Week 4 - Audition

Question 1

what is the most common cause of hearing loss?

Answer 1

death of hair cells

Question 2

how do cochlear implants work?

Answer 2

1. cochlear prostheses bypass hair cells and excite appropriate nerve fibers
2. general method is to pass multiple electrodes into Scala Tympani
3. stimulating electrodes are connected to a sound or speech processor, which can determine which of the electrodes should be activated in accordance w/ its analysis of the sound waves being presented
   - frequencies, durations, other variables defining particular sounds or speech patterns

Question 3

how does the auditory system hear “sound”?

Answer 3

1. responds to series of rapid fluctuations in air pressure brought about by compression and rarefaction (decompression) of air molecules
2. for any simple tone (single frequency), a plot of the pressure as a function of time yields a sine wave
   - max of sine wave corresponds to greatest density/pressure; min corresponds to least
   - amplitude represents magnitude of pressure change (greater amplitude = greater physical magnitude of sound)

Question 4

how is the physical magnitude of a sound-producing disturbance measured?

Answer 4

decibel scale

• compares physical magnitude of any given sound to a standard physical magnitude
• standard physical magnitude chosen is one near normal human threshold for tones to which humans are most sensitive (1000-3000 Hz)

Question 5

what is the equation of decibels?

Answer 5

20 x log (test P / reference P)

-reference P always = 1, so if had to increase test P to 100, would mean 40 decibel hearing loss

Question 6

what is the structure of the inner ear? how is “hearing energy” transmitted within?

Answer 6

contains auditory receptors as part of Organ of Corti, and filled with aqueous medium

• rarefactions and compressions are associated with molecules of air, and must be transmitted to molecules of aqueous medium
• at oval window, only 1/1000 of incident energy transmitted, meaning a 30 dB transmission loss
• to compensate for this loss, you need a 32-fold increase in pressure

Question 7

where is the increase in pressure (to compensate for 30 dB loss) accomplished?

Answer 7

in the middle air, by principle of P = F / A

• effective area of tympanic membrane is 60 mm2, and the oval window is 3 mm2, meaning there’s a 20-fold increase in P at oval window
• lever action of ossicular chain supplies additional increase
• force at oval window is not equal to force at tympanic membrane (exceeds by 1.3)
• total increase in pressure by 26-fold

Question 8

what is tympanometry? how does it work? what happens in conductive hearing loss?

Answer 8

clinical technique that measures impedance of middle ear to sound

• sounding sorce and microphone are introduced to ear canal
• amt of sound absorbed thru middle ear, or reflected from middle ear, is measured in microphone
• in conductive hearing loss, more sound is reflected than normal

Question 9

what are pathologies that result in conductive losses?

Answer 9

otitis media, otosclerosis, and ear wax in external ear canal

Question 10

what does the fact that “bone and soft tissue have impedances close to water” mean?

Answer 10

even without a tympanic membrane or middle ear ossicles, sound can be transmitted thru bone and soft tissues to head of cochlea

Question 11

what does the analysis of frequency depend on?

Answer 11

the way sound waves of different frequency affect the basilar membrane in the cochlea

Question 12

what are some properties of the basilar membrane? what does this allow? how does it change from the base to the apex? how are the hair cell cilia?

Answer 12

these allow different frequencies to affect it differently

• increases in width as it runs from the base to the apex of cochlea (.04 to .5 mm)
• concomitant decrease in stiffness (100 to 1)
• -for equally applied forces along membrane, regions of lower stiffness (wider apex) are displaced to a greater degree
• -decreased stiffness from base to apex causes an increase in phase lag from base to apex (more out of phase)
• hair cells are short/stiff at narrow/stiff base, and more than twice as long at apex

Question 13

what happens when the basilar membrane is coupled to the traveling wave through the fluid media around it?

Answer 13

1. stapes moves into oval window, compressing a volume of Scala Vestibuli
2. compensation occurs by bulging out of round window
3. downward bulge initially occurs near applied pressure at base
4. stiff base provides restoring force that brings membrane back to neutral position, requiring that volume of compressed fluid is taken up elsewhere and shunted farther along membrane to less stiff region, etc.
5. when stapes moves out, an upward bulge is produced, which follows downward bulge, producing a full traveling wave

Question 14

what does the distance a wave progress depend on?

Answer 14

frequency of sound wave producing that traveling wave

Question 15

what does maximum amplitude vary directly with?

