cords of the larynx) as a series of pressure waves of alternating compression and rarefaction of air molecules.
What is rarefraction?
Decreased air density (pressure)
What is compression?
Increased air density (pressure)
What is the intensity of a sound?
Loudness (the higher the intensity, the louder a sound is)
Increases when the air is compressed more forcefully during the peak compression in each cycle, resulting in increased density of air
What is the unit of intensity of a sound?
dB SPL (decibels Sound Pressure Level)
0 dB SPL= threshold
20dB SPL= 10 times the pressure of threshold
40 dB SPL= 100 times the pressure of threshold
What is frequency (in relation to sound)
Number of times per second that a sound wave reaches the peak of rarefaction (or compression)
What are the units of frequency?
Hertz (Hz; cylces/sec)
How do audiologists quantify hearing loss?
For each ear, check different frequencies to see the lowest intensity (dB SPL) that a person can detect (threshold)
Common in elderly
Loss of high-frequency hearing
Especially problematic for the perception of speech since fricative consonants (such as t, p, s, f) are distinguished by high frequency components that fall in the upper end of the human audiogram.
Transmission of sounds through the ear
Sound pressure waves reach the middle ear
Pressure changes move the tympanic membrane
Tympanic membrane pushes against the ossicles.
External ear components
pinna and external auditory meatus (ear canal) bounded by the tympanic membrane
Middle ear components
cavity containing the ossicular chain or 3 middle ear bones: malleus, incus and stapes
Inner ear components
cochlea and the semicircular canals
Fluid is much more resistant to movement than air; in mechanical terms, water is said to have a high impedance, and air a low impedance.
Most of the sound’s energy (>99.9%) reaching an air-water interface is reflected back, and and
How does the ear fix impedance mismatch
The middle ear!
The middle ear bones or ossicles, the malleus, incus and stapes, translate the airborne pressure waves into motion of the fluid of the inner ear.
The area of the foot of the stapes is much smaller than the tympanic membrane, so the smaller area means larger pressure amplitude.
The orientation of the middle ear bones confers a levering action resulting in a larger force (a gain of about 1.3:1)
Conductive hearing loss
Mechanical transmission of sound energy through the middle ear is degraded.
1) filling of the middle ear with fluid during otitis media (i.e., ear infection)
2) otosclerosis, in which arthritic bone growth impedes the movement of the ossicles
3) malformations of the ear canal (atresia), including “swimmer’s” and “cauliflower” ear
4) perforation/rupture of the tympanic membrane
5) interruption of the ossicular chain
6) static pressure in middle ear
Sensorineural hearing loss
Occurs from damage to or the loss of hair cells and or nerve fibers
1) excessively loud sounds
2) exposure to ototoxic drugs (diuretics, aminoglygocide antibiotics, aspirin, cancer therapy drugs)
3) age (presbycusis)
How can you test if hearing loss is sensorineural or conductive?
Compare the audibility of a 512 Hz tuning fork held in the air or pressed against the skull.
In conductive hearing loss fork against bone is effective at presenting sound by bone conduction, thus overcoming the conductive loss that pertains to air-borne sound.
3 compartments of the cochlea
scala vestibuli, scala media and scala tympani
fluid-filled, membranous compartments
separates the scala media and the scala tympani
Organ of Corti
Sits on top of the basilar membrane, within the scala media
Contains the inner hair cells that transduce sound into electrical signals
A hole in the BM located at the apex of the cochlea that connects the fluid filled scala vestibuli to the scala tympani. Relieves pressure.
Tonotopic arrangement or tonotopic map of basilar membrane
Mechanical properties of the BM vary along the length of the cochlea. As a result the compression of the oval window sets a traveling wave that will reach a maximum amplitude at a certain location along the length of the BM.
At the base of the cochlea (the end near the oval and round windows), the BM is thinner, narrower and more rigid (inner hair cells here vibrate best to high frequency sounds), while at the apex the BM is more flexible, wider and thicker (inner hair cells here vibrate best to low-frequency sounds).
What is the primary stimulus attribute that is mapped along the cochlea
Frequency because each IHC will respond best to a certain frequency determined by the mechanical properties of the BM at that particular location
Inner hair cell
Convert mechanical vibration into membrane potential changes
Stereocilia project out from apical surface of inner hair cells
Activation of hair cells
When the stereocilia bundle is pushed in the direction toward the longest stereocilia, the membrane potential depolarizes; when the bundle is pushed in the direction toward the shortest stereocilia, the potential hyperpolarizes.
Bending of the stereocilia results in altered gating of transduction channels located near the tips of the individual hairs. The transduction channel is a non-specific cation channel that is voltage-insensitive.
K+-rich fluid that fills the scala media and bathes the stereocilia on the apical end of hair cells
A fluid with ionic composition similar to blood (high Na, low K+) that fills the scala vestibuli and scala tympani and bathes the basal end of the cell
The positive potential inside the scala media due to active pumping of K+into the endolymph by the stria vascularis
Magnitude of +80 mV (endolymph positive with respect to perilymph).
Provides further driving force for influx of K+ ions into the cell. In essence the endocochlear potential is added to the cell’s membrane potential so that the total driving potential across the stereocilia membrane is a whopping –130 mV (the membrane potential, -50 mV, minus the endocochlear potential, +80 mV)