Answer 15

frequency of stimulus

• greater the frequency of sound wave, the less distance the traveling wave will move along the basilar membrane
• lower the frequency, the farther along the basilar membrane will the max displacement occur, and only the lowest frequencies produce waves traveling the full length of the membrane

Question 16

what is the character of the traveling wave reflected in?

Answer 16

including its points of max deflection, the character of the traveling wave is perfectly reflected in pattern of discharges transmitted by primary auditory neurons

Question 17

how are auditory receptor cells arranged?

Answer 17

in an orderly manner along basilar membrane, and are mechanically stimulated by displacements of traveling wave

• template of mechanical events occurring on basilar membrane is transformed into primary neuron discharge
• -receptor cells at points along basilar membrane traversed by traveling waves respond in proportion to amplitude of traveling wave
• -“peak” will have max sound b/c max discharge

Question 18

how wide is the peak displacement of the traveling wave?

Answer 18

as narrow as the width of a few hair cells

Question 19

what does the organ of Corti house, and rest upon?

Answer 19

houses the auditory receptors, and rests upon the basilar membrane

Question 20

what will displacement of the basilar membrane do?

Answer 20

cause movement of auditory receptor cells

Question 21

what are the tectorial and basilar membranes hinged to? what does this mean? what happens if you push them up? if you push them down?

Answer 21

hinged to rigid limbus, which means they operate relative to each other

• if you push them up, it will put a shearing action on cilia in one direction (depolarize, increase rate of discharge)
• if push them down, shears cilia in opposite direction (hyperpolarize, decrease rate of discharge)

Question 22

what does the electro-anatomy of the cochlea establish?

-what is the potential difference between scala media and ground? the inside of a hair cell? across reticular lamina?

Answer 22

a considerable battery for generation of receptor potential

• scala media: +80 mV
• inside of hair cell: -70 mV
• across RL: 150 mV

Question 23

what does shearing action on cilia alter?

Answer 23

the permeability (resistance) of auditory cell membrane, allowing for a change in current flow
-magnitude of change depends on magnitude of shearing force, which depends on magitude of basilar membrane displacements

Question 24

are hair cells spontaneously active? how does their frequency differ?

Answer 24

yes, their membrane potentials oscillate

• frequency at which different hair cells oscillate differs according to position along basilar membrane
• characteristic frequency matches frequency at which it would be most responsive to mechanical stimuli

Question 25

what does mechanical activation do to hair cell cilia?

Answer 25

amplifies spontaneous voltage oscillations

Question 26

what is the role of the central auditory system? how are they organized?

Answer 26

interpret discharge patterns of auditory nerve fibers to produce perception of sound
-cochlear nuclei are first in chain of nuclei that operates in both parallel and hierarchial fashion

Question 27

how is the central auditory system parallel?

Answer 27

each 8th nerve fiber terminates w/in cochlear nuclei by branching to each of its 3 divisions: dorsal, posteroventral, and anteroventral nuclei

Question 28

what is the most important principle governing 8th nerve fiber terminations?

Answer 28

the full range of frequencies transudced by cochlea is represented in each of the 3 nuclei
-high frequencies are represented dorsally, low frequencies ventrally (tonotopic)

Question 29

what is important about the ascending projections of auditory brainstem?

Answer 29

high degree of bilateral connectivity

-important for sound localization

Question 30

what is the ultimate target of auditory information from the periphery?

Answer 30

primary auditory cortex (A1) in transverse temporal gyrus (largely buried in Sylvian/lateral fissure) and surrounding belt of auditory subregions

Question 31

what happens within area A1?

Answer 31

neurons of similar best frequency are arrayed in a strip/belt-like structure that runs perpendicular (dorsoventrally for A1) to high-to-low frequency tonotopic axis

• auditory cortex is organized from cortical surface to white matter in columns
• transforms from 1D spatial frequency in cochlea to 3D spatial organization in cortex

Question 32

what happens if one passes an electrode perpendicular to the cortical surface of a cat?

Answer 32

individual cortical cells with different response properties

• simplest shows narrow tuning around particular best frequency
• others are tuned more broadly and have variable number of multiple best frequencies

Question 33

is the auditory cortex more involved with analysis of pitch patterns or pitch itself?

Answer 33

analysis of patterns
-this was discovered b/c after removal of entire auditory cortex, cats initially lost discrimination of frequencies, but could relearn it; however, they could not discriminate between frequency patterns, and could not relearn them